Odd behavior from Ameritron Amplifier

[jackson_r]

jackson_r;

I've been racking my memory banks here.  In my shop, we used a couple of AL1200s in an industrial process, and IIRC, one of them exhibited a similar problem.  I have a sneaking suspicion that what stretchyman sez about an open choke is spot on, and was the cause of the problem I found with the AL1200.

Again, IIRC, PC1 in the schematic you posted may be a strip wound choke using a high value power resistor as a form; this is probably pretty rugged.  RFC2 was a larger choke wound on a ceramic form, and this was the culprit; the winding had burnt out and needed to be re-wound.

This was quite a while ago, but I'm pretty sure that's how things went down.

Please exercise due caution around HV; it can be quite unforgiving!

I'll take a look under the hood later tonight for any obviously burned out or damaged chokes (from a safe distance).  I know all sorts of nasty things hide inside a tube amp like this that can hurl my fried carcass across the room long after it's been unplugged, so if I see anything that looks wrong I may just have to see if it's worth getting repaired by someone else.  I don't want to be poking around in there with a screwdriver, or anything for that matter.   

[ka1iic]

I looked at the diagram and sometime my eyes fixate on...

The centre tap from the filament transformer to the tube goes to a relay...  bad relay contact... perhaps... if that relay doesn't make contact to ground then no plate current...

That relay contact is taking all the current that the amp draws so there is a possibility that it is charred up/burnt etc etc

I may be wrong (most likely) but I am still praying it isn't the tube...

[Pigmeat]

A very good idea, Jackson. I had an Uncle by marriage who was TV repairman via correspondence school. Fred was a poke with one hand, keep the other in his back pocket kind of repairman. Hanging around his shop was a primer in self electrocution.

Good luck and be careful.

[James Brownyard]

Jackson, did you observe this amp working, putting out full power into a dummy load or antenna before you bought it?

Also, in the beginning, when your watt meter was pegging backwards, what did the meter on the amp read at that moment in the Grid (ig) and Plate (ip) positions?

Can you actually see the filament glowing?

[jackson_r]

Jackson, did you observe this amp working, putting out full power into a dummy load or antenna before you bought it?

Also, in the beginning, when your watt meter was pegging backwards, what did the meter on the amp read at that moment in the Grid (ig) and Plate (ip) positions?

Can you actually see the filament glowing?

James - Did I actually witness it working in person?  No.  I purchased it from a reputable online dealer of used Ham radios, parts, etc.  In fact it was guaranteed to be working so I could return it if it's truly DOA.  But given the hassle that would be involved in returning this boat anchor, I'd rather get it working if it's something easily fixed (or if it's something I'm doing wrong).  The grid and plate were both reading "0" during my initial tests.  I have not checked to see if the filament is glowing yet.  Somebody else recommended that as well and I think I'll do that tonight. 

[jackson_r]

So as far as whether the tube is glowing at all...negative.  I even took off the cover part way so I could visualize it (yes, I'm a death rebel!) and except for the light from the meter, it's as dark as a well digger's arse in there.  

[Stretchyman]

DING!

That's the lightbulb coming on type noise BTW!

And Yes, twas I who recommended the Heater be checked glowing.

More time wasted.....

It's a valve and needs to have a glowing heater, so simple to check and as per a previous suggestion could well be the relay.

I think we're there now and think it probably could have taken a fair fewer thoughts.

Q, is Heater Glowing?

A, NO.

As per my previous suggestion check for 5V across the heater terminals of the valve.

No scary voltages on the valve base so you should be OK.

OK!

[jackson_r]

Sorry again, stretchy.  The cabinet on this thing is such that you have absolutely no way to see if the tube is glowing without at least partially removing the top.  I had to work up the courage to remove the top part way and plug it in an turn it on.  I'll check the tube and the relay a bit later today. 

[Stretchyman]

NO Worries, Bud, but remember its a valve, they 'GLOW'........ Nicely!

 

You will get it working and in doing so will learn from the experience!

Most worthwhile!

 

Str.

[jackson_r]

Hi Everyone - I went to run a few more recommended tests this morning and found out my multimeter isn't up to the task.  I'll buy a new one on my lunch break today.  I took a few photos of the innards and noticed something:

https://drive.google.com/folderview?id=0B1Sx8UoqolwlWnVJVDdZZG5walk&usp=sharing

In one of the photos, you'll see that I circled an area in red.  That's a connection for the wire that goes to the tube "cap."  The solder joint is loose.  Not broken off completely, but loose.  I believe the wire is still making sufficient contact, and I'm not even sure it would stop the tube from working if it was broken completely, but I thought I'd point it out.

