Troubleshooting a Marshall 5203 Master Reverb 30 combo

[egj]

Hi all,

I have a Marshall 5203 Master Reverb 30 combo amp. It stopped working a while ago while we were playing bass through it (bad idea, I know). Now it doesn't turn on at all – no power light, no sound, no nothing. The amp sounds quite nice when working so I would like to get it fixed, but I know little to nothing about electronics.

There is a fuse inside but it is intact. Nothing else seems visibly off, either. I have a multimeter, so I can measure things, but I don't know where to start xP

Photos:






Here is the schematic.

Any help would be very much appreciated! 8|

[Enzo]

Start at the start.   First, did you have the fuse out before it went dead?  I don't know the amp closely, so where is the voltage selector?  I don't see one, it is just transformer wiring?   Make sure the thing is set for your voltage mains, are you a 240v area or 120v?

The fuse may LOOK intact, but did you pull it from its clip mand test it with an ohm meter?

Look at the schematic, the mains wiring comes into the amp, through the fuse, and on to the power switch.   DO you have mains voltage at the mains side of that switch?   And at the amp side of that switch?

Got mains voltage right at the transformer primary?   Got that far?   Now is there voltage at the red wires leaving the transformer?  The two that connect right beside the bridge rectifier.  And verify the center tap is also still grounded - blue wire?

Right next to the bridge are two caps, C31, C32.  Got DC voltage across each of those?   20 volts or so for each.

[egj]

Thanks for the reply!

Start at the start.   First, did you have the fuse out before it went dead?  I don't know the amp closely, so where is the voltage selector?  I don't see one, it is just transformer wiring?   Make sure the thing is set for your voltage mains, are you a 240v area or 120v?

I am in Sweden, which has 230V. Up until 1988 we had 220V, and this amp is set up for 220V. There is no voltage selector on the amp, but it pretty clearly states 220V on a sticker on the back.

The fuse may LOOK intact, but did you pull it from its clip mand test it with an ohm meter?

I did now and it seems fine. I tested it with the continuity thing on the multimeter too, and it beeps, so I'm pretty sure the fuse is good.

Look at the schematic, the mains wiring comes into the amp, through the fuse, and on to the power switch.   DO you have mains voltage at the mains side of that switch?   And at the amp side of that switch?

Got mains voltage right at the transformer primary?   Got that far?   Now is there voltage at the red wires leaving the transformer?  The two that connect right beside the bridge rectifier.  And verify the center tap is also still grounded - blue wire?

Right next to the bridge are two caps, C31, C32.  Got DC voltage across each of those?   20 volts or so for each.

OK – before I do any of this, this means I will have to plug the amp in to mains voltage before measuring. I'm a little concerned as I've never done anything like this, but I'm sure I can manage a few simple measurements like this with appropriate precautions. Is it alright to plug the amp in without the speaker attached for doing these measurements?

I apologize if I'm asking stupid questions but I'm not very knowledgeable about this stuff


Also, I've been measuring some of the resistors that are close to the output transistors (since at least this is something I know how to do) and some of them are pretty wildly off. R42 should be 1K5 and reads 605, R43 1K5 reads 612, R44 1K5 reads 1030, R45 1K5 reads 1390, and so forth... Surely this cannot be good?

[egj]

Okay, whoops, I just noticed something pretty significant.



This ain't right. :duh

[phatt]

Yep good work, you likely found it :tu:

Just remember to unplug the Amplifier before you re solder that connection, as it is the mains side of the transformer.
Phil.

[egj]

Yep, I just fixed it by resoldering that wire. Amp works great. Thanks for the advice, guys! It's always nice when a problem's something simple like this.

I do find it weird that some of the resistors measured pretty far out of spec, but the amp sounds good so I should probably leave it alone 8|

[g1]

  Measuring the resistors in circuit can give lower readings due to parallel resistances in the circuit.  To properly measure them you need to disconnect at least one side of the resistor from the circuit.
  I think yours are probably ok.  If you measure them in circuit and get a HIGHER reading than they should be, then they are suspect.

[egj]

  Measuring the resistors in circuit can give lower readings due to parallel resistances in the circuit.  To properly measure them you need to disconnect at least one side of the resistor from the circuit.
  I think yours are probably ok.  If you measure them in circuit and get a HIGHER reading than they should be, then they are suspect.

Thank you. Still learning, here