Peavey 260D Power Board Questions

[earache]

Hello
I am working on a Peavey 260D that hums on startup. It doesn't blow fuses. It also has 8VDC across the output jack. I removed the T03 power resistors to test them, which are (4) NPN's marked 67376 and they all test good. I have filter caps on order.

The transformers two legs (+/-15 and +/- 42V) test fine.

I am curious about a couple things - see picture



What is the large device between the power resistors with the twisted black wires attached?
What is the small diode- looking device that goes from the circuit board to the metal heat sink that the T03's are attached to?

I tested the sandstone resistors. They're big (10W) but small value (.33 and .1 ohms). They all test more than 100% over their marked values...should I replace them?

Finally, how else to proceed with troubleshooting after replacing the electrolytics?

Thanks in advance!

[The Dude]

The part pictured is a thermal breaker to cut power if the heat sink gets too hot. If it were bad, you would not hear anything.

On the resistors: Wire wound resistors rarely if ever are that far out of tolerance. I suspect your meter is not zeroed properly. Touch the meter leads together and see what the meter says. If it has a function to zero it, calibrate the meter. If not, you must account for the discrepancy when you check parts.

You measured 8VDC on the output. Did you measure any AC? Your meter could have been giving you an "average" DC reading from AC. You said it hums on start up. Does it then go away after a bit? A sign that caps are drying up.

[Enzo]

Yes^^^^



The odd part is a thermal switch. If there is only the one, it is likely wired to open if it gets too got, and kills AC power to the system. Follow the wires, are they involved with the power switch and transformer wiring?

The diode is a diode. It sits in a hole in the heat sink, but the two ends are wired to the board. Usually on a PV it will be aq special dual diode, and it is sued for the bias circuit.


The cement resistors are wire wound types. They are not likely off value, they mostly just fail to open or they are OK. Like Dude said, touch your meter probes together. If they read zero, then they are fine, but if they measure any small amount, that still must be subtracted from the reading. For that matter, just oxidation on the resistor wire lead might add a couple tenths of an ohm, your probe can;t get good enough contact.

Time to troubleshoot is before ordering caps. I'd check solder to the filter caps. I would check for ripple on all the supplies, preferably with a scope.

The transformers two legs (+/-15 and +/- 42V) test fine

OK, but either you have 15 and 42 volt rails OR the transformer puts out some AC voltages. But the transformer does not put out +/-15v and +/-42v.

[earache]

Yes^^^^



The odd part is a thermal switch. If there is only the one, it is likely wired to open if it gets too got, and kills AC power to the system. Follow the wires, are they involved with the power switch and transformer wiring?

Yes they are, and thanks for that clarification. It was not clear to me what the device's purpose was from looking at the schematic.


The cement resistors are wire wound types. They are not likely off value, they mostly just fail to open or they are OK. Like Dude said, touch your meter probes together. If they read zero, then they are fine, but if they measure any small amount, that still must be subtracted from the reading. For that matter, just oxidation on the resistor wire lead might add a couple tenths of an ohm, your probe can;t get good enough contact.

I took the .2ohm offset of my meter into account when measuring the resisters. Subtracting the .2 ohms, the .1 ohm resister measured .2 ohms and the .33 ohm measured .5 ohms.

The transformers two legs (+/-15 and +/- 42V) test fine
OK, but either you have 15 and 42 volt rails OR the transformer puts out some AC voltages. But the transformer does not put out +/-15v and +/-42v.

So if I unplug the transformer leads off of the board, I shouldn't expect to measure either 15 or 42 volts across the wires?

[52 Bill]

The transformers two legs (+/-15 and +/- 42V) test fine
OK, but either you have 15 and 42 volt rails OR the transformer puts out some AC voltages. But the transformer does not put out +/-15v and +/-42v.

So if I unplug the transformer leads off of the board, I shouldn't expect to measure either 15 or 42 volts across the wires?

You will measure voltage across the wires, but it will be AC voltage and not +/- DC voltage.

I don't know if you are not communicating clearly or if you are not sure of the concept here. Transformers only work with AC voltages. I think that Enzo's point was that your statement: "The transformers two legs (+/-15 and +/- 42V) test fine", is confusing. Do you mean that the output from the transformer is 30vct and 84vct or do you mean that the dc power supply rails/busses are reading +/-15vdc and +/- 42vdc?

[earache]

52 Bill, Enzo, Dude

Yes I misspoke, I should have said that the transformer's two legs when pulled from the board output both produced AC voltage. Of course the +/-15vdc and +/- 42vdc are after the power caps and diodes

Additionally, I put everything back together to try to get some DC voltage readings. The amp nows blows fuses on startup. I will plug it into the light bulb limiter (which I should have plugged into to begin with), and try to see which part of the power board is causing the problem.

[52 Bill]

The amp nows blows fuses on startup. I will plug it into the light bulb limiter (which I should have plugged into to begin with), and try to see which part of the power board is causing the problem.

Plug it into the limiter and pull either of the two transformer secondary plugs from the board and it will lead you to which of the power supplies is drawing too much current.

[Jazz P Bass]

Anyone have the schematic to post?

[Enzo]

Not here. I haven't been into my paper files, but I got nothing scanned. I went to my support website, but it goes by model, not boards.


SO it would be useful to know what model is the whole AMP?

I used the 260C just to wrap my head around, not sure how close that is.

OK, so you are blowing fuses. First, no speaker or load until we have the amp stable. Only so many things that can blow fuses. Check the power transistors for EC shorts. Check the main rectifiers for shorted. If you have already removed any power transistors, make sure any reinstalled ones are not shorted to chassis, as they would be if you left off a mica insulator. Make sure bias diode is on heatsink, the wires conected, and it is not open.

Having discussed the transformer, I assume you can power it with the secondary wires not connected, and it is OK by itself?

[TomCarlos]

Friends....

I have a 260C schematic... not sure if this comes close to what you have.

Tom

[Enzo]

OK, I found it. It is VERY similar to 260C,but all the part numbers will be different.

I don't know what amp we have here, but the 260D was used on the Standard MK3 guitar amp. There is also a 1981 version. A quick spotter: the 1981 has a 14 pin IC while the older original has an 8 pin IC.

[earache]

The amp is a 260D, and as Enzo stated, it is the older one with the 8 pin IC. It's a Standard Mark III.
By alternately pulling the pairs of transformet wires from the board I found which part of the power supply had problems.
Turns out that the 1N4003 diodes marked CR9 and CR10 were both blown, one even being split down the middle.
I had no 1N4003 so I used 1N4004s and the amp powers up ok now, without making the lite bulb limiter glow brightly.

The larger pair of caps have definitely been replaced as they are 4700uf not the original 5000uf which are no longer available. I may replace the pair of 1000uf as they look original

[TomCarlos]

Good job!! Congrats on the fix.

[earache]

What might have caused the diodes to blow in the first place? Devices around them seem ok.

I also neglected to thank everyone who helped point me to the problem - all the usual (great) suspects chimed in.
You do a great service to us amp tinkerers by lending us your time and expertise.
Thank you!
Earache

[52 Bill]

What might have caused the diodes to blow in the first place? Devices around them seem ok.

Line power surge, bad diode, shorted supply line in the amp, phase of the moon, etc. Sometimes there's no way to really know, you can only guess.

Glad to hear it's running again.