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Author Topic: Someone had Corsair II AM TX experience?  (Read 21042 times)

Offline netsmo62

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Someone had Corsair II AM TX experience?
« on: January 01, 2017, 2339 UTC »
Hi all, HI, I have made a Corsair II transmitter for MW operation (see pic attached) with a VFO DDS variant and power out is no problem, but modulation is always dirty and distorted at every input AF level, expecially when its transmitting music. Problem I think is in the LM386-TIP31C-BS170 section, because I have tried back xtal oscillator instead VFO DDS drive with always same result: power strenght is ok but modulation is distorted and coarse. I have tried to modify LM386 gain and/or to indrease R3 to 1,2 Kohm with no appreciated results. Someone that tried to built same transmitter could you give me some clue?

Have a good 2017. F.

Offline Terry

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Re: Someone had Corsair II AM TX experience?
« Reply #1 on: January 02, 2017, 0257 UTC »
I am really rusty on this stuff, but it looks like RV2 sets the bias on Q3. Have you tried adjusting it? The schematic shows it should be adjusted for 1.7 to 2.5 VDC at TP.

The next thing I would try is disabling Q3 (remove the 12 V) and listening to the signal without the benefit of the final amplifier.

Again, really rusty on this stuff, hopefully someone more knowledgeable will comment.
QTH Florida's Treasure Coast, near Stuart 100 mi N of Miami Grid locator EL97uf
Equipment: Kenwood TS-480SAT, R-600, Yaesu FT-857D, R. S. SW portable (Sangean), R.S. Pro-106 Scanner 25-1300 MHz, HyGain 18AVQII, M2 6M 3 el beam, Misc verticals and dipoles
73,
Terry

Offline Stretchyman

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Re: Someone had Corsair II AM TX experience?
« Reply #2 on: January 02, 2017, 1010 UTC »
A Scope would help?.

Not a good design however.

Str.
« Last Edit: January 28, 2021, 2034 UTC by Stretchyman »
'It's better to give than receive' so why Rx when you can Tx!

                                              ;)

Offline moof

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Re: Someone had Corsair II AM TX experience?
« Reply #3 on: January 03, 2017, 0257 UTC »
Same here. I tried several Corsair schematics and all three had by problems usually terrible mud audio or extremely low level. I futzed with every flavor of 386 replaced each individual component I couldn't test the value on.....Stick to the grenade for a tried and true works first time box.

Offline netsmo62

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Re: Someone had Corsair II AM TX experience?
« Reply #4 on: January 03, 2017, 0915 UTC »
Thank you Moof, so you confirm to me that anyone of three Corsair version appearing on Dave Martin's blog is really good working in modulation stage, neither that one use a transistor driver instead latest version with BS170 mosfet. Yes I have tried to change BIAS settings in all way possible, but only with voltage set very very low, modulation quality seems acceptable, so I have tried too to isolate AF LM386 stage before and LM 386+TIP31C stage from rest of transmitter and AF tone until there is regular. Well, what can I say? I will tried "last resort" to consult some experienced HAM in my neighborhood but better will be soon that faulty schemes disappear, 'cause we don't need garbage not really well tested and working that makes you waste time and money.

Thank you.
« Last Edit: January 03, 2017, 0919 UTC by netsmo62 »

Offline Pigmeat

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Re: Someone had Corsair II AM TX experience?
« Reply #5 on: January 04, 2017, 0141 UTC »
I was member of the homebrew group that the Corsair initially sprang from. I recall that during the tweaks on the design, the LM-386 was dropped in favor of another device. As my email box for the group was eaten in the the Great Yahoo Security Breaches, I no longer have access to that info at hand or can remember why the LM-386 was dropped.

There are more than few guys on this board who were members of the group. Perhaps they'll pop up with the info you need?

Offline FREI

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Re: Someone had Corsair II AM TX experience?
« Reply #6 on: January 05, 2017, 1142 UTC »
I was member of the homebrew group that the Corsair initially sprang from. I recall that during the tweaks on the design, the LM-386 was dropped in favor of another device. As my email box for the group was eaten in the the Great Yahoo Security Breaches, I no longer have access to that info at hand or can remember why the LM-386 was dropped.

There are more than few guys on this board who were members of the group. Perhaps they'll pop up with the info you need?

You can probably get in contact with Dave Martin via the Gary Stevens Pirate Radio Forum. He is on there from time to time

Offline netsmo62

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Re: Someone had Corsair II AM TX experience?
« Reply #7 on: January 05, 2017, 1158 UTC »
Ok, I have tweakening schematic doing many many tests and finally I think to have found what was wrong. In schematic AF output of LM386+TIP31C pass through a 17 uH inductance (L2 in schematic) in parallel with 560 Ohm resistor (R5) to BS170 drain. I have eliminate the resistor and substitute the 17 uH 1/2W "resistor-like" inductance I had placed before with a red toroid (T50/2) with 59 turns of 0.3 mm (AWG 28) copper wire.

But the real problem to solve was to separate input AF to LM386 input, so I have added a 1:1 600 ohm audio transformer, with one side directly connected to one RCA input, isolated from ground and other side connect to input of transmitter.

