Fender PA100 - smoke -> no sound

[ultrasparc]

Hi there

I'm new to this forum (is this the right thread, it could be "Maintenance, Troubleshooting & Repair" ?), and I'm hoping for some good advice with an old amp.

A friend asked if could take a look at his Fender PA100, now that I restored my old receiver successfully. I have never been inside an amp of this kind - only my old Pioneer receiver, so there are many things I don't know, but by now I can understand most of what is on a schematic (I have downloaded http://schems.com/manu/fender/pa100rev.pdf).

Also I have read that these amps aren't to fiddle with unless you know what you are doing. Well I hope I have enough sense to avoid the dangers, but if some caring person will point out what I should look out for I'd feel a whole lot more safe.

The story he (the owner) told me is, that he was playing his Gibson hooked into it, when he sensed the smell of smoke, the sound disappeared so he switched it off instantly. This happened about 3 years ago and it haven't been powered on since.

My plan is to open it up and look for obvious damaged components, and when I have peeked inside I will post my observations, in the hope that I can get some good advice.

Can there still be charge stored in it after 3 years, or can I safely put my fingers inside?
Maybe I should take some voltage readings just to be sure.

Thanks
Ole

[DrGonz78]

Can there still be charge stored in it after 3 years, or can I safely put my fingers inside?
Maybe I should take some voltage readings just to be sure.

Rule of thumb tells me to never trust the unknown when dealing with any equipment in regards to electricity. Perhaps someone powered it up yesterday for even 30 seconds. Don't risk your life touching something without using your test equipment to measure, since that is why you have that piece of equipment. Now if it was sitting in my garage for 3 years and I know (for certain) that it had not been powered up then yes it should be discharged. I think even then I would check the caps to see if there were still millivolt readings, which is healthy.

[ultrasparc]

Rule of thumb tells me to never trust the unknown when dealing with any equipment in regards to electricity. Perhaps someone powered it up yesterday for even 30 seconds. Don't risk your life touching something without using your test equipment to measure, since that is why you have that piece of equipment. Now if it was sitting in my garage for 3 years and I know (for certain) that it had not been powered up then yes it should be discharged. I think even then I would check the caps to see if there were still millivolt readings, which is healthy.

True, better safe than dead - I'll deal with it as has it been powered up.

[Leo_Gnardo]

Little chance old filter caps held charge for years BUT treat 'em as iif they did.

More than likely the smoke noted years ago was from failing hi voltage filter caps anyway. You'll get experience putting in new ones soon enough. Pay attention to polarity or - - - more smoke.

[ultrasparc]

Little chance old filter caps held charge for years BUT treat 'em as iif they did.

More than likely the smoke noted years ago was from failing hi voltage filter caps anyway. You'll get experience putting in new ones soon enough. Pay attention to polarity or - - - more smoke.

Agree - treat as they were charged.
I will know more once I get it opened, and you are probably right - there is likely a leaking or bulging cap or two in there.

Yes - observe polarity is important

[ultrasparc]

OK, now I've had a look into it and there's a couple of obvious failures

First it seems one of the 6L6'es is gone - it has melted metal on the inside of the glass.


Further visual inspection did not reveal anything, so I powered on, keeping my finger on the switch so I could switch off quickly. Which I did, because there was sparks coming from the Hum-pot, and one of the tube sockets.

A closer look revealed a very blown resistor in the socket of the blown tube (470 ohm)


So two easy targets to act on, but what would the forum do in this situation ?

I see these scenarios:

1. replace only the failed parts
2. replace all 4 tubes and corresponding 470 ohm res?
3. replace all 4 tubes and the 4 470 Ohms stepping up the Wattage to 3 or 5 watts?
4. take it further and replace all caps too?
5. other?

The Hum-pot - which has a little blackening by the contact surfaces

- is connected by the light-green wires to the tube sockets/470 Ohms - can I expect that the sparks that came from Hum-pot will be gone after replacement of the resistor/tubes.

What would have caused the failure: first the tube went, then the resistor, or the other way around?

