Peavey PA-200 Powerup With Dim Bulb Tester and Variac

Well nothing is going to pretend to work right until all 4 dead components, the diodes and resistors, are replaced.
IMHO 1n4003 may not have enough current rating for these positions. I'd use 2 or 3 amp rated ordinary diodes. Not schottky, but UF might be okay here. MR752 like in the bridge would do it but those might be pricey. You want at least 100 piv rating diodes.
Something took those diodes out, I'd check the .02 uf cap to the preamp power supply is not shorted. I'd also pull the base of each output transistor and make sure it is not shorted b to e or b to c.
Well, seeing Mr. Fahey's post, I see my prejudice against 30 year old rubber sealed wet capacitors is justified.
I hope the interstage transformer is not damaged. It has a dark spot on it.
Happy hunting.
 
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Those dead bias diodes are probably 1N4001. You don’t want/need high current, ultrafast, or highly overrated voltage ones in there. Any of the above would result in the output stage bias being wrong. Too high a voltage drop and you get excessive bias. Too low and you get crossover distortion. There doesn’t appear to be a pot anywhere to adjust this. Fried output transistors are the likely culprits if the diodes are blown.

Let’s hope that interstate transformer isn’t damaged. You will likely have to MAKE one if it needs to be replaced. Highly unlikely, though.
 
Let’s hope that interstate transformer isn’t damaged. You will likely have to MAKE one if it needs to be replaced. Highly unlikely, though.
Lots of these old monaural Peavey devices show up on ebay $20-30. consumer electronic & musical instrument category both. The freight to ship it in is probably more than the purchase price. They are cheaper than a new power transformer, case, pots, power switch, fuseholder all that stuff.
 
The TO3 output transistors, looking at the bottom from the screw in the long direction, the base is the one on the left. Emitter is the one on the right. Collector is the screw on the end, unless it is insulated from case by a plastic ferrule.
With power off, with the diode scale of your meter, should read 600 to 700 mv b to e and b to c on the output transistors. Higher backwards. If zero, remove & recheck.
With power on, as shown on the print, lower output transistors e should be .05 v and b .6 v. Upper transistor e could be 37 + .05 above emitter resistor, 37.6 on the base. This does not show on the print. Whatever, the upper bases should be ~.6 v higher than the input of the output cap. The current across the upper emitter resistor should be minimal, also. ie .05 v.
Note shorted output transistors often take the emitter resistors with them, which go open.
Note also, if mica washers are under the power transistors, they are 30 year life parts and should be replaced. If you use mica again, you'll also need heat sink compound. If you use silicon rubber insulators, those can be used without heat sink compound. If you get modern TO3 output transistors, the holes on the ends won't fit the old #6 screws; you'll need new #4 screws & nuts or 3 mm screws. Try to get those in the same box for the $9 freight charge, too.
In replacement On output transistors, from cheapest to most expensive is probably MJ15015, MJ21194, MJ15003, 2n3773, MJ15024. If other brand do not use 2n3055, the 75 v rail requires specially selected ones available in 1970 (Like 40406) and not now. In other brands 2n5629 or 2n5630 are probably good enough. Other brands 2n6338 39 40 41 are probably good enough.
 
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Progress update: I replaced the 2x diodes and 15 ohm resistors with new parts (1N4001 for the diode). Only the burnt diode measured as faulty once removed from the circuit (other diode and resistors were ok). I also replaced the 470uf and 50 uf caps whose values had drifted far out of original range. The 3500uf, 1000uf, 100uf, and 25uf caps measured ok, so I left them in.

Measuring the output transistors showed that 3 out of 4 were continuous (0V drop in diode test) from base (green wire) to emitter (black wire). I'll remove the transistors from the circuit and retest. The emitter resistors also seem to be much higher value than expected (but not open), so I'll remove them from circuit and retest as well.