[jackson_r]

Tube appears to be "good" from a continuity standpoint.  However, testing probe "flickers" on the pin depending on where it's placed.  Makes me think there's some corrosion that could be causing some problems with conducting voltage to the tube.  Socket seems to be fine at right around 5 volts.  Weekend project will be to clean up the pins on the tube, re-solder the connection to the tube cap and then see where we're at. 

[jackson_r]

First time in over a week that I've had to try to dink around with this thing.  I spent about an hour giving it the twice over today, and here is the only thing that I found that looks amiss:

http://s14.postimg.org/6ib0d08gx/IMG_5549.jpg

The circled connection in red looks fried.  Charred with the insulation of the wires melted.  This goes in to what the manual describes as the "primary connections terminal board."  Here it is, as seen from the rear of the unit:

http://s30.postimg.org/uwwri08gh/IMG_5550.jpg

This is over on the high voltage side of the amp, so I've yet to poke around for continuity with a tester.  Anyone have advice on my next step? 

[syfr]

I got in late but let me try some comments

1) that fried wire is likely due to someone changing the primary voltage from 240 to 120 (I'm guessing since you said the lights dim, that it's running off a 120V line). My guess is it has nothing to do with the issue you're observing.

2) Cleaning the socket pins is a good idea, as is tightening up the plate connections that you said that you've done.

Now that you've gotten as far as you have (reseating the 3-500, cleaning the pins) put the cover back on and see if the filaments light up when you have the amp ON but without any drive?

There's not much use going beyond that point.

Be very careful with this (and any) amp. Keep in mind that plate voltage can stay in the filter caps for quite some time after you've shut the power off , depending on the condition of the bleeder resistors and the internal resistance of the capacitors. 

Let us know what you find....

[ka1iic]

I got in late but let me try some comments

1) that fried wire is likely due to someone changing the primary voltage from 240 to 120 (I'm guessing since you said the lights dim, that it's running off a 120V line). My guess is it has nothing to do with the issue you're observing.

2) Cleaning the socket pins is a good idea, as is tightening up the plate connections that you said that you've done.

Now that you've gotten as far as you have (reseating the 3-500, cleaning the pins) put the cover back on and see if the filaments light up when you have the amp ON but without any drive?

There's not much use going beyond that point.

Be very careful with this (and any) amp. Keep in mind that plate voltage can stay in the filter caps for quite some time after you've shut the power off , depending on the condition of the bleeder resistors and the internal resistance of the capacitors. 

Let us know what you find....

I agree with you Syfr 100 %.  The connection in question looks like a re-connection from 220 v to 120 v and the melting was caused by a heavy duty soldering gun.  Also notice rosin specks on the chassis, that is a good indication of a soldering for the voltage change.

Syfr is also very very correct about the high voltage capacitors holding voltage for a long time after the amp is shut down... handle with great care.  Sure there are bleeder resistors but still the voltage is slow to discharge.

With something like this I would dis-able the high voltage totally because that doesn't seem to be the problem.  Dis-able-ing the High Voltage makes it much safer for you to poke around in the unit.  But you will still have to use great care as to where or what you poke.

If I were faced with this problem I would apply a very low amount of RF drive to the unit and put the Amp in the transmit mode and use an RF probe to see if the RF is getting to the input of the tube (cathode).  If RF is getting to the tube the next thing to check would be the connection of the tube grid to ground, if that connection is OK then.... errrrr...  the tube might have a problem with the Grid... no Grid, no amplify...  That's never a good thing, in the past I have vaporized the Grid element in tubes...  Yes it can be done and much easier than you might think...

Check the relays, mechanical things tend to be a very weak point in any device.  Look at the contacts of the relays they should be shiny but if they are black then they need cleaning.  If they are silver then they can turn black just by sitting around... many years ago I used mercury to clean up relay contacts but mercury is something you really shouldn't play with.  Sometimes relays just get tired (the springs and alignment) and don't do what they should.

I have a gut feeling the RF drive just isn't getting to the tube so check the schematic and look at the RF input flow, all circuits and connections from the RF input to the cathode of the tube.   If RF is getting to the tube and the connection from the tube's grid to ground are ok... then the tubes grid might be the problem.

Anyway.... have a Merry Christmas and a happy new year!

later...

[radiogaga]

C/P from somewhere:
 Carefully remove # 500Z tube..check... amp plate volts should be indicated. Key amp and observe reading on volt scale..should be same as unkeyed amp with tube Removed. Continue to move meter switch to current ranges..no idle current 150/200 indicates that the 3 500Z is the shorted proublem...D117 is also the meter protection diode and the 2 small caps beside often fail together (tube arking inside, tube short's ,to lite of loading control can cause arcking inside tube and tank stage. B+ is on tube at standby or operate, "operate switch" controls the bypass relay remote key line and with tube removed the procedure will help pin down what often is a internal tube defect that is a shorted 3 500Z.
     

KC7QNM, Apr 28, 2011

Any updates?
rgg