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Technical Topics => The RF Workbench => Topic started by: Charlie_Dont_Surf on May 30, 2020, 2144 UTC

Title: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on May 30, 2020, 2144 UTC
I struggled with whether it was worth going to the trouble to write all this up. Stretchy won't read this and if he does, he won't heed it, because, "not invented here" is very much a thing with him, or as a very crude former colleague of mine would say, "he thinks that his shit don't stink." However I also realized that there might be a few of these transmitters out there ("hundreds" according to him) and those owners might like to know what will happen to the transmitter in due time. So I decided to go into detail and lay it all out for people.

Stretchy wants to be taken seriously as a commercial manufacturer, because it apparently means there is some sort of aura around him when he does that, but yet he wants none of the responsibilities that come with being a commercial manufacturer of a technical product, among which are attention to detail, not being dismissive and having a conscience. So to be accepted as legit, he has to put his big-boy pants on and do the necessary validation work before putting something on the market. Then if he misses something, suck it up, apologize and make it right after the fact. Don't argue about it. If he half-asses this stuff, people like me will hound him.

Recognizing that a 23-page report is likely TL;DR (Too long; didn't read) for the most of you, I will present the introduction and the conclusions below and if that piques your interest, then download the report and read it or at least look at the pretty pictures. And yeah, I want your comments good and bad. Maybe I missed something too.

Introduction
 
This document details the investigation into a "40W TX" failure (twice) and remediation efforts. I strongly suggest that anyone who is considering buying one read this beforehand and consider other options. If you have already purchased one, I strongly suggest you employ the improvements detailed below. As I describe below, the TX may not fail initially but eventually it will likely succumb with enough accumulated stress. This is probably much more detailed than most people would expect on a hobby forum, but I want there to be no ambiguity as to the veracity of my claims and impact of the issue.

Conclusions

   I have witnessed two catastrophic failures on one transmitter and observed evidence of one of the same failures previously on another transmitter.

   Though determination the exact root causes at the device physics level is outside the scope and budget here, it is clear that transients at transmitter turn on with the on/off switch while the DC input is at 24 V are at fault. I have provided documentary evidence for likely failure scenarios.

   I have observed the failure of C1 on my transmitter and observed evidence of the same failure on another transmitter. The failure of C1 was likely the result of accumulated transient stress at transmitter turn on.

   I also observed the failure of the transmitters SiC FET which is likely the result of a Miller-type event.

   I have also presented several methods to ameliorate the transients, with documentation of improvement of the methods I implemented.

   It is important to note that the transmitter may work well initially but if allowed to run their course without mitigation, transients will likely eventually cause catastrophic failure of every transmitter, either by accumulated stress on C1 or possibly another Miller-type event on the SiC FET.

   These failures are the result of a fundamental design flaw, poor turn-on sequencing. The maker's failure to pay attention to necessary up front work and design validation suggests ignorance or negligence. It would essentially impossible for this problem to not be present in other "40 W TX" transmitters without changes to the design or modifications similar to what I have done.


Here is the download link: https://app.box.com/v/3ysyx-444reop
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: ION Radio on May 31, 2020, 1037 UTC
I know very little about electronics and designs and I certainly don't want to get in the middle of any war here but would just like to add my experience with Stretchyman and his transmitters.

I have been using Stretchy's original xtal TX for 3 1/2 years.

Also I have had the 40 watt GAN TX for 1 1/2 years. Yes I have always powered up at 12v and then ramped up to 24v. Just seems to make sense.

I have an order in with Stretchyman for his new 100 Watt transmitter, hopefully that will happen soon.


I have countless hours on both transmitters and have had nothing but a positive experience with Stretchyman and his transmitters.
I will buy every design he comes up with because I know he stands behind his work. What more could you ask for.

There are several of his TXs on the air here in the US and I highly recommend them. They are easy to operate and sound fantastic!

I thought people should know the positive side also.

-IR



Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on May 31, 2020, 1237 UTC
Many thanks for you hard work and dillegence. Quite a read and I will certainly incorporate your findings into my future designs.

 re 'The Lurker' what a plonker, spent hours turning it on and off to see if it blew up, it didn't, so he left it for a year, turned it on and it blew up. Yes I agreed to repair it for free but because of his stupidity and labeling the returned unit as 'Radio transmitter, burned up' with a value of $200, I had to pay fees to receive it. So whilst I'll heed your technical critique (some of it at least) you need to get your facts straight.

Anyway rather than slagging of other people's designs, we're all waiting for yours!

Regards to all out there...

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Kage on May 31, 2020, 1650 UTC
OP definitely doesn't want to investigate the chaos of my homebrew gear without their head exploding lol, but I'm a hobbyist so that's another story.

I want to feel bad for Stretchyman but this could be a learning moment here. Honestly with my own designs I wish I had someone scrutinize my work for free like this, wrong or right, it's still useful to go over things again and analyze. Part of the design process, and why version 2, 3 and so on of a product come to exist.

I do understand just enough about class E/D transmitters and power transients, gate protection, switch-on timing/ramp-up protection (like the big class E ham stuff) and so on to follow along with that long document. So maybe the best idea here is to show how to reproduce the problem easily in the lab, then find a way to quickly fix and put an end to it like the document suggests if this can be reproduced in a similar lab utilizing compatible but other equipment to rule out external faults?
Here I thought my worst fear was switching on a PA FET before the PLL/DSS got running and a pull down resistor on the gate was enough. Ugh, problems show up everywhere even in the most well planned out designs. Then when you think all is working flawlessly something causes a parasitic oscillation where you didn't expect and magic smoke is filling the room.

You know you two might be at odds with one another over this but maybe it would be beneficial to the community at large to work with one another. You're obviously both quite educated by the posts I read like this. Maybe put the egos aside and work together?
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: JimIO on May 31, 2020, 1654 UTC
How is the ON/OFF switch connected in the circuit?
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: ChrisSmolinski on May 31, 2020, 1708 UTC
A very interesting analysis / post mortem, so to speak, thanks for posting it. It also brought back some memories... 