Thanks to all for the help so far! I'm glad this amp is so easy to work on :)
 

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Ok, I removed the transistors and emitter resistors from the circuit with the attached results: it looks like 2x transistors and 2x resistors are dead. Interestingly, the dead parts are not from the same pairs (a resistor from dead transistor is ok, and vice versa).

Would this transistor be a suitable replacement? I'll replace the mica washer with rubber or mica+heat transfer compound as well. Is it good practice to replace all transistors such that they all are from the same series?
 

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Yes, zero voltage drop across b-e or c-e on a silicon transistor on diode scale is the wrong number.
Yes, MJ15024 is a superior npn transistor that will replace these. All 4 transistors should be changed at the same time, with the same part number, from the same batch. MJ21194 may be cheaper, and would be entirely adequate. MJ15015 is also adequate and very inexpensive, if they still stock it. I got my 15015 at $1.40 each years ago.
About the "good" capacitors. What goes wrong on electrolytic capacitors is ESR, which can only be measured by an ESR meter. I measure the things with a calender (>20 years is wrong), but I prefer my equipment to not be broken all the time as the rubber capacitors fail one by one.
I replace the screw terminal caps with snap-in caps, mounted through holes on a piece of bare board, with the board mounted to the case with a couple of screws and a stand off. Much cheaper. Also multi-gang can caps, I replace with several single snap in or leaded caps, mounted on a solder type cinch terminal strip. Which terminal strip can be mounted to the case with screws.
 
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I’d only replace the big can if it needs it. They generally have decent rubber seals on them compared to the little PCB mount caps. They can be pricey, too. If you get full output power, it’s fine for now. What you might do is keep your eyes peeled for similar units on the surplus market - they show up from time to time.
 
What indianajo and wgski said.
When you replace caps and transistors, remeasure (noload, still with dim bulbI) and post results.

As of :
Interestingly, the dead parts are not from the same pairs (a resistor from dead transistor is ok, and vice versa).
probably dead resistors opened fast and worked as fuses, protecting "their" transistors, and viceversa.
 
And if you still have one working pair of output transistors, you can use them to make sure everything else is ok while you wait on new ones to arrive. Parallel pairs are only needed to handle a heavy load. A single in each side will work to check everything else, once dead diodes and resistors are replaced. The voltage on the hot side of the output cap should still center, and the amp should pass a signal into a light load (say 100 ohms). The light bulb limiter will prevent any other damage if there is still something that was missed, and it would be better to find it now before putting in new 15024s.
 
Hi all, I am installing my parts from Digikey and wanted to confirm the pinout of my transistors (datasheet here). The datasheet is not entirely clear which pin is the base and which is the emitter; the picture seems to show an arbitrary orientation. Do I have it correct in the picture below?

In addition, I'm a bit confused about the electrical connection of the case (red wire). The cases of the transistors aren't connected electrically to the middle pins of the transistor sockets, so what exactly is the purpose of the red wires?

Thanks again for the help.
 

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For TO3 pin identification see post 25. I think this picture is looking from the long direction at the pins, but if not, other way.
I didn't take it apart so I don't know where the red wire goes. Possibly to one of the collectors but you have to get the right one. Each has an individual wire. Two of the collectors go to the centerline, the + of the speaker capacitor. Two of the collectors go to the +74 supply. Peavey later used the screws on the ends of the transistors to connect to a PCB; I don't know if they were doing it in 197? The screws must be insulated from the aluminum plate since all 4 transistors are mounted on it.
If this was disassembled and abandoned by a previous owner, you'll have to trace the base & emitter leads and compare to the schematic diagram to figure out which pair of output transistors is which. Then install red wire for the +74 and some other color (I use orange) for the centerline.
 
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Those cases ARE connected to the middle strip by the mounting screws.
And you got E and B right, at least on the last picture.
Your 45 degree angle is somewhat confusing, a 90 degree picture would be much better, then there is no doubt whatsoever about E-B-C pinout.