Marshall part PCBCD0053 needed, Please help!

[Dave Johnson]

Hi all, as the title says, I'm in need of a Marshall power module.
Part number PCBCD0053. All my searches have turned into a dead end.
I have replaced the chip TDA7293 but it looks like I need the whole thing this time.
The amp is acting as if the module isn't even connected. No output.
Could this part be obsolete and no longer available?

[nickb]

What is the amplifier model number? It would be very unusual to need to replace the PCB unless it has burned through. Try some basic trouble shooting. Once you give us the model number we can look up the schematic, or better you find and post it. The we can tell you where to measure a few voltages to see what might be going on.

If there any noise form the speaker at all? Have you checked for the send / return switch jack problem by using a lead from send to return? Is the speaker OK?

[Dave Johnson]

The amp is the dreaded MG100DFX
When I replaced the TDA 7293V chip on the Power module, that screws to the heatsink, last time I had sound but it was short lived. It was just a loud hum, then the chip smoked again.
When I replaced the chip this time it's as though the module isn't even connected.
I fear I lifted a couple of traces with all the soldering & unsoldering.
The whole power module does not seem to exist in anyones parts departments anymore.
The only information that I could find was some ancient threads on this forum when searching.

[nickb]

The amp is the dreaded MG100DFX
When I replaced the TDA 7293V chip on the Power module, that screws to the heatsink, last time I had sound but it was short lived. It was just a loud hum, then the chip smoked again.
When I replaced the chip this time it's as though the module isn't even connected.
I fear I lifted a couple of traces with all the soldering & unsoldering.
The whole power module does not seem to exist in anyones parts departments anymore.
The only information that I could find was some ancient threads on this forum when searching.

The first golden rule is not to connect the speaker after a chip change. The reason is that if there is a problem you risk the new chip and the speaker. Have you tried connecting the speaker to a different amp to test it? If that is OK then take a really good look at your rework to see if there are any open tracks or solder shorts. Once you are sure all is OK you can power up but leave the speaker disconnected. Now measure and report the voltages on all the pins 1-15 of the chip relative to ground.

[Dave Johnson]

Thanks for taking the time to help.
The speaker is fine. Just checked it with a working amp.
Now the odd thing is that I can't get any voltage readings on any pin of the chip.
As though the power module was disconnected from the main board???
However it IS connected. The amp is on, without speaker. The fan is running, pilot light is on and fuse is not blown. Now what?

[J M Fahey]

The module is available, maybe just not under that part number, but simply as "Marshall power module".
MG100HDFX 100w Power Amp Output Module For Marshall Amp | eBay


BONUS POINTS: Made in USA and seller offers to help you troubleshooot your amp

[Dave Johnson]

Thanks a bunch for that. I think I'm going to start there with a known good part.
That looks exactly like what I'm after. Going to snag one now.
God only knows what harm that I've done to this one. The chip has been soldered & unsoldered 3 times. I know for sure that some of the solder points have lifted.
Thanks again Mr. Fahey, I owe you a cold beer.

[Enzo]

And when the TDA burns up SOMETIMES it damages the mute circuit, (STBY on the drawing). In the power supply page bottom center is a transistor, a cap, and a zener all in parallel. They make the STBY signal. If one of those shorts (from high voltage coming from the blown TDA chip), it leaves the amp chip muted. Always check those.

[Dave Johnson]

Thank you Enzo. I'll check into that in the AM.
You guys are great.

[nickb]

Thanks for taking the time to help.
The speaker is fine. Just checked it with a working amp.
Now the odd thing is that I can't get any voltage readings on any pin of the chip.
As though the power module was disconnected from the main board???
However it IS connected. The amp is on, without speaker. The fan is running, pilot light is on and fuse is not blown. Now what?

Well the new module won't work without power either...

The power transformer has two secondaries, one for the lower voltage low power stuff and another for the power amp. The power amp supply is a simple bridge rectifier and center tapped transformer to give the +ve and -ve rails. Probe to see if there is AC out of the transformer (W4/W5/W6), then DC out of the power supply (on CON3). If OK and then re-check at the power amp and of the cable.

I don't seem to have a a schematic for the MG100DFX but here is the MG100HDFX. I think it's the same in this area. See page 2.

Marshall_MG100HDFX.pdf

[Enzo]

The amps are the same. The MG100DFX is a combo, the MG100HDFX is a head.

[Dave Johnson]

Thanks Nick. I have a couple of things to tend to this morning, but I'll be back into this thing later today.
The schematic is exactly what I'm working on, I downloaded it. Much easier to use that way then having to try to blow up online drawings.

And Enzo, I did locate the mute circuit on the schematic and the amp.
Will report back with my findings later.

Thanks again, for all the help.

[mikeydee77]

Just a thought but isn't this one of those amp modules where the heatsink is isolated from the chassis. Perhaps worth checking that out.

[Dave Johnson]

It is. And that may be what I had missed when the last chip blew. One of the plastic insulators was faulty and it was indeed grounded when this new chip blew. Now I checked for continuity between the heatsink screw and ground and it read open, as it should be.
Thanks for the tip.

[J M Fahey]

One last detail: when plugging/unplugging that module into the main board be certain amp not only is OFF but also that main power rails have fully discharged to zero.