Quite some time ago in a former life I worked in industrial process controls, equipment for steel mills and the like. Nearly 24/7 uptime and high reliability was obviously a requirement. One of my projects was designing a power supply for x-ray sources used for non contact measurement of the thickness of metals while being rolled. Somewhat related to transmitters in fact, as the process we used was to generate a PWM signal (a few hundred volts at around 12 kHz instead of several MHz) which was then sent through up to a few hundred feet of cable to the oil filled source tank itself, where it went through a transformer to generate 5 or 10 kV which then went through a capacitor/diode voltage multiplier bank which produced up to +/- 120 kV of DC at several hundred watts (a few mA of HV current).

It's difficult enough to make this work on the bench. But then to make it work in the real work is a different level of non trivial. Especially since x-ray tubes/sources have a nasty habit of occasionally deciding to arc, resulting in significant currents that like to blow things up. Possibly the tube itself (thousands of dollars) or voltage multiplier components, or the power supply itself. Or everything. Plus now the rolling mill has to shut down while you fix it. Transient detection/prevention (or at least mitigation) is crucial here. Also looking at the entire circuit and determining where/how currents should flow, and what could go wrong to make them flow in undesirable ways/paths.

Years ago, I came across the concept of "Muntzing", I think from a Bob Pease article. Google can find a better description of the process and history behind the term. Basically it's removing parts from a design until you end up with the bare minimum required, to save costs. That is getting rid of the safety factor stuff the engineer added to make the design as robust as possible. Originally practiced by the owner of a TV set company in NYC back in the 50s, now I think common in China  :)   I think it's possible to accidentally Muntz a design by stopping when you get to the point that it starts working, or by squeezing out as much as you can from the components, and then some. Hey, I can push the transistor a little harder, seems to work OK, just runs a little warm. We've all been there, we've all done that, I know I have. 

In the case of transmitters, I am not sure of the value of extracting the last watt vs reliability. You have to quadruple power to get another S unit, double the power to get half an S unit. Can you tell half an S unit difference? Perhaps under marginal conditions, when you really need that extra dB or three. Under most conditions, I am not so sure. S8 vs S9? Probably not.

In the case of my x-ray power supply, the moment of excitement was when our company CEO decided to run the torture tests on it. We set up a test rig of a complete source unit, except the x-ray tube was replaced with a resistive load (no need to nuke everyone with radiation). The lid of the source tank was left off, it was still filled with transformer oil. When powered up, the oil swirled around due to the high electric fields, like a deep fryer.  He used an 8 ft lucite rod with a ground wire attached to the far end, and proceeded to directly short various things out. Lots of bangs and sparks and arcs were produced, and a good time was had by all. He insisted that only he was allowed to do this for "safety" reasons as he did not want anyone else to risk being injured. But we knew he wanted to have all the fun himself.

And yes, it passed the test.  :)

p.s. None of this is meant as a criticism of Stretchy or his transmitters. I suspect there are a lot of them in operation here in the USA, and they give us a lot of fun stations to listen to. So kudos for that!
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on May 31, 2020, 1824 UTC
I don't mind critique at all as long as it's well founded and in the majority I'd say it is. Any improvements will be incorporated and some already have, sequencing for instance. I wasn't go to say anything till CDS had finished his findings. I appreciate the effort he has gone to. My next offering already has sequencing and starts up in low power mode, swr protection, audio monitoring etc.

Few points worthy of note.

I never fitted a 15A fuse, I'm not that stupid and a 3A was fitted when sold.
The 'exploding' C1 was a 630V X7R type and still surprised it gave way.
There is no reason to use gate Res and have found they just slow down the gate waveform hampering efficiency and causing the drivers to run hotter.

I'm continuing with my experiments and have a selection of ops who are willing to test out and abuse (if nesessary) any further designs I make.

Thanks again for your time and trouble.

Regards.

Stretchy.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on May 31, 2020, 2237 UTC
I know very little about electronics and designs...
...Yes I have always powered up at 12v and then ramped up to 24v. Just seems to make sense.

Let me ask this then: if you know very little about electronics, how and why does that make sense to you?

There are some areas where that makes sense and in fact it is critical (GaN transistors, for example, come to mind) but it's rare in my experience to have "low-voltage" equipment that needs the user to do this.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 01, 2020, 0006 UTC
Honestly with my own designs I wish I had someone scrutinize my work for free like this, wrong or right, it's still useful to go over things again and analyze.

I did it because his transmitter caught fire right in front me and once I dug into the details, it was so stupid that it was maddening because it isn't that hard to find and address if you are as capable as he claims to be. I have to investigate some weird stuff and sometimes we never figure out the root cause. Rarely am I confronted with something this obvious. I hooked up the 'scope probes, turned on the power switch, looked at the DSO screen that very first time and said to myself, "are you freaking kidding me?"

It should be said that something catching fire is not a laughing matter in some parts of the world. If you are in a remote location with your transmitter and it starts a fire, there will be hell to pay. Whatever penalty you may get from the communications regulators will pale in comparison to what will happen to you from the standard legal system due to causing a bush fire.

If it was just a question of some esoteric design approach to something, then meh, it's not worth getting wrapped around the axle about it. This is a bit more important than that. I'm more concerned for peoples' safety than anything else.


So maybe the best idea here is to show how to reproduce the problem easily in the lab, then find a way to quickly fix and put an end to it like the document suggests if this can be reproduced in a similar lab utilizing compatible but other equipment to rule out external faults?

Given that the same issue appears on two different transmitters, with two different fabrication processes (SiC and GaN) used for the FET, I'm confident that you will see it with essentially all transmitters of this type because the problem is in the start-up circuitry, or rather lack thereof. Plopping in an outboard 12 V regulator to replace what is coming from the modulator is a bandage but it works well enough to keep the gate calm (but not "still") until the other stuff around it has settled down.

Maybe I will get around to writing that up at some point.

Here I thought my worst fear was switching on a PA FET before the PLL/DSS got running and a pull down resistor on the gate was enough. Ugh, problems show up everywhere even in the most well planned out designs. Then when you think all is working flawlessly something causes a parasitic oscillation where you didn't expect and magic smoke is filling the room.

