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Reply With QuoteIf 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?
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.
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?
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.
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...
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
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.
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
Clean the FX send & return jacks.
Or jumper them together.
A dirty contact in the jack may be the problem.
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
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.
Update: Power Module backordered. Will try to find an alternate source. Hopefully Marshall stills supplies this part as a repair part.
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.
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
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.
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!
I'll keep that in mind as an option if I can't get the entire power module anytime soon. Although, I'm not the best at soldering, so it may be a last ditch effort to replace the caps and/or the chip. The caps are intact visually, with no bulging or burn marks, but of course there could be internal problems. Thanks.
Yea....all d best for da further repairing of ur amp!
Regardless of well meaning advice on the web, changing parts without proving a need to only add to the ambiguity of troubleshooting. As you admit, you are not the best at soldering so the chances of introducing more defects, they are unrelated to the original, and often more difficult to track down because tech induced faults are not the usual failure types found in simple defects in a circuit.
What test instruments do you have at your disposal? Do you have an oscilloscope, signal generator and multi-meter? There are some simple checks that can be done with only a meter that would point you in the right direction. If heat is suspected(the time delay of the on-set of symptoms is consistent with heating problems), the bond of the IC to the heat sink is very important to keeping cool and out of thermal shutdown. The IC has a shutdown circuit in case of the die temperature raising too high. Make sure the mounting is secure and the heat sink is absorbing most of the heat generated. Try disconnecting the speaker and notice if the time span that the power reduction starts at changes. It can be a load related problem and the IC is doing its job of protecting itself. If the amp, with no load stays working......try connecting the speaker every few minutes if you have no other means of monitoring the signal. If you have an AC voltmeter, you can monitor the signal at the speaker wires that are disconnected from the speaker. If it still shuts down, focus on the heatsink connection as allowing the thermal protection to drop the power out. Since the amp works when not hot, there is less to suggest a faulty power IC, the vast majority of failures in ICs are catastrophic die failures that would not return to operation when cool.
While I agree completely with the systematic approach to troubleshooting, I personally would not advise you to plug and unplug the speaker on that amp as a test procedure. My personal experience has been that this chip will catastrophically fail when treated this way or when looked at funny or when it rains on a Tuesday.
It's failing solder on patching jacks, not always visible upon inspection. Reflow all pots & jacks next time you're in there.
CAUTION!!! Solid state and tube amplifiers contain lethal voltages and can retain them even after unplugged. Refer service to qualified technician.
I would assume that the gain would be turned down while connecting the speaker, but even if not, it should not cause failure of the device. If it blew from having a moderate load applied with no signal, it is best to find out early that the amp is going to blow for any reason, or no reason. I never found them to be as unstable as you suggest. I don't think the manufacturers have either since warranty repairs add up very quickly when there is a major design fault or poorly spec'd component, and a field revision would be issued.
That is a pretty firm diagnosis but does it fit the symptoms described? Not really, a mechanical failure of a solder joint does not behave so regularly with such precise step changes. He really should focus on the clear clues which point to high probability causes instead of highly improbable ones. There was no report of the drop being erratic, noisy, distorted or total, one or more of which would be expected results from mechanical intermittent connections. The timing and predictability suggests a step change based on localized temperature increase, or a very long time constant electrical cause which would be a rare conditional also. A few simple measurements should narrow the choices down very easily and quickly.Originally Posted by pecorporation
Indeed. This IC does not like to see a short on it's output. If there is any level of current on it's output, the brief short created at the speaker jack while plugging & unplugging during operation can be enough to cause failure. I've alsoi seen alot of slow fan operation or failure on this model causing overheating issues resulting in IC failure.
Yep, you're right. I guess I read through things a little fast. Sorry for the mis-information.
Thanks everyone so far for the advice. I'm not an electronic tech, but I am a mechanical engineer, so I know just enough to be dangerous. I don't trust my soldering skills enough to work with the IC. I got this amp real cheap, so it's also a cost-benefit analysis. Replacing the Power Module for about $20-30 is a cheap troubleshooting method for my needs. If it's not the power module, a new one doesn't hurt anyway. Unfortunately, I don't own the necessary electrical diagnosis equipment. While I'm waiting for the ordered Power Module to come in (which may take weeks), I thought about the connection of the chip to the heat sink. I can get some thermal grease and re-attach it as I would a PC's CPU. Again, it seems to work better when the chasis out, which points to thermal management, but I don't have enough tests to know conclusively. It might still be random volume dropping. If it was initially caused by insufficient cooling, and proper thermal management is restored, do you think the existingchip/power module is damaged, or just operating as designed when overheated?
Thanks again.
Update: Received a replacement Power Module and it did the trick! Seems to work great (at least after one test). (Hard to find right now though due to Marshall part distribution issues.) Thanks to everyone for all your help, and especially techno-rachit who nailed the problem.
Definately worth getting the entire Power Module vice the chip if you are not skilled at soldering.
Great advice! This is a very useful forum. Thanks again.
OK, I spoke too soon. The problem is back. This time, the amp starts at low volume, then after about 5-10 minutes, goes to normal, then can go low again. Pretty random problem. It does sound better with the new power module, so that probably helped. Also, when I turn it on cold, there's a small "thunp" sound through the speaker right at startup. If I turn it off and back on, this sound doesn't appear, only from cold startup.
Any suggetsions? Thanks again.
I don't want to muddy the waters here, but I believe I read a post by Enzo where he warned to drain the caps before hooking up one of these chips. If so, it's possible that the new chip is kerflooie (I hope not).
Thanks for the info. How do I do that? The unit was off of course and unplugged for a while when I replaced the module. I applied thermal greese as i would connecting a heat sink to a PC's CPU. Is there a way to easily check if the new chip is bad (either bad when I got it or messed up since?)
Short the positive side of the power supply caps to ground. You can use anything - a screwdriver, a wire .... Just make sure that you stay insulated from the "SNAP"!
Update - had an electronics expert work the amp. Basically, many bad solders. Looked good to the naked eye, but if you tapped the circuit board, you could have the volume go in and out. This guy re-slodered many connections and the problem is gone! Appears that the power module was not the problem (but a new one doesn't hurt). Everything works now, except the input jack is a little flaky - needs a little jiggling initially to work (separate issue). Anyone know of the correct part number for the jack or a compatible replacement? (Looks like there are still issues getting Marshall parts in the US right now.)
Also, now that the amp is working fine, I did notice a buzzing sound when not playing using OD2 only. Clean/Crunch/OD1 is perfectly silent when not playing. Used different guitars and cables, turned off all lights, tried different outlet, no change. Is this normal?
The OD channel IS buzzy. That is the way it is.
Parts Is Parts will have/get the jack for you.
Link: Fender Stratocaster Parts,Fender Telecaster Parts,Fender Bass Parts,Marshall Amp Parts, Guitar and Amplifier parts,Gretsch Guitar Parts,Gibson Guitar Parts, Hofner Bass Parts. Parts is Parts has more Guitar parts and amp parts.
I have attached the MG100HDFX parts list.
It is a stereo jack labeled foot switch on the list.
... BUT ...
make sure that the jack problem isn't also just a solder joint problem.
Indeed...
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