Also it needs some cleaning up - there are traces of - I don't know what - dirty (black) dust, mainly around the wiring out of the main trafo, which, by the way has a small crack at wire-entry point

- should I care about that?
Should I give it a shot from the glue-gun to seal it, or just leave it be?

Lots of questions, I know.
/Ole

[g1]

What you called molten metal is the getter flash. It is normal, some tubes have it on the side, some on top. Look at the preamp tubes and you will see all the tops have it.
The 470ohm 2 watt resistor is the screen resistor for the power tube. They usually blow because of tube failure. So that tube that was in that spot needs to be replaced, along with the resistor. The same tube issue is probably what fried the hum balance pot, which should also be replaced.
I wouldn't worry about the transformer, it looks like it may be whatever kind of goop they sealed the wire entry hole with, rather than a crack in the metal.

[trobbins]

Why would the hum balance pot get stressed? The schematic shows ss rectifier, and fixed bias circuitry (so output valves cathodes are at 0V). Perhaps pins 2-3 tracking across one of the output valves?

[J M Fahey]

Why would the hum balance pot get stressed? The schematic shows ss rectifier, and fixed bias circuitry (so output valves cathodes are at 0V). Perhaps pins 2-3 tracking across one of the output valves?

The schematic does not show the shorted tube

[Jazz P Bass]

If the bad tube pucked high voltage into the heater circuit, that would certainly 'challenge' the hum balance pot.

[Leo_Gnardo]

If the bad tube pucked high voltage into the heater circuit, that would certainly 'challenge' the hum balance pot.

Roger THAT! Not only in-tube shorts, the occasional pin3 - pin2 lightning bolt "arc".

Seems half the hum balance pots I see, have to replace.

[trobbins]

I can appreciate pin 2-3 tracking. That amp doesn't have an indirectly heated valve rectifier, with heater powering other valves/humdinger, so not that mechanism. That amp is fixed bias, so an internal plate or screen short to cathode won't stress the hum balance pot. So does that only leave a pin 2-3 failure as the only 'common' mechanism for hum pot stress?

The shorted screen valve could have been to cathode, or even anode I guess.

[ultrasparc]

What you called molten metal is the getter flash. It is normal, some tubes have it on the side, some on top. Look at the preamp tubes and you will see all the tops have it.

well, this tube is the only one with getter flash on the sides, and it is the one in the position with the fried 470 Ohm resistor, so I believe it is failed.

The 470ohm 2 watt resistor is the screen resistor for the power tube. They usually blow because of tube failure. So that tube that was in that spot needs to be replaced, along with the resistor. The same tube issue is probably what fried the hum balance pot, which should also be replaced.
I wouldn't worry about the transformer, it looks like it may be whatever kind of goop they sealed the wire entry hole with, rather than a crack in the metal.

Agree with your assement on the trafo - it doesn't look nice, but must be harmless.

I will get some resistors, and probably a full quad of 6L6GC's - owner agrees to spend the cost, and I suppose they must have a limited time left, before the others will fail as well - or, am I wrong here?

[Leo_Gnardo]

well, this tube is the only one with getter flash on the sides, and it is the one in the position with the fried 470 Ohm resistor, so I believe it is failed.

I will get some resistors, and probably a full quad of 6L6GC's - owner agrees to spend the cost, and I suppose they must have a limited time left, before the others will fail as well - or, am I wrong here?

Since there's one "odd" output tube, I'm guessing it was used as a replacement for a previous failure. Have a close look at the tube socket, scanning especially for the track of an arc between pins 2 & 3. Have a look at the other 3 while you're at it. IF you replaced output tubes, SG resistors, whatever, and left an arc path behind, it would be all for naught. If you do find an arc track, your choice to replace the socket or grind away the affected part with a Dremel or similar mini grinder tool. Sparks coming out of the balance adjust pot, I'd assume that's going to need a replacement too.

Who knows what lifetime might be left on the remaining three output tubes. What's more important is the new ones should be matched.