As another former colleague would say (sarcastically), "it's all part of the fun." Sounds like you know what you are doing though and what to look for. Don't be discouraged.


You know you two might be at odds with one another over this but maybe it would be beneficial to the community at large to work with one another. You're obviously both quite educated by the posts I read like this. Maybe put the egos aside and work together?

My interaction with Mr. Perfect in Old Blighty has left a very bad taste in my mouth. I left my last job over similar abusive behavior, grandstanding and people that don't like their flow disrupted by truth, no matter how many times you say it. Don't count on a collab.

For anybody else, yeah, maybe. I'm kind of burned out on this stuff right now though.

I have been thinking about something cheap and (hopefully) "quick and dirty". (And might be pretty dirty.) At this point, I think it might work, it's just a question of how well. (I guess that you can say that about a lot of things.) We shall see.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 01, 2020, 0023 UTC
How is the ON/OFF switch connected in the circuit?

Download the doc and look at Figure 1, which I realize isn't a great portrayal of it but...

Think of an on/off switch at the end of the range of a volume control. So, with the knob all the way to the left, you turn it right and the first thing that happens is you hear a click and you have turned on the power. From there, the volume knob just increases the gain of the stereo amp, the Class-D stereo amp that is used as the modulator.

When you turn on the power, two things happen:
1) you turn on a 12 V regulator output on the modulator (the stereo amp). Think of it as a regulated supply to power whatever auxiliary equipment you may have along with the stereo amp. This comes on I think something like a few hundred microseconds or a few milliseconds later after the switch is thrown.
2) the audio outputs (what would normally be the speaker outputs) come on around ~500 milliseconds after the switch is thrown, if I recall correctly.

The modulator has its own built in boost converter and generates its own 24 V for its own use from a 12 to 24 V DC input. The internal 24 V is used to power the Class-D amp and the 12 V regulator output I spoke of above.

Does that help?
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 01, 2020, 0045 UTC
Years ago, I came across the concept of "Muntzing", I think from a Bob Pease article. Google can find a better description of the process and history behind the term. Basically it's removing parts from a design until you end up with the bare minimum required, to save costs. That is getting rid of the safety factor stuff the engineer added to make the design as robust as possible. Originally practiced by the owner of a TV set company in NYC back in the 50s, now I think common in China  :)   I think it's possible to accidentally Muntz a design by stopping when you get to the point that it starts working, or by squeezing out as much as you can from the components, and then some. Hey, I can push the transistor a little harder, seems to work OK, just runs a little warm. We've all been there, we've all done that, I know I have. 

Yeah, I know the principle. I sometimes have employed it when I am debugging and have created a mess and have to go back to something barebones just to get to a known working state. From there rebuild gradually, checking everything step by step to figure out the problem. It works in either hardware or software.

I wasn't looking over his shoulder when he was doing this so I don't know but my guess is that sequencing wasn't a concern, other than at a very gross level.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 01, 2020, 0743 UTC
re 'The Lurker' what a plonker, spent hours turning it on and off to see if it blew up, it didn't, so he left it for a year, turned it on and it blew up. Yes I agreed to repair it for free but because of his stupidity and labeling the returned unit as 'Radio transmitter, burned up' with a value of $200, I had to pay fees to receive it. So whilst I'll heed your technical critique (some of it at least) you need to get your facts straight.

As far as methods go, turning on and off a few times (I dont see a mention of hours there so I don't know where you are getting that) is a fairly standard test to evaluate power-on behavior and see if something locks up. It should behave pretty much the same way each time. If you do the on/off cycle faster, it's a way of evaluating the bandwidth of your bias network, which is a way of checking for propensity to oscillate, power supply rejection, etc. Also, look up "double-pulse testing" in the topic of switch-mode circuits. (A class-E TX is a switch-mode circuit.) It's about looking at the circuit under hard-switching transitions without incurring so much power dissipation. So, no, I don't think that what this person did is wrong. I get where they are coming from because I've seen stuff like this this too. Unusual outside the lab perhaps but on the other hand, if your transmitter is stable, why would you care and what do you have to worry about? Maybe you could learn something from them instead of rejecting out of hand.

The lurker forwarded the email traffic between the two of you about this. [All personal info was redacted before forwarding.] Your interpretation of events above does not fit what I am reading below and somehow, I am not at all surprised by that. I think that you need to get your facts straight. It looks like they were pretty angry at you too. Why do you have so many enemies?

Note that the lurker's transmitter also had a 15 A fuse too. So both of your transmitters that I have seen have a 15 A fuse, more than 2x the maximum expected current demand. What an interesting coincidence! Because you just said that you put a 3 A fuse in both transmitters before you shipped them. Out of all the possible fuse values there are in that size too. Wow. Amazing coincidence that two different owners both swapped their 3 A fuses for 15 A. Hmmm. (Shuffles feet. Uncomfortable silence.) Nah, that doesn't make sense, does it? I guess the fuse fairy must have gone into the packages when they were waiting in the cargo area at Heathrow and swapped the 3 A fuses for 15 A fuses and done this on two separate occasions, huh? Yeah, that's probably what happened.

Also, you charged $50US for a cap? Doooooode! Those caps are like $3 each at Mouser in the US. You need to find a better distributor that can cut some better deals for you and your high volume business. You know, 'cuz you're like McDonald's with "hundreds of transmitters shipped" and all

I started forming a picture weeks ago based upon the following and it's starting to gel more:

- The quite obvious transient turn-on pulse issues your transmitters have, that, to me, stick out like a sore thumb.
- The time-dependent nature of the failures those issues will generate.
- The lie you told about the fuse ratings in another post in this thread, trying to make yourself look better.
- The huge current ratings of the actual fuses you deliver with the transmitters that virtually assure internal damage when the shit hits the fan.
- The huge lies you told above about your interaction with the lurker. (See the emails below.)
- The defensive posture you took when the lurker described the tests they did to check out their transmitter, which also happened to reveal the transient issue that I found too. (Not surprising since they all have this problem.)
- The snide, defensive posture you took when I started poking my nose into your business weeks ago.
- The money you charged for a repair (after saying you would only charge postage), like $50 for a $3 capacitor and the presence of a big transient pulse inside the transmitters that ensure that all the transmitters will most likely need a repair at some point, thus guaranteeing a revenue stream.

