1979 Peavey Mark 3 Bass 400 B/G Module Blowing fuses

[twilightofthedogs]

Hi all, I am trying to sort out a Peavey Mark 3 bass amp that is blowing fuses. I have been coming back to this amp for a while now. If memory serves me I have already checked and replaced faulty power transistors. I have disconnected the preamp board and am just working with the power section. I have it hooked up to a current limiting light bulb so at least not going through fuses any more. The one thing I have to go on is that when I remove U1, which is an TL074 op amp it stops blowing fuses. I did try replacing the op amp and it didn't solve the problem. A little hard to trouble shoot without being able to turn the amp on and look at voltages! I did check continuity between the terminals of the U1 socket and the chassis just to see if i could find any obvious shorts, but nothing glaringly obvious. Any thoughts on particular areas I might investigate for issues or troubleshooting techniques would be greatly appreciated. Schematic attached

[Helmholtz]

Disconnect speaker AND "autoformer" from output.
Hopefully this stops the amp from blowing fuses.
Do you see DCV at the output?

[twilightofthedogs]

Thanks. Still blowing fuses with speakers and auto transformer disconnected. I actually don't think my version has an auto transformer. As I bring up the voltage on a variac and current limiting bulb I am seeing about 2vdc at 40% line voltage on the variac. Havent turned it up much more than that as the light is pretty bright by then

[Helmholtz]

... auto transformer disconnected. I actually don't think my version has an auto transformer.

That sounds a bit contradictory. You should be able to spot an output transformer.

[twilightofthedogs]

Ha. Sorry. I just meant that it's not connected in that it's not there. There is a note on the schematic that only the "g" version has one. It's not there in mine. Definitely the PT is the only xformer present

[Helmholtz]

So without a load connected to the amp the question is: Where is the large current flowing that makes the amp blow fuses?

[Enzo]

Any voltage readings at 40% of mains voltage are meaningless.

If an amp goes to DC, a load will cause it to draw heavy current. But if the bias string opens - like say the dual diode is unplugged - then both sides of the output can turn on and then excess current flows.

Correct, the autoformer is only used in some amp models using this power amp. And at DC it represents a dead short across the output.

And all the cold tests can be done. Shorted main rectifiers? Any of the power transistors shorted to the heat sink? (As in the insulating mica wafer is missing or broken)

[Jon Snell]

Due to the inherrant design of these amplifiers, if the output transistors fail, the driver side usually fails as well.
With no spealer load,
Monitor the voltage across R26, you are looking for no more than 0.7v. If there is more than that, check the pre-drivers also.
The VBE on Q3 and also Q13 should stay around 0.6v as well.
Are the output transistors electrically isolated from the heat sinks.

[J M Fahey]

Checking output transistors is fine but not enough, perfect ones but nuked drivers will make them hog current, open biasing network the same, etc.

Just "my" checklist, others might have their own:

1) connect amp through bulb limiter, even if to make it "automatic", Variacs are fine but requite you looking at a meter,manually turning the lever/button, etc. ; bulb will protect amp 24/7 and has its own warning light even if you are not looking at it.

2) measure rail voltages.

Post what you find.

Further suggestions depending on results.

[twilightofthedogs]

Thanks all. A couple dummish questions:

1) The bias line... is that the 15/-15v line coming off the power transformer? Is that represented by the - or + for instance the + symbol at R6 leading to pin 5 of U1b?

2) Is it possible to measure correct voltages within the circuit with the amp fully powered and plugged into a current limiting bulb with the bulb lit from excess current draw?

Thanks

[Jon Snell]

Thanks all. A couple dummish questions:

1) The bias line... is that the 15/-15v line coming off the power transformer? Is that represented by the - or + for instance the + symbol at R6 leading to pin 5 of U1b?

2) Is it possible to measure correct voltages within the circuit with the amp fully powered and plugged into a current limiting bulb with the bulb lit from excess current draw?

Thanks

1) sadly no. It is the potential difference that sets up the idle current in the output stage.
2) Not likely at all.
How deep is your understanding in this field of electronics?

[glebert]

Have you tried pulling U2 or U3? Wondering if opening up that feedback path might tell us something. Also, have you tried it with the compressor in and out and with different threshold settings?

As far as electronics expertise, I would say that this circuit is not the most straightforward to understand. There are lots of loops that are interconnected.

[Enzo]

When the bulb is bright, it means the amp is trying to draw current, so it reduces the mains voltage the amp sees. SO voltage readings mean nothing.

The bias string in this amp is essentially: R19, CR9, CR14. Those 15v supplies are just that...supplies.

Most solid state power amps are one big loop. Step back and look at how it works. Pretty much everything from the bias diodes to the right is one big current amp. It is driven by Q9. Whatever voltage appears at the collector of Q9 is going to be the voltage at the output. (more or less) The signal size at Q9 and the output should be about the same. The main loop is the output sampled back into U1d-13. Where it is compared to the input signal at U1d-12.. The three op amps upper left, the rest of U1? Those are the compressor circuit. Unplug the four-pin plug, and they are "gone". Note the output of that string feeds the control pin of U2, not the signal input. Pull U2 and all that is gone.

COnsider the output. If it goes to DC, absent a load, it will just sit there that way. Put a load on that DC and it will draw hard. A shorted transistor can also put DC on the output, That we can discover with an ohm meter.

The bias string is tied to V+, and the other end pulled down so the output sits at zero. If Q9 shorts, then its collector sits at V-, and so does the output. If Q9 is open, then it all pulls up to V+, and so does the output. Of course we can also have DC on the output with a good Q9. It does what its base is told, so if the signal there says DC, so will the outputs. SO it pays to determine up front whether the output is following its Q9 or whether it has issues on its own.

This is just how I intuit the thing, of course there is more to the amp.

[twilightofthedogs]

Thanks all... this is helpful. I have a pretty good understanding of tube amps and have fumbled my way through some solid state fixes but I definitely don't have as good an understanding of the theory behind it especially when op amps and ICs start showing up. Unplugging U2 has no effect. There actually is no U3 on this particular amp. I have never had the 4 pin compressor connector connected while i've been trouble shooting, so from my reading of the above posts parts A,B,C of U1 are "gone" anyway. I will direct my attention to Q9 and the bias rail. Thanks again, always helps to narrow things down a bit!

[J M Fahey]

it reduces the mains voltage the amp sees. SO voltage readings mean nothing.

With due respect , I´d change that to "voltages measured will not match what is shown in the schematic" which I guess is what you meant, but of course voltages measured even with bulb limiter present have a diagnostic value.

It is not the same having, say, +/-30V rails (which of course were reduced by series bulb present), symmetrical, no DC at the output, faint hiss on speaker, etc. all of which indicate a healthy amp, than having, say, +/- 2 or 3 Vdc, +1.5V at the output, etc. which point at a very sick amp.

I much prefer reading voltages and taking deductions out of that, than maddeningly pulling every single part to be checked outside.

So again: please set all knobs to zero, no signal input. no load whatsoever, neither speaker nor output transformer, although that already seems to having been ruled out.

* what rail voltages do you measure?
* what do you measure at speaker out?

Thanks.

PS: a lot of voltage measurements will follow, but which ones depend on first results.
PS2: just asking what I would start measuring if it were on my bench.
Obviously each Tech has his pet preferences.

Here an old Gaucho/popular saying is "cada maestrito con su librito" literally meaning: "every Teacher has his own favourite textbook"

Brazilians have a similar meaning, but somewhat rougher one: "a opinião é como o cú, cada um tem o seu" literally "opinion is like an *ss, everybody has his own"