Marshall 3540 Bass Amp fault / help me learn about chokes

wwrrss · Post by **wwrrss** » Mon Apr 13, 2015 7:21 am

So I currently have a Marshall 3540 that my bands other bassist managed to kill opened up and ready to fix, I had a proper nose around inside it and spotted this...

...after some research and advice from friends, I'm pretty sure that burnt out component (labeled CH on the board) is a 5 ÂµH Choke.

I can't seem to find a direct replacement anywhere though, so what I'd like to know is can I run 5x 1ÂµH Chokes in series or parallel to do the same job?

Thanks!

NickS · Post by **NickS** » Mon Apr 13, 2015 1:24 pm

That looks like it's in series with the output. The 2540 delivers 200W RMS into 2 ohms, so that's 10A RMS. Your replacement would have to pass that current comfortably. You have to ask yourself why it's screwed as well - is one of the 2SK134 MOSFETs short-circuit?

Your best solution to replacing the inductor may be to rewind it yourself. It will have a rod down the middle (a ferrite bar core). You can cut off the orange heat-shrink sleeving and count the number of turns of wire on the core. Strip the burnt wire off and replace with some lacquered copper wire of the same gauge.

wwrrss · Post by **wwrrss** » Tue Apr 14, 2015 8:48 am

NickS wrote:That looks like it's in series with the output. The 2540 delivers 200W RMS into 2 ohms, so that's 10A RMS. Your replacement would have to pass that JEFF comfortably. You have to ask yourself why it's screwed as well - is one of the 2SK134 MOSFETs short-circuit?

Your best solution to replacing the inductor may be to rewind it yourself. It will have a rod down the middle (a ferrite bar core). You can cut off the orange heat-shrink sleeving and count the number of turns of wire on the core. Strip the burnt wire off and replace with some lacquered copper wire of the same gauge.

WHAT IS AN JEFF?

how can i test the mosfets incase they're dead too?

thanks for the info.

NickS · Post by **NickS** » Tue Apr 14, 2015 11:48 am

wwrrss wrote:WHAT IS AN JEFF?

Mezz is filtering the facts. That should read current. I relaxed. He got me.

how can i test the mosfets incase they're dead too?

I could do with a better pic than this, cropped from an ad on the net.

Does any of the four big resistors look as thought it's had a hard life? That would be a clue if one MOSFET had gone short-circuit.
First cut; measure with a meter. You could really do with one that measures low resistances. Measure between the case of the 2SK134/2SJ49 and the end of the nearest big resistor to each transistor. You may be looking for the difference between a dead short and .66 ohms (+/- 10%)

If you know your way around electronics, circuit diagram is at Dr. Tube though this is slightly different to yours as they have two output inductors, CH1 and CH2, in parallel.

If the choke got that hot, you may also find that it's affected the solder joints to the board. Hopefully no tracks on the board are fried.

NickS · Post by **NickS** » Tue Apr 14, 2015 12:29 pm

Actually, that isn't right. Though listed as 3540 and sharing some similarities, the diagram says it's for the 6010, 6020 & 6040. I'd be interested in better pics of both sides of the power module if you take it off.

Post by **Doog** » Tue Apr 14, 2015 1:11 pm

[youtube][/youtube]

NickS · Post by **NickS** » Mon May 11, 2015 1:20 pm

OK, I have your higher quality pics.
What does the amp do at the moment, or have you been unwilling to switch it on??
Do you have a multimeter?
If you remove the nuts and bolts nearest the PCB on the output transistors, you should be able to measure resistance from the case to the other two pins on each transistor.
Would you feel confident to unsolder the A (+VE RD) and E (-VE BL) wires and tape them up so they're not touching anything?

wwrrss · Post by **wwrrss** » Mon May 11, 2015 2:39 pm

NickS wrote:OK, I have your higher quality pics.
What does the amp do at the moment, or have you been unwilling to switch it on??

It blows the main fuse in the back of the amp.

NickS wrote:Do you have a multimeter?

Hell yeah!

NickS wrote:If you remove the nuts and bolts nearest the PCB on the output transistors, you should be able to measure resistance from the case to the other two pins on each transistor.

Of course!

NickS wrote:Would you feel confident to unsolder the A (+VE RD) and E (-VE BL) wires and tape them up so they're not touching anything?

Yeah, no worries.

NickS · Post by **NickS** » Mon May 11, 2015 3:48 pm

Well, take the resistance measurements and give us a table of results - that might tell us whether you've got a short in a MOSFET

If you're happy to do it, you could unsolder and tape up those leads. If it still blows a fuse there's other damage, such as maybe the rectifier bridge gone short-circuit. If it powers up you can check the other channel's still good.

The diagram for the 3540 power amp (slave, I guess, as it has a little op-amp stage on the front and all the component numbers are different) shows separate transformer windings, rectifiers and smoothing for each channel, so if it still blows fuses the next step is probably to identify the power supply components for that channel and disconnect the bridge from the transformer.

NickS · Post by **NickS** » Tue Jun 23, 2015 10:54 am

Bump.. how are you doing?

NickS · Post by **NickS** » Fri Oct 14, 2016 9:47 am

NickS wrote:Bump.. how are you doing?