Marshall VS15 DC on Output

[misterc57]

In my never ending quest to learn more and have fun, I am going through a pile of broken amps.

This one is a Valvestate 15.

It is missing its speaker. Seeing around -20 VDC at the output, which may explain the missing speaker.

Anyone have a schematic they can share? I have not found one yet for this model.

This will be more than a schematic request.

Thank you, MC

[The Dude]

I don't have a schematic, but that's a relatively simple amp- probably a TDA2030 or similar for an output device (on the big heat sink). Get the part number off the chip. Look up the datasheet. Check supplies. If both rails are good and you have DC on the output, you most likely have a bad output IC. It's not an uncommon failure for these little TDA output amps.

[sugeda]

...

[The Dude]

Thanks sugeda! So the output IC is LM1875. Check pins 3 and 5 to confirm both rails are there and approximately equal and opposite. If they are, I'd replace the IC.

[TomCarlos]

Thank you Sugeda for posting the correct schematic. I deleted my post.

[misterc57]

Thank you all. That schematic is not a match.

So IC4 is a TDA2030. I read the following VDC

- 15.8 pin 3
+ 20 pin 5

Should V be that far apart? Bad IC4 or other issue to check?

IC2 and IC3 have almost matching - 8.8 VDC pin 4 and + 9.1 VDC pin 8

Thank you

[TomCarlos]

Just a note... the LM1875 and the tda2030 are somewhat identical chips. I wonder if that chip has been replaced.

UPDATE: Changing "nearly" to "somewhat" identical.

[g1]

Just a note... the LM1875 and the tda2030 are nearly identical chips. I wonder if that chip has been replaced.

Looking at the supply voltages, I would think you are right. The 2030 can only handle 36V, the LM1875 is good for 60V.

[misterc57]

But even the IC number designation is off. That schematic states IC3. The amp and board is IC4.

Also I think I saw some Cap numbers that did not match. And on this board the processing starts at IC2 (IC1 is not installed on the board).

Based on the V alone do you all feel this is still the final output IC issue and it should be replaced or are there other things to investigate?

Thank you

[The Dude]

That is a bit of discrepancy on the rails. it could be because the negative supply is loaded by a shorted IC. It could also be that filtering is poor on the negative rail. I would remove the IC and recheck rails with the IC out of circuit. If the negative rail is still low with no IC, check filter caps and connections on the caps. Check ripple with scope if you have one.

[g1]

But even the IC number designation is off. That schematic states IC3. The amp and board is IC4.

Also I think I saw some Cap numbers that did not match. And on this board the processing starts at IC2 (IC1 is not installed on the board).

Maybe like G15RCD without reverb?

[misterc57]

IC4 removed. I have balanced 22V in the first stage and balanced 9.3V in the second stage rails.

New IC is on its way.

[g1]

New IC is on its way.

LM1875 ?

[misterc57]

LM1875 ?

TDA2030, same as what was in there. Looked original.

[g1]

TDA2030, same as what was in there. Looked original.

You have a 44V supply and the TDA2030 is a 36V part. It won't work there (or not for long). Did you look at the schematic in post #11 ?