I'm developing a picture of your "business model" and if that picture is accurate, your "business model", sir, stinks to high heaven. But I guess once you're in the business of profuse lying, all other forms of deceit come naturally.

Ta-Ta

-----------------------------------------

 
Hi Stretchyman,
 
Long time, no talk. It's all my fault and sorry for the long "radio silence" (see what I did there?). I had some personal stuff take over my life for the majority of the year, however, with that mostly resolved, I had some time for the important stuff again. :-)
 
I never got around to soldering in the extra cap for better low frequency audio response nor did I get around to trying out the programmer that you sent yet. I will though. I got a little anxious to do something so I decided to skip those steps for now.
 
I recently put together a kit to take with me to operate at a remote site and made the first tests last Saturday with your TX at 12 V/10 W. All good and signal on SDRs seemed good considering what I was using for power and antenna. I upgraded the antenna a bit today and made a cable to series connect two 12 V batteries for ~25 V for tests  but when I turned on the TX at the remote site today, I got nothing on the RF power meter and the distinct odor of something smoked (I then looked down and could see the smoke coming out the vent holes.)
 
I have no idea what happened. I turned it on into a perfect load (I had just finished tuning up the antenna with an antenna analyzer and antenna tuner before switching over to the TX. I had a VSWR of 1.05:1.) The DC supply was good. (Immediately after the smoke I verified that I didn't hook the batteries up wrong. I saw ~25.5 V at the DC connector with the correct polarity, + is on the center pin.) There was no audio connected at the time, i.e., nothing in the audio input jack.
 
When I got home I opened up the TX. (See below. The fuse is removed.) The fuse (15 A) is blown. I do not see any anything smoked on the top side of the boards. For what it's worth, I do not see a DC short from the center pin of the DC connector to ground. That's been the extent of my failure analysis.
 
I had this thing working at 24 V a year ago when it first arrived and all was fine then.
 
I'm half inclined to go buy some more fuses (I don't have any of that size laying around right now), put one in and see if the problem persists even at 12 V but I have a feeling that the problem didn't just go away. Fuses don't make that kind of smell - or any smell - in my experience so there must have been something else serious going on.
 
I'm happy to poke around a little more if you think it would be helpful.
 
(lurker)
 
-------------------
 
Hi (lurker), it's probably the O/P FET, just send it back I'll fix it for you. I'll only charge postage.
 
Regards and Happy Xmas.
 
Stretchy
 
-------------------
 
(Stretchy,)
 
OK, sounds good.
 
Any idea why the output PA would blow like that? Do you have a way of remediating this?
 
Do you think that the slew rate of the of the power supply has anything to do with it and maybe there is an inrush issue? I ask because there was no issue at 12 V. Would either slowing the slew rate or turning on at 12 V then going to 24 V help that?
 
Thanks!
 
------------------------------
 
I don't know (lurker). Only other I know blew because user was switching swr meter whilst txing.
 
So only the second in 60+ units.
 
Don't worry!
 
Stretchy
 
---------------------------------
 
Fair enough.
 
I need your address since I've never sent you anything.
 
(lurker)
 
---------------------------------
 
(Stretchy's home address redacted)
 
Regards
 
Stretchy
 
Mark as GIFT and DONT mention anything radio related on the customs lable.
 
---------------------------------
(Stretchy,)
 
It's on it's way to you.
 
Happy New Year,
 
(lurker)
 
--------------------------------
 
(Lurker,)
 
Looks like you didn't mark the item as a gift and customs are requesting 80.62.
I'm not willing to pay that so you have 2 choices.
1; it will be returned to you and you can send it again, marked as a gift.
2; you put the balance into my PayPal account via friends and family.
I'm sure I mentioned to mark as gift? Maybe you've never sent anything outside of the US?
Cheers.
 
Stretchy
 
-------------------------------
 
Absolutely marked it as a gift and marked it as "audio equipment". See the lower left of this attachment.
 I've sent plenty of stuff outside the US. Perhaps UK customs folks are illiterate?
If it's not too late, present this to them. If they won't accept it, I will pay for that too.
 
-----------------------------------------
 
 
Ok but it's not marked at ZERO value so they'll want their money I'm afraid.
My PayPal is.
(email address redacted)
 
Cheers.
 
Stretchy
 
-------------------------------------
Money has been sent just now.
I know that you don't care but I'll tell you what happened anyway. I marked the value as " $0.00 " and checked "gift" on the customs form and walked to the counter. I asked the clerk if the shipment was covered under insurance, she said that it was not covered unless I indicate a value. I know from experience that indicating a value might trigger a customs duty but she claimed that there would not be one because it was marked as gift, and I stupidly believed her, so I wrote in digits ahead of the " $0.00 ". That's the last time I do it that way.
 
(Lurker)
 
-------------------------------------
 
No worries, I made the same mistake sending something to Russia last year. It was worth 2K however (a rare item!) and insured it for 300 and the customs charged on the insurance value.
Mother F*ckers!
Anyways don't worry I won't be charging you much for the repair.
Regards.
 
Stretchy
 
----------------------------------
 
Hi (lurker).
OK, it's in poor shape.
Modulator is broken, not sure what's up but have changed the chip and it still wont modulate 100% without cutting out.
Mod tran has caught fire, severly burnt.
Pa and driver blown.
Ive done as much as I can this weekend so will finish next when I have more time.
The modulator is a probem as they are no longer made and the new ones dont fit the case.
I'll have to see what I come up with.
BTW was it hit by lightening!!??
Regards
Stretchy.
 
-----------------------
 
BTW - when I first got this rig a year ago, I put it through a series of stress tests, one being a series of turn on/turn off over and over again at 24 V either with it "cold" (room temperature) or "hot" (after 30 minutes continuous at 40 W carrier with ~100% modulation). Everything was fine. Then it was put away and untouched for almost a year. I took it out for test at Xmas and all was good. Then a week later, a second test and that's when it blew, right at turn on. Air temp was ~12C.
 
