Another Marshall valvestate 8004 blowing up Tda1514's

[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]

Yes master Jedi sir. 🤟

[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 😫

[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?

[Jazz P Bass]

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

[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

[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 )

[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?