Marshall MG100DFX Volume Drop

[nleavitt]

I just acquired a MG100DFX in great condition except it for an intermittent volume drop. At startup, the amp is fine. After about 15-20 minutes of playing (I've only tested at low volumes), the volume drops lower and stays there. I can turn the amp off, then back on and the volume resumes. The condition is the same on both channels in all settings (clean/crunch, OD1/OD2). Based on other posts for this and other amps, I've tried the following with no luck:

-Cleaned all pots and jacks with electronic cleaner (from Radio Shack) The interior is very clean in appearance to begin with, and it looks like a well maintained amp in general.
-Patched the FX send and Return with a single patch cable - no difference. Volume problem persists.
-Inspected for bad solders/loose connections. Everything looks fine.

Any help or suggestions would be appreciated. Thanks in advance.

[techno-rachit]

If such an issue persists, it is possible that there is no problem in the preamp section, the problem is in the power output stage, what are the transistors that are there in the power output stage?

[nleavitt]

Sorry, I wouldn't know. I'm an ameteur, so if you can tell me what to look for on the board, I can tell you. I'm hoping maybe a capacitor or something that i can easily get can be replaced. Thanks for the reply.

[techno-rachit]

Try to troubleshoot the output IC TDA7293V , it is the heart of the power output section, check for dry solders....if not working, change the IC. It is a 15 pin IC that will be mounted on a heatsink. Looks something like this one Semiconductor: TDA7293V (TDA 7293V) - POWER AMPLIFIER / 100W / 4E / 120V+MUT...

[techno-rachit]

Also sometimes, if the IC burns or is damaged, it damages the little circuit on which it is mounted. So if you want you can buy the replacement circuit from power module mg100 dfx

[nleavitt]

Thanks for the advice. I can easily replace the entire power module and am more comfortable doing that than messing with the indicidual chips/components. WHat do you think the chances are that this is where the problem resides? SOme additional information:

When the volume drops, I dried connecting to the FX return. The drop is still present, so I think that confirms your suspicion that it's not in the preamp.

Last night I took it apart and inspected the circuit board. No appearant loose connections or bad solders on the main board or the power module.

Thanks again, I appreciate the help.

[techno-rachit]

as i said..if there is a volume drop problem....the problem is mostly only in the pre amp section.......the TDA7293V is damaged...dats da reason its happenin....n such problems only occur in amps with a single IC output..
to confirm the problem of the module, connect the line out of ur amp to the input of another amp.....n check whether the volume drops.
fell free to ask anything else

[Jazz P Bass]

Clean the FX send & return jacks.
Or jumper them together.
A dirty contact in the jack may be the problem.

[techno-rachit]

if dis wud hav been da problem, y only after sum time there is a volume drop?
it should hav happened immediately
n the volume resumes after a restart of the amp

[nleavitt]

techno-rachit - Thanks for the clarification. I've ordered the power module (even found it for less here ($14.95): Antique Electronic Supply) When it comes in and I install it, I'll let you know if it solves the problem.

In the mean time, I'll try the line out to another amp if I can. Will this test work with headphones as well? The "emulated line out" and headphone output are the same jack. The individual channel volumes control this output. The master volume only controls the speaker (according to the manual).

Jazz P Bass: Good suggestion. I did try that, but it didn't change anything. I also plugged my guitar directly into the FX Return and the volume still dropped (although it was just a clean guitar signal). I have an older Valvestate 8040 in worse condition that had similar volume drop problems. Patching the send/return jacks did work for that amp. Thanks for your help.

[nleavitt]

Update: Power Module backordered. Will try to find an alternate source. Hopefully Marshall stills supplies this part as a repair part.

[Jazz P Bass]

I beleive the Emulated Line Out is a separate IC from the output to the speaker.
You should be o/k to run that jack.
As to the module: make sure the power supply is drained (ie: 0 volts) before you attach the connector to the amp.
Check the fan for dirt/ obstructions & that it spins freely.

[techno-rachit]

Yes the headphone test will work , coz after seeing the schematic, the output section of the line out is totally separate from the main output section

[nleavitt]

I did an interesting test last night. I had the amp apart to inspect the main circuit board for loose solders. I put it back together but kept the metal chasis (whatever it's called) just on top of the amp, open to air. I connected the power, speaker and guitar and started playing to wait for the volume drop which I consistently had, then would hook up headphones to see if they are affected. Well after 30 minutes of playing, no drop. Seemed to play normally. I figure one of three things is happening here:

1. The problem is intermittent and it was just random luck that it didn't occur. (Like taking your car to the mechanic so he can hear the noise you hear all the time, but it doesn;t show up during the test drive.)

2. I cleaned or knocked a piece of dust out and that was the problem.

3. The TDA7293V was being cooled beter with the "top off" and the problem results from overheating of the chip. Now as mentioned I replaced the fan and the volume problem had shown up with the new fan operating. If sensitive to heat, maybe there's an air flow issue or maybe I should reattahc the chip to the heatsink with the heat conducting gel that is used for computer CPUs. The TDA7293V/heatsink/fan arrangement is very similar to a standard CPU cooling system.

I'm very curious about 3, but will test further to see of it's not 1.

Replacing the power module is still probably a good idea, however it appears backordered everywhere. Appearantly Marshall changed US parts suppliers and this is causing part availability issues across the board. I might try messing with replacing the chip itself.

[techno-rachit]

Well supply proper ventilation to the IC and if possible change it.
Befor that try chaging the capacitors on the small module. If capacitors r busted, then this problem occurs. check the electrolytic ones. this happens if the capacitor drains. Cheers!