Was given a Marshall MG 50DFX but no volume

[RickBlacker]

I'm using a regular mini stereo headphone output. It's measuring roughly 1.x vac

[RickBlacker]

I think that you are focusing on the wrong
Write down the measured Vac on the schematic.
When you get to C22 it should be obvious where the signal is failing.

Set multimeter to 200vac.

Pin1 = op-ampA
Pin7 = op-ampB

Readings are :

IC1A : 0.0
IC1B : 0.0

IC3A : 0.2
IC3B : 0.0

[Jazz P Bass]

Set multimeter to 200vac.

Pin1 = op-ampA
Pin7 = op-ampB

Readings are :

IC1A : 0.0
IC1B : 0.0

IC3A : 0.2
IC3B : 0.0

Is this with the 1 Vac IPod signal input into the amp?

[RickBlacker]

Output from stereo headphone jack is bouncing between 1.0 and 0.7 vac. I measured on the other tip of the cable.

[RickBlacker]

Thanks all for the great help!

I got it fixed. It was a bad solder joint on one of the input plug's pins.

[Jazz P Bass]

Marshall 50

Way to go!
Yeah, those voltage readings at the first ICs did not look too good.
Glad you got it fixed.
John G

[cjlectronics]

Just wanted to add to this thread that I recently repaired an MG50DFX with the same low volume problem and the issue was a broken trace on the input jack. I had to solder a wire from the jack to the inductor and the amp volume problem was solved. Now the only wierd thing is that the fan will rattle and flutter for the first 10 seconds after power on but then starts running with no issues. I removed the fan and power amp IC, then cleaned all the dust, re-flowed all solder joints on the IC, and bolted it back on (verifying there was no short between the heat sink and the chassis). The fan doesn't rattle as much but still starts noisy but quickly goes away.

[bignaughtydog]

Volume issue

Just wanted to add to this thread that I recently repaired an MG50DFX with the same low volume problem and the issue was a broken trace on the input jack. I had to solder a wire from the jack to the inductor and the amp volume problem was solved. Now the only wierd thing is that the fan will rattle and flutter for the first 10 seconds after power on but then starts running with no issues. I removed the fan and power amp IC, then cleaned all the dust, re-flowed all solder joints on the IC, and bolted it back on (verifying there was no short between the heat sink and the chassis). The fan doesn't rattle as much but still starts noisy but quickly goes away.

Hi all.I did read somewhere a while back that the issue with the mg50dfx volume is directly related to the transistor C2120Y9E.I have the same problem and am about to order a replacement (or minimum quantity) If we remove the electronics tray from the amp and look behind the channel selector button about half way across the pcb, its the black half moon shape object.It has three legs and so should be fairly easy to remove/replace.I will post results here as soon as the job is done.Good luck to all you involuntary silent shredders out there.

[Enzo]

Hi, welcome to the forum.

I'm looking at the schematic, and I can't find a 2SC2120 on it. What does it connect to? Does it have a part number printed by it, like "TR2"? I only see two small transistors, TR1 is a 2N7000 in the footswitch circuits, and TR2, an MPSA06 in the mute circuit. But I gyess a 2SC2120 would work just as well as an MPSA06. Is it TR2?


If in fact it is TR2, then nearby should be D19 and C129. The three are in parallel. SO if one is shorted, they all will measure shorted. Use an ohm meter and see if you get a short across the transistor or the diode.

Those parts are the mute circuit. If you think the transistor is bad, just remove it and power up the amp. If the amp now works, then yes that transistor was bad. If that does not restore sound, then don;t bother ordering that part, unless you are ordering other stuff too.

[J M Fahey]

Your fan rattling and then becoming silent when picking up speed probably means it has not much life left.
Typical symptom of a worn bushing (no expensive roller bearings there) plus a healthy dose of dust/lint.
IŽd replace it (itŽs cheap anyway); it stalling in the middle of a show may burn your power chip needlessly.
Buy cheap insurance by replacing it now.
Good luck.

[Jazz P Bass]

"Sometimes" the cranky fan can be recoverd by peeling back the label & applying a drop of good machine oil.

[Enzo]

I have to say, I can't recall a time when a fan came back on me after the drop of oil on the chattering bushing quieted her down.

[J M Fahey]

Well, *I* do that, but didnŽt want it to be stoned as a "Little shop in the Pampas" solution.
Besides, everybody talks about USA as the "IŽll SUE you to death" Country, so I preferred to suggest a "Court proof" solution.
Oh well.
WeŽre not *that* different then

[bignaughtydog]

Hi, yes it is the TR1 you speak of.It has C2120Y9E printed on it but i have seen the schematic with 2N7000 indicated.Im fixated with this because it was mentioned somewhere or other as the prime cause of the problem .Im under the impression that parallel components are arranged as such because the failure of one dosent affect the others (as well as resistance values etc).A little like parallel xmas tree lights.The bulb can be identified by its lack of illumination whilst the others are illminated.Perhaps im wrong im my assumption but i will have a go at your suggestions as they seem plausable and are worth a go.I will try and thanks.

[Jazz P Bass]

TR1 should be a 2N7000 FET.
It is used in the footswitch circuit.
The parallel circuit Enzo was describing is in the Standby Voltage (ie: mute) circuit for the power amp IC. TR2 (2SC2120).
The SB voltage is slowly ramped up to the SB pin of the output IC.
What are you trying to fix?
A footswitch problem or a "no sound" problem?