(Charlie clarified this with the lurker. The "over and over again" was "5 or 6 times", he thinks, the second one after 30 minutes at 100% sinewave modulation. Shouldn't be a problem, and it wasn't. What he means by "tests": 30 minutes with music into a dummy load. Then went out to the remote site for a second test and BOOM.)
 
-----------------------
 
OK it's all fixed and sending out today.
I'm somewhat perplexed at your series of tests
Why?
If you purchased a $6K ham transceiver are you going to sit there turning it on and off to see if it breaks?
Please don't do that again...
Postage it $30 and $50 for the repair.
So $80 via friends and family to paypal, [redacted].
Regards
Str.
 
---------------------------
 
Thanks for the repair. Please confirm that you sent it the correct address, which was on the box and is listed in Paypal: [ redacted]
 
A few things:
1) I'm concerned by the repair cost as you said before I sent it that you would only charge for postage.
 
2) As for your concern over my tests, let me tell you what I do for a living. I am a [redacted] engineer for [redacted], mostly amplifiers. I take first article products, make sure that they work correctly, make sure that they meet specs and if they don't, make changes. Part of determining whether things work is finding marginal designs. Bias networks and DC power management are notorious for having issues that can be revealed by a small amount of power cycling. Not fatal flaws that need to be fixed, especially in the context of a homebrew project (which this is) where overlooking issues is necessary, but issues that nonetheless I would want to know about. Does it latch up? Does it get stuck in a undefined state due to unequal power-on time constants of the different portions of the rig? Am I ever going to see it go into a weird state when I power it on when I am in the field, in the dark, without equipment? That's one of the points behind doing this. Since this rig goes straight to TX at max power supply current at turn on, issues from large inrush currents are a potential issue. Cycling the power is another way to find these.
 
Stressing things slightly outside the norm but within reasonable bounds is about making sure that things aren't marginal and I'm unlikely to run into problems later. It is standard practice. The point is never to break it. That is a fundamental misunderstanding but I get that from some people who don't have a background in evaluation.
If I had a $6k XCVR, I would power cycle it a few times. I do this with most equipment. However, your question only partially relevant. For one thing, most rigs that I am familiar with will go through a safe power-on/boot-up sequence that takes a half a second or so, so many of the sequencing issues are less likely and the time constants are longer. Also, max current draw is in CW mode at max power.and if a modern commercial rig cannot do QSK at max output (thus max power supply current), then it shouldn't' be on the market. It is less likely to have an inrush issue for this reason. .Also, I'm quite confident that the engineers at Yeasu, Icom and Kenwood will do some power cycling as part of their release to production readiness approval process. Other commercial equipment manufacturers do this for the reasons above.I've had customers in their market segment complain to my employers because my products turn-on sequence did not "play nice" with other vendors stuff.
 
(lurker)
 
-------------------------------
 
Whatever 
You blew it up by abusing the crap out of it.
If it was just the output FET I would have fixed it for nothing.
I've had to replace virtually every component in there at cost to me.
Pay up .
Str.
 
-------------------------------
 
"abusing the crap out of it."  What a load of bollocks.
 
So cycling the power a few times is "abusing the crap out of it"? Then we're all in trouble because that's pretty mild and the modern world of electronics is going to come crumbling down. The semiconductors by themselves can take a lot more abuse than that. Also, since you're pretending that I did it, explain to me how the TX went merrily along it's way for a fucking year after that test with about 2 hours of on-time sprinkled in there between the time I "abused the crap out it" and the time it actually died. The reliability models would say that it would have croaked earlier.
 
Jeez, you would think that the mod transformer would isolate the RF side from the modulator, but no apparently the whole fucking thing got taken down. Think about that for a moment. Think about what that would require to take the whole fucking thing down. How would that happen? It would have to be a major issue.
But you're a fucking dipshit who think that I shelled out $300 to get my jollies by "abusing" electronics. Why would I spend $300 just to do some sort of pretty extensive damage the day after I took it out of the box? Newsflash #1 for you: I've got much more exciting things to do with $300 that don't involve electronics. Newsflash #2 for you: you've got a vulnerable design with insufficient protection, whizkid.
So you're a fucking liar.
Also, so you promised to fix it for the cost of postage and then you only told me that you're going to charge me extra AFTER you did the repair and sent it to me (to the wrong address, of course). You never gave me the opportunity to refuse that extra charge. Here we call that "bait and switch" and it's illegal in commercial transactions. It's deceit, plain and simple. Had you written to me and asked me if I wished to proceed, I would have said no.
 
So you're a liar and you're deceitful. It's no wonder your reputation is in the gutter over here.
 
I will pay you for the postage (as I agreed to) and I will pay half of your repair charge (that I never agreed to) because it does work now, though I will probably never use it again. You won't get any more money out of me. I'm done with you.
 
Also, FUCK RIGHT OFF, MOTHERFUCKER.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on June 01, 2020, 1040 UTC
Yep that's him, well you always get one, or two.

I really don't know what you're trying to achieve by posting all this?

Your spending all this time and effort, why?

Do you really think anyone is going to read thru all of someone else's ramblings?

I couldn't be bothered the first time.

Well, whatever you're trying to achieve good luck to you.

I've tried to be civilised and heed your wise words but you seem to be going head long into slagging me off.

I'm s big boy, I can take it and frankly care not for you or whatever it is you are trying to do.

I'll leave it there.

I'll delete my posts in due course and leave yours floundering in nonsense.

Bye bye.

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Azimuth Coordinator on June 02, 2020, 0330 UTC
Interesting failure analysis.. Nicely done..   But.. the design is basically a hobbyist kit and well... you know how that can go...   Now that you know the shortfalls I'm sure you can fix it yourself and improve the basic design and share your findings with other who also bought this Tx so they can also benefit from it.   Now if you want Broadcast Quality... I know a guy..

caveat emptor

tAC
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Radio Station on June 02, 2020, 1752 UTC
This forum is use to make money (by some) that leads to the all problems noted here (finally exposed).  Instead of being a free exchange of designs and ideas of many people to achieve a goal that all can benefit from.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: ChrisSmolinski on June 02, 2020, 1800 UTC
This forum is use to make money (by some) that leads to the all problems noted here (finally exposed).  Instead of being a free exchange of designs and ideas of many people to achieve a goal that all can benefit from.

