Another Marshall valvestate 8004 blowing up Tda1514's

[888guitars]

I just need a bit of help with a Zener question. As I wait for parts, I can't get the two rails to match. One side of the amp ( left/chan A) works. But the other is whacked. I did replace TR2 but still have too much of a voltage difference on either side of c4 and c6. They should be similar right? Well one is negative. But I see 27v on A side and -34 on B side. Is ZD2 most likely the culprit of not dropping enough?

[The Dude]

It's a good idea to post a schematic or schematic link when you start a thread so we know what you're talking about.

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[888guitars]

Oops. I thought I did 😏

[The Dude]

Do you measure any AC on the low rail? You might have a bad filter cap. Do you have the shorted TDA or TDA's removed? If one is defective, it might load down a rail. If you suspect a zener is bad, measure the voltage drop across it. It should be around 27VDC. Are either TR1 or TR2 shorted?

Edit: Also check that C12 &13 are soldered in well- no cold/cracked joints.

[888guitars]

Yes the TDA is pulled out.My meter won't read 2200 uf. Maybe c12 is bad? I will just replace him. The Zener must be bad , I will double check the voltage drop when I get back to the bench tomorrow. I do remember seeing zero volts on one side of the diode?

[The Dude]

Check the AC voltage across the cap. If it's not doing it's job, there will be ripple/AC there. If the supply is clean, no need to replace it. One side of each zener is ground, so yes there will be zero volts on one side of each. It's the other side you need to measure. If there's 27 volts there, no need to change that either. There's no need to shotgun it. Troubleshoot your way through the supply and figure out what's wrong with it.

[Jazz P Bass]

I am confused as to the question of what is wrong.

The OP stated that "One side of the amp ( left/chan A) works. But the other is whacked."

The power supply is common to both TDA1514's, is it not?

I would also like to know where the OP is going to find a non- bogus TDA1514.

[888guitars]

I have blown up 2 TdA's from China. I now ordered one from a USA supplier that is a confirmed Phillips. We shall see.

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....?

[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

[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.

[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?

[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]

C4 27v and c6 is -34

[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]

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