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Thread: Marshall part PCBCD0053 needed, Please help!

  1. #1
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    Marshall part PCBCD0053 needed, Please help!

    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?

  2. #2
    "Thermionic Apocalypse" -JT nickb's Avatar
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    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?
    Experience is something you get, just after you really needed it.

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    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.

  4. #4
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Dave Johnson View Post
    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.
    Experience is something you get, just after you really needed it.

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    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?

  6. #6
    Old Timer J M Fahey's Avatar
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    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
    g1 likes this.
    Juan Manuel Fahey

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    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.

  8. #8
    Senior Member Enzo's Avatar
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    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.
    J M Fahey likes this.
    Education is what you're left with after you have forgotten what you have learned.

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

  10. #10
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Dave Johnson View Post
    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
    Experience is something you get, just after you really needed it.

  11. #11
    Senior Member Enzo's Avatar
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    The amps are the same. The MG100DFX is a combo, the MG100HDFX is a head.
    J M Fahey likes this.
    Education is what you're left with after you have forgotten what you have learned.

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    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.

  13. #13
    Supporting Member mikeydee77's Avatar
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    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.

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    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.

  15. #15
    Old Timer J M Fahey's Avatar
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    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.
    Juan Manuel Fahey

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    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???

  17. #17
    "Thermionic Apocalypse" -JT nickb's Avatar
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    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.
    Experience is something you get, just after you really needed it.

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    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.

  19. #19
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Dave Johnson View Post
    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.
    Experience is something you get, just after you really needed it.

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    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.

  21. #21
    Senior Member Enzo's Avatar
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    If one was shorted, it would measure so while in the circuit.
    Education is what you're left with after you have forgotten what you have learned.

  22. #22
    "Thermionic Apocalypse" -JT nickb's Avatar
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    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.
    Experience is something you get, just after you really needed it.

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    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.

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

  25. #25
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Looks like TR2 is bad. Pull and re-test.
    Experience is something you get, just after you really needed it.

  26. #26
    Senior Member Enzo's Avatar
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    Remember TR2 has C98 and D14 in parallel. ANy one of the three could be shorted. Or two or all three for that matter.
    Education is what you're left with after you have forgotten what you have learned.

  27. #27
    "Thermionic Apocalypse" -JT nickb's Avatar
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    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.
    Last edited by nickb; 07-26-2017 at 09:01 AM.
    Experience is something you get, just after you really needed it.

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