I think there's plenty of room here for both. Commercial products can exist side by side with "free" (I assume you mean open / public domain). The former doesn't preclude discussions of the latter. Some people do not have the equipment (or technical ability) to build their own equipment, or don't want to.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Radio Station on June 02, 2020, 2357 UTC
This forum is used to make money (by some) that leads to the all problems noted here (finally exposed).  Instead of being a free exchange of designs and ideas of many people to achieve a goal that all can benefit from.

I think there's plenty of room here for both. Commercial products can exist side by side with "free" (I assume you mean open / public domain). The former doesn't preclude discussions of the latter. Some people do not have the equipment (or technical ability) to build their own equipment, or don't want to.


Well coming from someone that also sales stuff I can understand your view as you understand mine.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: syfr on June 04, 2020, 1937 UTC
Is there supposed to be something wrong with people who "sale" stuff?  Is that a bad thing?

People "sale" stuff because there's people who want to buy it... that's true for Stretch and for Chris. I'm pretty grateful for the opportunity.  There's about 10,001 schematics on the web that are free. A lot of them are posted here too, but not everyone wants to assemble a BOM, lay out a circuit board, order parts etc.

  I don't recall either Chris or Stretch refusing to answer any technical questions because they want you to buy their gear, because that never has happened.

Now, we return you to the regularly scheduled axe grinding.


Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 06, 2020, 1618 UTC
I really don't know what you're trying to achieve by posting all this?
Your spending all this time and effort, why?

I have explained the safety issues and concerns. Since you either don't seem to understand or (more likely) don't want to understand, then I can't help you.

I also don't like being lied to and I don't like to see other people being lied to either.


I really don't know what you're trying to achieve by posting all this?
Do you really think anyone is going to read thru all of someone else's ramblings?

I have provided the evidence of what you said and evidence refuting what you claimed. Whether someone feels like reading it is another question but I am being transparent and providing proof, unlike you who is fast and loose and never seem to provide proof for what you claim.

So I ask you, who would you be more likely to trust, someone who brings proof or a bullshit artist?


I couldn't be bothered the first time.

That's the difference between you and me. Your attention to detail is lacking for someone who wants to be a commercial provider with a happy customer base. I'm not expecting to make everyone happy but I'm also not making "hundreds" of faulty transmitters.

Let's also note that after admitting the issue exists, you are not seeking to make things right for the owners of your transmitters. You have only said that you will make improvements for future transmitters. You are doing your current customers a disservice.


I've tried to be civilised and heed your wise words but you seem to be going head long into slagging me off.

See above. You deserve it after all the abuse I and others took from you.

Also, nice attempt to make yourself the victim here. You have a future in public relations.


I'll delete my posts in due course and leave yours floundering in nonsense.

Nothing like covering up the evidence.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 06, 2020, 1649 UTC
Interesting failure analysis.. Nicely done..   But.. the design is basically a hobbyist kit and well... you know how that can go...   Now that you know the shortfalls I'm sure you can fix it yourself and improve the basic design and share your findings with other who also bought this Tx so they can also benefit from it.   Now if you want Broadcast Quality... I know a guy..

caveat emptor

tAC

In general, I agree that a hobby project has lower standards but once you start selling "hundreds" (as he claims), I suggest that it is stepping out of strictly hobby territory and start bleeding into commercial-product territory. In my opinion, the standards should be a bit higher for a reputable commercial product. Even if you don't agree with my previous sentence, I think that you would agree that having "hundreds" of ticking time bombs out there is undesirable.

The report was written with the backend of it providing various avenues to improve the issue. However, I understand that some will not be able to weigh the various options and just want a prescribed solution. Other tasks need my immediate attention but once I get some time in a few weeks or months, I can boil it down.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: redhat on June 06, 2020, 1844 UTC
FWIW, I know of PLENTY of commercial products I would consider ticking timebombs....PTEK comes to mind ;D  Point is, even in the commercial world, there is good stuff, and junk.

+-RH
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 06, 2020, 1900 UTC
Is there supposed to be something wrong with people who "sale" stuff?  Is that a bad thing?

People "sale" stuff because there's people who want to buy it...

Absolutely nothing wrong with selling your wares. The seller is providing a service and it's fair to get some money for that. Don't misinterpret my criticism of his faulty design and his lying as criticism of selling things in general.


I don't recall either Chris or Stretch refusing to answer any technical questions because they want you to buy their gear, because that never has happened.

Again, there are answers and then there is easily disprovable false information. Those are two separate things. False information isn't doing anyone any good.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 06, 2020, 1928 UTC
FWIW, I know of PLENTY of commercial products I would consider ticking timebombs....PTEK comes to mind ;D  Point is, even in the commercial world, there is good stuff, and junk.

+-RH

Fair point. In that case, let's say "reputable commercial products" then.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: davemartin on June 06, 2020, 2152 UTC
Has he gone?
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 07, 2020, 0520 UTC
Has he gone?

Who? Me? I'm still here.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on June 07, 2020, 0834 UTC
Me too!

BTW if anyone is having any problems with any of my Tx's please get in touch. I have developed a PCB which is simple to fit and gives power sequencing and current limit/slow starting to any design using a MT.

PM for details.

Regards

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: davemartin on June 07, 2020, 2033 UTC
Has he gone?

Who? Me? I'm still here.
No not you.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on June 07, 2020, 2234 UTC
That'll be me then, otherwise I'm not sure what's going on....  -...--.-

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Antennae on June 21, 2020, 1653 UTC
Charlie Don't Surf, have you read my post back in October?  My TX went pop with the powersupply. I proposed a solution for the powersupply too: 
https://www.hfunderground.com/board/index.php/topic,58324.0.html

On one hand I like the fact that Stretchy is making cool transmitters that are small, efficient and powerful. But I don't like when he does things like bring your friend The Lurker into the light.  That's not very discrete. Some people use these things for pirate transmissions, unlike myself, and they could get unwanted attention from the govt.s that are no doubt reading these pages too.

