Was given a Marshall MG 50DFX but no volume

[RickBlacker]

Hi all...

Today I was just given a Marshall MG50DFX. The guy who had it said he was playing through it, then the volume went out on it. Said it's about 8 months old. I know this guy and know he would be kind to his equipment.

What I've noticed since I've had it here at home is that, I have to crank it all the way up to get any audio out of it.

With the clean channel, there is very little audio
With the dirty channel, I can get more audio out of it when I turn the gain all the way up. Still very slight volume. Also I can hear the hum of the amp increase in volume as I have the master volume up and turn up the gain.


When I pluged my Line6 PODx3 in the return on the FX loop, then I got volume. Still have to have main volume cranked all the way up.

Also tried plugging in my pedals to the FX return. I have a distortion, eq, delay and chorus. That too will drive the amp but again, the master volume needs to be cranked all the way up.

One more thing, for grins I tried to plug into the headphones jack... Still no good. In fact, I couldn't even tell if anything was coming through, but there was definitly still volume coming through the speaker. Odd... i would have assumed a headphone jack would have bypassed the output stage to the power amp.

I'm really hoping that I can get this thing up and running for cheap.

Any help will be greatly appreciated.


Thanks
Rick

[Enzo]

Plug a cord from FX send to FX return. ANy help?

RUna cord from the FX send to some other amp. I assume the signal is weak there?

If that output is weak, then I suspect something late in the preamp. Since both preamp channels are affected, I'd look to the preamp out, like a bad IC9, open C22, or open R20.

[RickBlacker]

Hi Enzo, thanks for the help, greatly appreciated.

I did bridge the FX loop and that made no difference. I've also ensured that the problem is not with the speaker itself. I took the speaker out of a different amp and plugged the speaker from the Marshall into it, the speaker worked just fine

When you say open, what does that mean? Blown?

[Enzo]

Open, having no continuity, measures the same as waving your ohm meter probes in the air. Could be burnt out, could be broken wire inside from vibration, who knows.

[RickBlacker]

Last night I did a little probing around with my multimeter

I found C22 which is towards the end of the preamp. Put multimeter on volts, put black lead to chassi for ground, put the red lead on one side then the other side of C22. Nothing, no reading.

I did this same thing on all the pots, never got a voltage. Being being new to this that I am, am not sure if that is expected or not.

I found D2 which is towards the beginning of the preamp, got a reading of 14.9v.

I checked all the IC chips, the ones labled as IC1A, IC2A, IC3A... The little op-amps... They all seemed to register about the same 14.9v

For shits and grins, I pulled the connection out of CDN2. I measured what I thought would have had voltage which was pin 5, it didn't have anything... However, pins 1 and 2 registered 15v.

I checked REG1 and REG2, they had these voltages
REG 1
24.5
0.0
14.5

REG 2
0.0
-24.7
-14.5


I dunno... I was just poking around... Maybe this can give some clues....

[Jazz P Bass]

Voltage Regulators

The voltages that you show for the voltage regulators are perfect.
+ - 24 in / + - 15 out.

[RickBlacker]

Durring my lunch today I will be taking some random readings.
Do you by chance have any suggestions of test points?

[Jazz P Bass]

C22

Just curious about how you measured C22.
There will not be any Vdc on either side.
What you want to check is the Vac signal when you have a signal coming in to the preamp.
The preamps main reason for existence is to raise (and allow change to) the feeble 100mv signal that is coming from your guitar and send it to the power amp section.
If you trace the signal down through the schematic I am sure you will see where it is failing.

Did you try, as Enzo has suggested, to FX Send the signal to another amps input?
This is how you tell if the problem is in the preamp or the power amp.

[RickBlacker]

Just curious about how you measured C22.
There will not be any Vdc on either side.
What you want to check is the Vac signal when you have a signal coming in to the preamp.

Well... I'm new to this and didn't realize at the time I needed to have an input signal coming in. I was simply trying to test voltage by poking around.

I'm about to do some readings with a steady input source from my boss loop pedal

If you trace the signal down through the schematic I am sure you will see where it is failing.

Hope so.

Did you try, as Enzo has suggested, to FX Send the signal to another amps input?
This is how you tell if the problem is in the preamp or the power amp.

Yes I did... I've done several things.

Tried headphones, same output level
Took the SEND from the FX loop, put it to the input of another amp, same output level low.

Tested teh Marshalls internal speaker by driving it with another amp, that worked fine.

Have taken my Line6 PODx3 and or FX pedals and hooked them directly to the RETURN on the FX loop and things worked fine.

[RickBlacker]

With the multimeter set at 200AC
C22 bounces between 3.3 and 1.5ish
C62 bounces between 0.1 and 0.2


For grins, I took a RESISTANCE reading of them.
R20 reads 4.67 ohms with meter set at 20k
R62 reads 1.0 ohms with meter set at 2K.


Took AC voltage readings going INTO
R20 reads average of 1.6
R62 reads average of 0.1

Took AC voltage readings OUT of
R20 reads 0.1
R62 reads 0.1

[Jazz P Bass]

Marshall MG50

I think that you are focusing on the wrong section of the schematic.
We know FX out is weak.
We know FX In works.
That proves out R62 onwards works.
Start at the begining of the circuit. The input jack.
Next, pick a channel & follow it through.
Using your Vac scale on your meter look at the output pins of each IC. (Pin 1 or 7)
Write down the measured Vac on the schematic.
When you get to C22 it should be obvious where the signal is failing.

[RickBlacker]

So, what should the AC output of C22 be?

[52 Bill]

Took AC voltage readings going INTO
R20 reads average of 1.6
R62 reads average of 0.1

Took AC voltage readings OUT of
R20 reads 0.1
R62 reads 0.1

Seems the signal dies at output of R20.

What resistance reading do you get across R73?

[Jazz P Bass]

Marshall MG50

I do not know what the output voltage "should" be.
At the FX input, if a 100mv guitar signal could not drive the amp to full power and the IPod could, that is a hint.
Try to measure the IPod output voltage.
It will most probably be over 1 Vac.
Possibly 2 Vac.

[RickBlacker]

Seems the signal dies at output of R20.

What resistance reading do you get across R73?

.97k