Now modulation is good (see short video linked) normal BIAS regulation without rustles and whistles, IRF530N power mosfet runs from warm to moderately hot depends from BIAS voltage, reach nominal 10W output, 1-1,3 A absorption.

https://youtu.be/DN_Q6t4k530
« Last Edit: January 05, 2017, 1628 UTC by netsmo62 »

Offline redhat

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Re: Someone had Corsair II AM TX experience?
« Reply #8 on: January 05, 2017, 1726 UTC »
I love the recycled PSU heatsink  ;D

I still prefer high level modulation of the final, but I'm glad you got it running.  Someday when I tire of high power (not real likely) I'll start building rigs like this just for fun.

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Offline netsmo62

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Re: Someone had Corsair II AM TX experience?
« Reply #9 on: January 05, 2017, 2346 UTC »
Oh, this was an old prototype when I was thinking final Mosfet become hot like hell and in the back of heatsink put a PC fan  ;D Now is a simple heatsink in the back of metallic box...

I think that problem in the end is the impedance mismatch from AF input through LM386+TIP31C to intermediate Mosfet BS170 and then to final mosfet. When I was testing, with an higher impedance signal injector square wave input signal (without transformer) audio was clear and no distorted. Could be instead use a 1:1 audio transformer same result can be achieved with a little one-transistor preamplifier in front of LM386 input...

Ciao F.

Offline Zazzle

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Re: Someone had Corsair II AM TX experience?
« Reply #10 on: January 06, 2017, 0943 UTC »
Hi there!

But the real problem to solve was to separate input AF to LM386 input, so I have added a 1:1 600 ohm audio transformer, with one side directly connected to one RCA input, isolated from ground and other side connect to input of transmitter.

Now modulation is good (see short video linked) normal BIAS regulation without rustles and whistles, IRF530N power mosfet runs from warm to moderately hot depends from BIAS voltage, reach nominal 10W output, 1-1,3 A absorption.

Late me is late. Good you found a workaround. Feels god to get stuff done, hm? :)

Anyway, may I add my opinion on that? It may be a bit tech heavy, so feel free to ask if I'm confusing you

I've to agree, that the modulation circuit around the LM386 is prone to cause issues. With a distorted modulation I assume that the amplifier goes into clipping (is overdriven). The design is kinda ugly and one would be better off with a circuit that feeds the signal from the Emitter back to the OpAmp/Driver input. Along with a potentiometer to adjust the gain (and modulation grade). Makes it easy to run with input levels from - say - 100mVss to 800mVss.

I've got two theories why the transformer solved the issue:
  • 1) You have a ground loop problem. You can check this by feeding an Audio signal to the input (without transformer) by using a entirly battery driven device (that has no other connections than the one to the transmitter).
  • 2) The input signal is too high and overdrives the amp. The transformer solves it because it's input of 600R is too low for a regular audio source and causes the signal to drop on a way smaler level (which doesn't overdrive the Amp).

Kind regards,
Zazzle.
Trans-/Receivers: JRC NRD-525,  ICOM IC-R72,  YAESU VR-5000,  YAESU FT-897D
Antennas: 80M Halfwave Dipole,  40m Inverted-V,  5/8λ Groundplane,  20M Longwire,  misc. UHF/VHF Scanner Antennas.

Offline netsmo62

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Re: Someone had Corsair II AM TX experience?
« Reply #11 on: January 07, 2017, 1822 UTC »
Interesting that ground loop question. I will try battery operated, however (I don't know if it count) audio source come all from battery operated device (CD Player, Smartphone MP3 player).

Thanks, F.

Offline Pigmeat

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Re: Someone had Corsair II AM TX experience?
« Reply #12 on: January 07, 2017, 1919 UTC »
Hmmm... CD players are notorious for having problems with RF, I've had them go "phht" using quality insulated audio cable wrapped around a ferrite. The things innards just aren't robust enough to handle the RF field around the transmitter, IMO. And I'm talking about frying them exclusively with a transmitter that outputted around 14 W of carrier. Not much power at all. If they'll fry at 14 W, they'll fry at 10. Not enough difference to matter.

If you can still find audio cassettes to record on, nothing beats a simple cassette deck. I've never had a problem with RF at these power levels with an MP3 player either.

Offline Zazzle

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Re: Someone had Corsair II AM TX experience?
« Reply #13 on: January 08, 2017, 1928 UTC »
Hey,

(...) IMO. And I'm talking about frying them exclusively with a transmitter that outputted around 14 W of carrier. (...)

Actually, yes. Very possible. I haven't thought about that because, on my wordesk, I always use a dummy loads and short wires. So no high levels of RF escape. But yes, now that I remember the Video with "a lot of open wires"...

Happened to me outdoors when testing the 30W CW-Beacon. I was using a Laboratory Power Supply. The distance between Antenna and PSU was about 5M. But depending on where I stood the µC in PSU went mad and locked up in emergency shutdown mode.

Kind greetings,
Zazzle
Trans-/Receivers: JRC NRD-525,  ICOM IC-R72,  YAESU VR-5000,  YAESU FT-897D
Antennas: 80M Halfwave Dipole,  40m Inverted-V,  5/8λ Groundplane,  20M Longwire,  misc. UHF/VHF Scanner Antennas.

Offline Pigmeat

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Re: Someone had Corsair II AM TX experience?
« Reply #14 on: January 09, 2017, 0743 UTC »
Believe me Zazzle, it's no fun to go out in prime transmitting season, this time of year, walk in to your tx site in temps well below freezing, be tuned up and ready to go and have one of the things fry on you.

You don't need a radio to transmit then. People can hear you cussing for kilometers as you pack up and tromp out in the dark, hoping your flashlight doesn't die.

The things I used to go through to put on shows for this bunch.