FYI, I just checked my fuse, its a 15A and I managed to blow it with my last attempted repair. 
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on June 23, 2020, 1507 UTC
Wasn't me who bought him into the light, it was the eejit..

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on June 27, 2020, 2102 UTC
Wasn't me who bought him into the light, it was the eejit..

Str.

Thanks for the flowers, asshole. You don't know when to STFU, do you?

While I am here, for the record, The Lurker contacted me and he volunteered his TX and all the information.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on September 10, 2020, 0635 UTC
Following up:https://www.hfunderground.com/board/index.php/topic,71438.msg240668.html#msg240668
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Mickpeasoup on June 21, 2021, 1503 UTC
I was going to purchase a Transmitter from Stretchyman, but I'm now going to bow out after seeing several posts including this thread.

Aftersales service plays a huge part in my purchase, & as what I've read here is pretty grim I'll be looking elsewhere.

Many thanks for the heads up.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on June 21, 2021, 2159 UTC
Such a shame you believe everything you read.

Never mind..

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: CoolAM Radio on June 23, 2021, 0134 UTC
https://www.youtube.com/watch?v=Pf98INtkBeI (https://www.youtube.com/watch?v=Pf98INtkBeI)



Andr
CoolAM Radio - Shortwave
the Netherlands
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on June 23, 2021, 0556 UTC
Thanks...!

Even the low power versions are a different design to the 40W but hey if folk want to believe the Charlie TW*T good luck to them..

Things have improved considerably.

Str
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: syfr on June 23, 2021, 1941 UTC
I was going to purchase a Transmitter from Stretchyman, but I'm now going to bow out after seeing several posts including this thread.

Aftersales service plays a huge part in my purchase, & as what I've read here is pretty grim I'll be looking elsewhere.



I'm not going to slog through the above, but I can tell you  that I got excellent support after the sale in helping me complete my 40M AM transmitter kit.


Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on July 02, 2021, 1708 UTC
but hey if folk want to believe the Charlie TW*T good luck to them..

Things have improved considerably.

Str

What is there to not believe?

You, yourself, are admitting right here (and in previous posts) that you had problems with your 40W TX. You improved it by changing the power sequencing. I wrote up 36 pages in two documents detailing the issues and why they are design flaws. You even thanked me for the first report at the time. I took a different approach to improving it but that's OK; I just didn't want to change the sequencing and make it harder for people to modify.

You admitted that you have a frequency shift issue with your 10 W (I guess) crystal controlled transmitters and believe it to be the result of some specious claim that crystals oscillators axiomatically randomly drifted 1 KHz or more since the days when then they were first invented ~100 years ago, which is completely ridiculous and I explained why. I suggested that the frequency shift issue is more likely that the crystal is dissipating too much power and provided a link to technical articles suggesting the same. This was met by no counter evidence from you.

If this wasn't enough, we have the folks that chimed in here and privately to me that told me that they had the same issue with your 40W TX. The frequency shift issue is further validated every time certain stations go on the air.

Both of these issues could have been found if you had spent a modicum of time actually examining them in the prototype stage before you shipped any. It's fairly basic in the professional world to evaluate your product in the customer use case. In this case, it means actually turning the TX on and leaving it on for an hour or two and make sure that it doesn't break, it doesn't drift, the power output stays within expectations, power dissipation stays within expectations, and on and on. It's also pretty normal in this customer use case to turn the transmitter on and off each time you use it and had you done that, you no doubt would have broken a transmitter, which should have caused you to investigate. Even if that somehow did not turn up anything, when the evidence was mounting that normal customer use was breaking the the 40W TX, I showed that you were just sweeping it under the rug and blaming the user instead of actually addressing it. It was only after I wrote up my expos that you came clean on it.

So, again, what is there to not believe?

It is perhaps no wonder that, with your behavior as it is, that Chris banned you from advertising on this forum. He didn't want to be a part of it. (Side note: I suggest that your signature and Andr posting the video above should also be considered advertising.)


Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on July 02, 2021, 1950 UTC
Please just pxss off.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on July 03, 2021, 0053 UTC
Please just pxss off.

Thank you for your educated, thoughtful and enlightening answer that clears up all questions that anyone on here might have about you and your cheap little fraudulent "work".
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: flexoman61 on July 03, 2021, 0959 UTC
I think everyone is tired of this Charlie. Find another venture....PLEASE!
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: CoolAM Radio on July 03, 2021, 1203 UTC
Please handle further in PM's to eachother - not here anymore!


Andr
CoolAM Radio - Shortwave
the Netherlands
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Radio Station on July 03, 2021, 1529 UTC
Anyone who buys from Stretchyman deserves what they get!!! :o :'( :-\
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Antennae on July 03, 2021, 1541 UTC
When my old model worked, it performed super great. He offered to fix it too but I declined. He's capable of making a better design.  I bet you can trust him if he says he made the design better it.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on July 03, 2021, 1901 UTC
I think everyone is tired of this Charlie. Find another venture....PLEASE!

Take a look at the timeline. I was letting things lie for a long, long time. I've moved on to other things but he brought it up again, not me. Your complaint is not with me. If there ever was an "own goal" (as they say in the UK) here, this is it.

Also, I didn't see anyone complain when he brought it up but the minute I reply with actual facts to set the record straight, I'm mobbed by his fanboys and apparently become the aggressor, somehow. It's good that you've all gotten your biases out in the open.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: outhouse radio on July 03, 2021, 2040 UTC
never have seen a video of one of these things actually performing while hooked to any equipment like a scope and meters etc ,
that's one reason why i never bought one of his things, i get a snicker every time someone airs with one of his things and you see the comment STRETCHY MOD on rocket chat  ;)
-
ps - i decided not to buy from him  or recommend one of his things to anyone long before this thread ever started ,
everyone has to make their own decisions ..

