Yet another Marshall AVT150 No Output

[Glenn63]

Hello Gents,
I've tried scouring all the AVT150 threads for answers before posting my own, I'm stumped!
This AVT was not blowing the fuse, merely no output.
It took me a little while to understand that this circuit is a modular slaved configuration and I made the mistake of trying a test on Amp A with Amp B disconnected.
So after replacing:
both the TDA7293 chips
the 47uf caps C3 & C5, and 10nf caps C2 & C4 on the modular boards
the 22uf caps C191, C216 on main board bootstrap circuit
I 'have not' replaced the 10pf and 22pf caps C6 & C1 on the modular boards (Yet)

I have confirmed that Main board connectors 102 & 103 are supplying 49V +&- to the chips, which also is present to raise the mute function at pin 10.
A signal nearly proportional to the input signal is present at pin 2 of Amp A.
No AC signal is present at output pins!
No DC voltage is present at output pin3 connector 102 & 103 main board.
Speaker load is connected (slight noises heard on speaker but not a pop from meter probe on outputs)

I've been wondering if the MUTE or STNDBY functions are open, but as I said there is a voltage greater than +5V at TDA pin 10.
I don't understand why Marshall has the STNDBY pins of both amps linked together via Link107 with no voltage to raise the STNDBY ?
(It must have worked before so why try to fix what isn't broken, I just don't understand why?)

Any pointers on where to go from here would be greatly appreciated! Gotta get this one outta here!

Glenn.

[Enzo]

Link 107 is not on the mute pins. Standby is pin 4 of the connector (pin 9 of the IC), those are linked my a wire labelled STBY. Note the other wires are not labelled. They labelled it because it is connected to any other points in the schematic also labelled STBY. Find the power supply page, see below the tube is transistor T104? That is the standby circuit. See R218, 219 that don't seem to go anywhere? That point is connected to the STBY on the other page.

Mute is tied up right at the IC, but you say there is no voltage at STBY. Now are you measuring no voltage there? Or are you assuming it is not present because of how you are reading the drawing? If STBY is stuck to ground, then T104, ZD100, or C217 has shorted. (or much less likely, the negative volts at the base of T104 are absent.)

[Glenn63]

Link 107 is not on the mute pins. Standby is pin 4 of the connector (pin 9 of the IC), those are linked my a wire labelled STBY. Note the other wires are not labelled. They labelled it because it is connected to any other points in the schematic also labelled STBY. Find the power supply page, see below the tube is transistor T104? That is the standby circuit. See R218, 219 that don't seem to go anywhere? That point is connected to the STBY on the other page.

Mute is tied up right at the IC, but you say there is no voltage at STBY. Now are you measuring no voltage there? Or are you assuming it is not present because of how you are reading the drawing? If STBY is stuck to ground, then T104, ZD100, or C217 has shorted. (or much less likely, the negative volts at the base of T104 are absent.)

Thanks for help Enzo. I see now (funny how many times you can look at something and be wrong) that LK 107 is not on the STBY pins, but I did check for and verify that the 9 pins on the amps have continuity and are not stuck to ground. I knew that pinout from the data sheet. Meter testing showed no voltage present and I did not see the STBY note on the power supply circuit. (Eureka! How many times you can look and not see.)
I will dig into this tomorrow! Thanks for your input.

[Glenn63]

Thanks for your help Enzo! Your "less likely" scenario proved to be true.
-V was not present at D139. The board traces from REG100 to D139 and to Fan connectors look stained but not broken. (Possible leakage from C193 when hot?)
I have placed jumper wires across both traces to be sure.
-V is restored to T104 and +6V is present at junction R218/219 STBY.
I've reasoned this circuit ensures both Regulators are functional or STBY will be cut off at amps?
The - and + voltages at the regulators read a 1V difference, possibly due to capacitance loss at C193?
Logic tells me I should replace the 2200uf filter caps before assembly. Correct?
I just realized these caps are rated 25V! Seems grossly underrated for use with these regulators. I have a couple of 35V caps but their size is considerably larger. Perhaps I will drill mounting holes in the large ground trace at the proper space for these larger caps.

A few good lessons learned here in troubleshooting and schematic reading.
Thanks,
Glenn.

[Enzo]

C193 and REG100 do not connect to D139. Note the line from the cap goes to the fan connector, not the diode. The lines cross but there is no dot joining them. The negative voltage to REG100 is totally separate. D139 takes AC from the transformer through R223. That makes a simple half wave supply by charging C218.

C193 will not be an issue. It feeds regulators which have several volts of headroom. Leave the 2200uf caps alone. WHy is a 25v cap under spec for a 15v regulator? What voltage is there?

[Glenn63]

The input to REG100 is at -22.2V, hence my questioning the value of the 25V caps.

Anyway, in regard to these connections, this board layout is opposite of the schematic.
The traces are in proximity of C193 and have corrosion from leakage?
C139 to D193 failed, and
R223 to fans may fail.
Jumpers on bottom of board are hot glued for stability.

The repair is a success with a test signal, though I have not measured the output.
If replacement of C139/221 wouldn't be a necessary maintenance, I will be closing this one up!
Thanks for your help.
Glenn

[Enzo]

I see the discolored trace, but all it does is go past C193, I see no connection to it.

[Glenn63]

Tough to see clearly with photo, but 'REG100' text is printed directly on top of the trace from C193 negative to D139.
It's also possible that the failure was due to a solder connection, though it wasn't intermittent.

Still, my biggest concern is if this is leakage damage? Or possibly from regulator heat?
I want to be confident that I've addressed the issue fully.

[Enzo]

REG100 is printed atop the trace because the trace is near the REG100 part, it does not label the trace, it notes the nearby part.

That trace is discolored because it got hot from excess current, it bubbled the green paint.