Marshall 3520 bass amp help, blowing fuses...

[HTH]

Hi all,

My bass player got a 3520 model Marshall bass head (rack) off eBay a few weeks ago, gigged it a few times then it went off during rehearsals last week with the 2A mains fuse blown.

I've isolated the secondary windings from the diode bridge to see if its the toroidal PT shorted and the mains fuse doesn't blow with the secondaries isolated. Also measured the AC voltage across the secondary with 94vac end to end and 47vac from either end to the CT. I'm therefore thinking that the PT is fine and its something else thats drawing too much current in the power amp section - maybe the big 10,000uF can caps (they're 26 years old after all).

Any help/advice appreciated - I have minimal experience with solid state gear, I'm just a hobbyist amp guy into valve gear.

Schematic link... http://www.drtube.com/schematics/marshall/3520pwr.gif

[mozwell]

From the schematic it looks like you can disconenct the +/- rails from the 10000u caps to the power amp.
First measure the bridge rectifier diodes with a multimeter, check for a shorted diode, all ok, then check the 4 power mosfets for shorts in the same way. Also measure to see if the 4 gate resistors (470 ohm) are ok.
Search here for light bulb limiter & build one (very easy)
Disconnect the power rails to the power amp & turn the power on using the bulb limiter, the bulb should flash brightly at turn on, then gradually become much dimmer. If it stays bright you have a problem with the transformer / bridge rectifier / capacitors.
Measure the +/-DC rails, are they correct ? even some value vaugely close is ok when testing using the bulb limiter.

If this is all ok, we can move onto the next step.

[Enzo]

You have so far verified that the most reliable part of the amp - the transformer - is in fact OK.

Your amp blows fuses, the most common cause of that is shorted output transistors. Check them.

Then check the main power supply rectifiers for shorts. May be a bridge or may be discrete diodes, either way.

The filter caps may be old and dried up, but that results in hum. They don't usually short. Easy enough to check to see if they did though.

[Jazz P Bass]

Check the output mosfets for shorts.

[HTH]

Thanks for the suggestions guys - I'm been out gigging all weekend, so this is my first chance to check up on this repair.

When checking MOSFETs for shorts, what am I looking for? - i.e. how do I test them for shorts? Are we talking using the diode function on a DMM to ensure the diode junction is in-tact?

[HTH]

or use the continuity function?

[Enzo]

It has three leads. We hope none of the three are shorted to any of the others. OHm meter, continuity tester, whatever. I'd use the ohm meter myself so I don;t overlook the continuity tester's threshold.

[HTH]

I've found these links on testing MOSFETs (note to self: Google is your friend, ha ha)...

http://music-electronics-forum.com/t16032/
Testing transistors with DMM or VOM - diyAudio

I also have a Peak semiconductor analyser which would be perfect, but I'd need to remove all of the power MOSFETs to do that - bit of a PITA of a job (will try testing in-circuit first).

[HTH]

Just found this in the Ampage archives.... http://music-electronics-forum.com/t28830/

Same amp blowing fuses - shorted rectifier module (as suggested earlier in this thread).

[Jazz P Bass]

When trying to find a fault for 'blows fuses', your best bet is to look for a gross short.
The first place to look is the output section (the transistors are basically varying the power supply).
You don't need to know how, at this point, to 'test' the component.
It's either low ohms or high ohms between the pins.
It helps to know the circuit or have the schematic at hand as there may be resistors in circuit that will throw off the readings.
If nothing looks obviously shorted you will want to check the power supply itself.
Again, you are looking for gross, obvious shorted conditions.

[HTH]

A big thank you to everyone who's contributed to this, I will save the suggestions for future reference.

I've just tested the rectifier module and theres a short, so I'm replacing it now and hoping thats the limit of the problem.

[HTH]

All fixed, I've put a bigger rectifier module in as its all I had anyway - one of those big-ass metal ones Marshall used back in the day (25A or something daft).

Much love to everyone who helped.