HAVE A NICE DAY
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on July 04, 2021, 1841 UTC
You haven't watched YouTube then?

The newer designs are fine.

I'm trying my best and as suggested just ignore the the negative Twxt

You have a nice a nice day too!

Go away Charlie, we're all tired of you.

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: outhouse radio on July 05, 2021, 0232 UTC
You haven't watched YouTube then?

The newer designs are fine.

 then do a video with them hooked to a scope etc and prove it . it's that simple .
because 13 minutes of you jabbering and waving your hands around doesn't prove a thing .
https://www.youtube.com/watch?v=Pf98INtkBeI
neither does 4 minutes..
https://www.youtube.com/watch?v=WS3opIkvGac
and neither does 3 minutes
https://www.youtube.com/watch?v=myWBTN394g4
-
come on now ,post a video with them working hooked to un-doctored equipment and show the masses how they actually work . because those video's don't do it and your word doesn't mean much .


if a picture is worth a thousands words , then just think what video proof is worth .  ;)

have a nice day

Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: KaySeeks on July 05, 2021, 1853 UTC
I'm trying my best and as suggested just ignore the the negative Twxt

Go away Charlie, we're all tired of you.

We're not looking for you to be performing on-the-job training and you've shown yourself to be less than transparent. If you can't be trusted to get basic, important details right the first time, why should we trust you now?

I, for one, would like to see more details and inspection, not less. Your attitude leaves me with the impression that you are trying to prevent inquiry and hard questioning, and it's clear that I am not alone.

Reading back through some of these posts over the past 12-18 months just now, I can tell from your statements and answers that you might not be the right person to be making transmitters for anyone but yourself. I don't get a confident feeling that you know what you are doing.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on July 06, 2021, 0625 UTC
We're not looking for you to be performing on-the-job training

You hit a sore spot. You told the truth.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on July 06, 2021, 0645 UTC
because those video's don't do it and your word doesn't mean much .

With talk like that you better watch out because the fanboy army is at this moment circling the wagons. They're coming for you.  :D
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Stretchyman on July 06, 2021, 1341 UTC
Let someone else do it then an put up with all the crap you lot spit out.

Fact is no one else will!

Good job I'm thick skinned.

I will leave you all alone.

Bye.

Str.
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: radiogaga on July 06, 2021, 1557 UTC
Dont let the door hit you on the way out................ ;D ;D ;D

rgg
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: outhouse radio on July 07, 2021, 2105 UTC
Let someone else do it then an put up with all the crap you lot spit out. <<<< TRUTH HURTS DON'T IT ...

Fact is no one else will!

Good job I'm thick skinned. <<< YOU SPELLED ARROGANT A&&HOLE WRONG ....

I will leave you all alone. <<< WE CAN ALL HOPE ...

Bye. <<< PLEASE LET IT BE TRUE ...


Str.

i knew he wouldn't post video proof ..

no offense intended to anyone running one of his things ...
like i said earlier , we all make are own decisions ...
have a great day/night
 
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: redhat on July 08, 2021, 1542 UTC
I guess I should have read this document Sooner.

Some thoughts;

I have lost a handful of SiC devices over the last few years while building my transmitter, and usually they failed because of my assumptions in design leading to a few deficiencies.  One thing I did find during my research is that most of the failures occurred during drive loss events.  This seems to be evident in the transient nature of the waveforms you observed.  Most DDS devices produce a DC output when not programmed, and this is specified in the Analog Devices datasheet for the AD9850.  I think Stretchy was using a 9800 series DDS in his devices, mainly for cost reasons.  Until the program is loaded into the DDS, it will latch the output of the PA on, causing an overcurrent failure of the transistor.  My solution was to add a drive loss protection circuit, and I have not lost another device since.

The later versions of his transmitters I noted the elimination of the modulation transmitter, and instead feeding the PA directly from the modulator.  This may well have solved the problem, as the modulator has overcurrent sensing and protection built in.

+-RH
Title: Re: Stretchyman 40W TX Failure Investigation and Remediation
Post by: Charlie_Dont_Surf on July 08, 2021, 2151 UTC

Until the program is loaded into the DDS, it will latch the output of the PA on, causing an overcurrent failure of the transistor.


In this doc (https://app.box.com/v/3ysyx-444reop (https://app.box.com/v/3ysyx-444reop)) in Figure 17 on page 18,
I show the behavior of the gate (which means that it is coming straight from the DDS through the FET driver). It goes through some sort of power-on behavior which causes the gate to pulse and since the drain voltage is already applied, the transistor turns on, etc. Unfortunately, the gate (and thus the DDS) behavior was not at all consistent - each time you turned it on, it looked slightly different (different pulse length, different number of pulses, etc.), which lead me to believe that it is not the result of some sort of UVLO or POR, which I would think would be the same each time you turned it on.

My solution was to add a drive loss protection circuit, and I have not lost another device since.

What you describe is the elegant and preferred solution, IMO. The caveman solution in this case (in addition to protection passives all over the place) was to change the power supply sequencing so that the DDS supply goes on first to let the DDS and the gate go through its (unpredictable) turn on sequence then - after some period of time where it is safe - apply the drain voltage while the gate is on but still idle. Obviously if you wait too long to apply the drain voltage, the gate will already have a squarewave drive applied and you risk problems not unlike the problem you are trying to solve.

I did not do this in the end because I thought that the modifications would be a bit advanced for some and I was trying to keep things on the simple side.

The later versions of his transmitters I noted the elimination of the modulation transmitter, and instead feeding the PA directly from the modulator.  This may well have solved the problem, as the modulator has overcurrent sensing and protection built in.

My understanding is that his 100W TX uses a PWM.

I have a PWM circuit in the works that is meant for a wide-variety of mid and low-power transmitters, and I suppose could be an add-on to replace the Class-D amp and modulation transformer in a Stretchy 40W TX. It generates its own internal and external DC voltages and has a simple power sequencer (TI LM3880) to apply those voltages in an ordered manner, creating a safe and predictable turn-on and turn-off sequence for the whole TX. The revision 2 PC board is back from the fab house but I am focused on other things at the moment and haven't populated it yet.