Marshall 8004 Woes

[stereomonostereo]

I've been stuck on a Marshall 8004 for a while. When i started working on it, both TDA1514s were blown. I replaced them both and powered the unit up. Channel A is working as it should, but Channel B keeps blowing the TDA1514. My readings on my TDA1514A chips (with my variac set at 50v) are as follows:

IC1

1 = 0
2 = -5.7
3 = -5.7
4 = -11.4
5 = 0
6 = +11.5
7 = +8.9
8 = 0
9 = 0

IC2

1 = 0
2 = -10
3 = -10
4 = -11.5
5 = 0
6 = +11.5
7 = +11.4
8 = 0
9 = 0

My + and - rail seem to be alright, but i'm not sure about the discrepancies between pins 2 and 3, and 7.

I'm getting -26 at my BDV64 rails and +26 at my BDV65 rails. My zener diodes seem to check out okay. I replaced R26 and R27 as they were burnt.

I'm pretty stumped. Anyone have any ideas?

[Amp Kat]

Maybe this will help.

http://music-electronics-forum.com/t9735/

[stereomonostereo]

Thanks for the link, i was checking that out before. Most of what was checked out seems right, i just can't figure out where this extra voltage is coming from. I'm beginning to think it's the transformer.

[52 Bill]

If one side works ok, I can't believe that the transformer would be the problem, unless this amp has separate dual power supplies.

[stereomonostereo]

Bill: yeah i think you're right about the transformer

I'm getting different reading across my Zener diodes. I'm reading -12 and -10 across ZD4 and -12 and -5.4 across ZD3. Both zeners were replaced though and seem to be testing okay.

R17 and R5 are showing the same voltages. R17 is -10V and R5 is -5.4. My rail voltages on pins 4 and 6 of both ICs read identically.

It seems the only culprits could be C18 and C19, but i've replaced C19 to no avail. And i don't see C18 being a problem.

Any idea what i'm missing?

[52 Bill]

I've not seen one of these in a while, so help me out here.
You say channel A works fine and B blows the chip.

Is Channel A IC1 or IC2?

Are both chips installed new?

[stereomonostereo]

IC2 is channel A, IC1 is channel B

Channel B works fine and channel A keeps blowing the chip.

I just tested the voltages with the IC2 removed and the voltages across the zener measure properly. The only real differences i was seeing were coming off of pin 7.

[Amp Kat]

Shouldn't you be getting +/- 28 volts and not 10 & 12. Something is either shorted or open causing the voltage to drop. Look at the voltages in that other thread as a reference for what you should be getting.

[stereomonostereo]

I'm still testing it with the variac set at 50v. When i run at regular voltages i blow the TDA1514 chip

[52 Bill]

I would assume that the voltages on pin 7 without the chip would be higher than with the chip installed?

Have you checked all of the parts of the bootstrap circuit? R20, R26 and R27 and C24?

Has your new TDA1514A already been damaged?

[stereomonostereo]

The three resistors and the cap all check out okay. It's a new TDA1514 that hasn't been damaged.

I'm stumped. I'm beginning to wonder if the board is conductive somewhere it shouldn't be.