Another Marshall valvestate 8004 blowing up Tda1514's

[J M Fahey]

That.
You are measuring DC at pad 5 using a 1M or 10M impedance meter,a said above it is connected to +V, an electrolytic is not the best insulator in the World and the tiny leak is enough to show important voltage.

Now you connect speaker and that dissappears.

Try connecting a 1k resistor from pin 5 to ground, what happens?

[g1]

My question is: from this and an older thread it seems ok to see 22v at the speaker jack with both ICs out. Why is that?

I just skimmed through this thread, but don't really see anywhere that DC on the output being ok was mentioned? Can you link the older thread?

I suppose there is a path from the + supply to the TDA pin5 by way of C24 and R20. Disconnecting one leg of either would tell you if that is where the DC is coming from.

Schematic attached.

(P.S. somebody at Marshall really needs to learn the difference between milli-farads and micro-farads )

[mikeskory]

Yes I did see that. Both chips are out. I want to make sure it's safe to put in new TDAs. Either way I measure the TDA pin 5 output lead it shows about 24V

[Jazz P Bass]

See the line that says 'do not ground'?
You cannot measure the output with respect to ground.

[mikeskory]

I hope it's ok to jump in on this older thread. I'm working on the same model with the same problem. I followed this thread successfully to find a shorted TR2. I had already replaced the TR1 along wth ZD3,ZD4 and some caps. My question is: from this and an older thread it seems ok to see 22v at the speaker jack with both ICs out. Why is that?

[888guitars]

After going through 5 bad TDA’s, I finally got one that seems legit. At
Least they gave me my money back but how frustrating 😫

[888guitars]

Yes master Jedi sir. 🤟

[J M Fahey]

Measure voltage on both ends (anode and cathode) of ZD2.
If track/solder is open between its anode and TR2 base or between its cathode and ground, it won´t do its job.

Like Enzo says, shotgunning parts is not enough, only half problems come from bad parts, other half comes from poor connections (cold solder, cracked tracks, dirty contacts, frayed wires, etc.)

Time and time again we read distressed posts like"I´m going mad, replaced *every single component* in the board and still the damn amp does not work!!!"

Personally I always insist on taking voltage readings all over the place and when finding a weird one, trying to find what caused it.

You read bad rails (fine) and indirectly read weird TR2 base voltage (2 V below raw supply voltage instead of 8V below like the other one) but failed to act on that useful piece of data.

I bet TR1 base is 27V away from ground, while TR2 base is some 32V away ... a clear indication that the 27V Zener is not doing its job.
Why? ... go find it

[888guitars]

Good thing I got 2 from mouser. I’ll pull it and check the zener again.

[Jazz P Bass]

Aaah. Thankyou.

TR2 is not doing anything at all.

It is either the incorrect part or maybe the zener is in backwards.

[888guitars]

C4 27v and c6 is -34

[Jazz P Bass]

The ultimate aim of the two "pass" voltage regulator circuits is to regulate the voltage down to + - 27 Vdc.
ON the Emitter Lead of the transistors.

I have yet to see a measurement at C4 & C6.

[888guitars]

Wait? I see it as R2 is the positive rail and r3 the negative. They should be similar except for the pnp vs npn. So when I look at either side of c4 and c6 they should be somewhat mirrored. Right? In that case the same voltage drop across R2 should be the same same as for r3? But they aren't even after replacing them. I replaced the Zener with a half watt 27v I had as I'm waiting for a bigger one, but nothing changed?
Everything is good after the bridge and after the filter caps. It gets goofed up on the " input right" side at the BDV64 and Zener which both have been replaced with no change in voltage?

[g1]

The weird thing I am seeing is the volt drop across R2 on channel A is about 8 volts and the drop on R3 is only 2 volts? I replaced r3 and still same 2 volt drop....?

This makes sense if the zener is defective or has bad connections.
Also, R2 and R3 are not on "channel A", but should be common to both channels, as was mentioned in post #7.

So there appear to be 2 separate problems, one is the zener problem, with which the amps may still work ok (as ch.A shows) for awhile.
The second problem is the bad amp.
Fix the zener problem first. It could be that the newer amp chips can't handle the excess V- supply, where the old one in ch.A seems to be ok with it for now. But the excess V- could end up blowing the ch.A amp chip as well.

[J M Fahey]

Agree.
If +rail is +27V *for both* amps, then negative rail (if regulator is shorted or Zener is open) , can be -34V, for both again.

I see this circuit as a (factory) kludge.
Clearly they have a ton of , say, 24+24VAC transformers , most probably ordered for some other more popular design (or surplus from a model which did NOT sell well) but TDA1514 insists on no more than 26V DC rails, so they added a couple BDV64/65 (which might also be overstock) to down regulate 34V rails to 26V ones.
99.998% of power amps do NOT use regulated supplies