Peavey PA-200 Powerup With Dim Bulb Tester and Variac

[Cheeto333]

Ok, thanks for the help. Should I replace the caps before measuring bias?

[JMFahey]

You seem to have a capacitor out single supply transformer driven power amplifier.
Can´t find its specific schematic but it "should" be close to this one:
[see attachment below]

Please confirm , I seem to see in your pictures a large "coffee can" main capacitor and a smaller board mounted axial cap, plus a driver transformer.

If so,it should be close enough.

Component designations might vary, but general structure should be similar.

Do not start by replacing caps, specially some expensive ones, at random, first you must troubleshoot, this is not a guessing game.

1) for now do not connect speaker or any load

2) what size bulb are you using? Too small it will always glow quite bright.
I suggest 40/60W .
"Real" Watts, not "equivalent":

Old style 60W:


Modernish higher efficiency "75W" bulb inside a bulb, actual 60W


we want a red hot wire inside.

3) connect amp to mains straight through dim bulb, no Variac.

4) all controls to 0, no signal input, measure DC voltages.
Specially +75V +V and +37V "halfway", at the positive of output capacitor.

For now I do not worry much about voltage at speaker out.

Please post voltages measured, which will be some 20% or 30% lower than expected, because DBT will eat some of your mains.

IF DBT shines *bright* , or you see smoke or smell very overheated components , turn amp OFF and post results.

Also a better PCB picture, end to end and showing all components clearly.

[Cheeto333]

Thanks JM; I will report back with voltage readings. I am using a 40W incandescent bulb in my tester. See below for additional images of the power board.

Just to confirm, the big can is the 3500 uf in your attached schematic, correct?

[JMFahey]

Yes.
The two largest caps there are the main filter one: 3500x80 in my schematic, yours should have *similar* specs, and 1000x50 output coupling one, same considerations.

That is a VERY well made amp, VERY robust, can easily be left as good as new 45 years after it was made, amazing.

[thimios]

Originally Posted by Cheeto333

Thanks JM; I will report back with voltage readings. I am using a 40W incandescent bulb in my tester. See below for additional images of the power board.

Just to confirm, the big can is the 3500 uf in your attached schematic, correct?

Picture isn't clear but this diode 1N4001 in the left side of the first picture, the uper one looks like hurned

[wg_ski]

Those two diodes and one of the 15 ohm resistors look pretty iffy. One diode does look bigger than the other - either it is not original, or suspect. Those four components and the two 470/6.3 are the critical bias components for the output stage. You can test the diodes, but if they are crusty I’d replace them. They are cheap, as are the 15 ohm resistors and two caps. The caps may be fine, but Illinois Capacitor from that vintage aren’t exactly the best. They get leaky, but still *work*. Any time I see those old IC caps, they go (they are what always fails in the protection circuits of the big Crest amps. Always). The big wire wound resistors won’t go open circuit unless the board is cooked, and it’s not. If all of those are good, there is nothing left that can result in blown output transistors. The output transistors are easy enough to pull and test, if for some reason you still don’t get about 37 volts (half the supply) on the hot side of the output cap. Either they are all good, or replace all. They are probably house numbered selected 2N3055’s, but I would use 2N3773’s or better. But if they are good, leave them. If the output cap is leaky replace it. It may be “leaky” if the hot side is stuck to the rail voltage due to an output transistor or bias circuit failure, but ok at normal voltage. I would use a bigger one to get better bass response, but you may want the original size to protect small speakers against low frequency overload like dropped mics.

If the last stage is clean you can move on to the two stages that drive it. They are isolated by the transformer and anything wrong here can’t result in a catastrophic cascade failure, like they can with more “modern” designs. That’s one reason why these old things were nearly bulletproof.

[Cheeto333]

Thanks thimios and wg-ski. Should I replace the diodes and 470/6.3 caps before checking the +75 and +37 voltages?

