Marshall part PCBCD0053 needed, Please help!

[Dave Johnson]

Haven't had much of a chance to mess with this thing this weekend, but did plug headphones into the headphone jack and everything worked normally thru the phones.
Not sure if that's a clue or not.
Checked some voltages as suggested by Nick B.
W4: 31.9vac
W5: 31.9vac
W6: 31.9vac
Con3 has +43.2vdc on 3 pins and -43.1vdc on 3 more. The remaining pins read 0
Any clues here???

[nickb]

Are you sure W6 was 31.9VAC? It's connected to ground and so should be zero. The CON3 power supply voltages are good. You said the power was missing from the board so go and recheck.

Now CON3 pin 4 is wrong. That is the standby signal. It should be at least +3.5V for the chip to work. Go and measure the voltages on TR2. Pin one is the emitter and is connected to ground. Pin 3 is the collector and is the standby signal. The last pin 1 is the base. The base should be about -0.7V.

[Dave Johnson]

You are correct Nick. I mistyped the results for W6, it is indeed 0volts.
Went and rechecked Con3 pins 1 thru 9 all read 0V.
TR2 had -0.6 at the base pin.

[nickb]

You are correct Nick. I mistyped the results for W6, it is indeed 0volts.
Went and rechecked Con3 pins 1 thru 9 all read 0V.
TR2 had -0.6 at the base pin.

Earlier you said

Con3 has +43.2vdc on 3 pins and -43.1vdc on 3 more. The remaining pins read 0
Any clues here???

So which is it?

If the standby pin 4 really is zero and the base of TR2 is -0.6V then either the transistor is bad or R47 (33K) is open. Check TR2 pin 1 to pin 3 with an ohmmeter for shorts. Also measure the resistance of R47.

[Dave Johnson]

Kind of a stupid question, but do I have to liftTR2 and R47 off the board to check them.
The reason I ask is when I check them on the board the resistance climbs as long as my meter probes are making contact on either component.
This is checking on a 200k setting.

[Enzo]

If one was shorted, it would measure so while in the circuit.

[nickb]

We're looking for a low resistance on TR2 or 33K on the resistor. You don't have to remove them. If your meter is climbing past 33k then the resistor is open. You can remove it and double check.

One thing to watch for on the MG100DFX is the heatsink is bolted the TDA7293 chip. The heatsink is then electrically isolated from the chassis by four bushes on the rear panel. It's easy for the mounting screws to short to the chassis. If that happens the fuse will blow.

[Dave Johnson]

That has indeed happened on this one. I was reading continuity between the screw that holds the chip to the heatsink, untill I removed the offending heatsink / fan mounting screw.
Now it's reading open as it should be.

[Dave Johnson]

R47 climbed to 31.5 ohms and stopped.
TR2 climbed to 37.5 ohms and stopped

[nickb]

Looks like TR2 is bad. Pull and re-test.

[Enzo]

Remember TR2 has C98 and D14 in parallel. ANy one of the three could be shorted. Or two or all three for that matter.

[nickb]

Yes, those components are candidates too, as is the power amp board as it is also connected to that node. At least that can be unplugged.