[indianajo]

Checking is fast and costs nothing. Do the checking of voltage first. That will determine whether the problem is inside the output cap (1000 uf 50v) (Bias or transistors) or the output cap itself.
In the end you'll want to replace all those ancient e-caps. The total value of caps is about $8, the freight from thief river falls mn or north carolina is $10. So buy them all at once, but replace one at a time and see if you made the amp better, or worse. Amateurs (including me) make a lot of bad solder joints. If you replace too many things without measurements, you don't know where you just inserted a problem. If you do one part, and it gets worse, you know just where the problem is.

Also checking the diode wg-ski complained about is a simple resistance check. Also the resistors he complained about. Color is nothing, resistance value is important. Diodes are of course 600 to 700 mv forwards, 9999 or ---- backwards. Backward won't be fully 9999 because the cap will charge up the parallel capacitor. If your DVM doesn't have diode scale, use the 2000 ohms scale (on mine).
If you take measurements, you'll get everything you need in one $10 box, instead of making several orders. If output transistors are bad, or diodes or other secondary components, you should find out now. Fortunately as wg-ski said, the transformer between input & output limits the size of output meltdowns. My direct connected PV-1.3k, it took 127 small parts to repair it because the rail voltage went all the way back to the DDT parts one step from the input.
IMHO parts from a local storefront have been lower quality than the parts I get from newark, digikey, or mouser. I can buy 5000 to 10000 hour rated electrolytic caps, instead of the 500 hour caps I used to have to buy at the TV parts store. Those had to be replaced 3 times since 1970.

[Cheeto333]

I measured ~+52VDC at both measurement points (across the cap can and from the positive of the 1000uf cap to ground, see picture). Bulb glowed brightly initially, then dimmed.

The two diodes and 15 ohm resistors appear to be toast (diodes measured ~0V forward voltage and resistors had ~3 ohm resistance).

Should my next step be to replace components one at a time as suggested by indianajo?

[JMFahey]

Originally Posted by thimios

Picture isn't clear but this diode 1N4001 in the left side of the first picture, the uper one looks like hurned

Good eye
It looks burst.
Also the first 470 x 6.3 grey capacitor on its left looks burst, its rubber seal looks dirty with salts, compare it to the one to its left.

In due time all electrolytics must be replaced, but it´s good practice FIRST to find current problem, simply to reduce number of variables, and then tidy it up.
That said, those visibly damaged of course must be replaced now.

Thanks thimios and wg-ski. Should I replace the diodes and 470/6.3 caps before checking the +75 and +37 voltages?

Check NOW, among other reasons so we can guess that is the actual problem.
Whay in you replace them, do other things and then it "works". You´ll never know whether you actually repaired it or itb started working by resoldring , moving wires or connectors, whatever.
Like Enzo says: "do not find excuses to NOT check something"
Besides, as indianajo says, it takes esconds.
And then RECHECk after you replaced them.
Replacing parts is easy, main job is troubleshooting, finding what/why something happened.

If you replace too many things without measurements, you don't know where you just inserted a problem. If you do one part, and it gets worse, you know just where the (new) problem is.

EXCELLENT advice.

I measured ~+52VDC at both measurement points (across the cap can and from the positive of the 1000uf cap to ground, see picture).

That hints at shorted top transistor, or open/unbiased bottom one, or open bottom emitter resistors.

Start by replacing visibly broken diode, anyway check that both drop about 700mV on diode test and replace visibly leaky 470uF cap.
To avoid ordering parts "one by one", you "should" have 1N400x diodes available, absolute worst case recycle one from a dead/abandoned power supply and replace cap by "any" cap you have, from 100uF up.
**all this just for testing** to know what to order.
If way out of value , also replace the 15ohm resistor.
If no 15 ohm resistor in stock, make it out of 2 x 33 ohm ones in parallel, 10+4.7 ohm in series, etc, its value is critical in thye series biasing string from +V to ground.

Voltages might go back to normal (although *something* made those parts burst) or transistors might be damaged (top ones shorted or bottom ones open) or bottom emitter resistors might be open.

Measure voltage from bottom transistor emitters to ground.

ABSOLUTE WORST CASE replace everything to the right of the driver transformer but you may be spending some money on power transistors which *might* be fine.