JCM2000 DSL401 hums after replacing rectifier, Help!

KaptinKangaru

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i wouldn't bother testing the caps and just assume they're bad. unlike the 50s when capacitors were more expensive than big hunks of iron (older amps are wild.... big horkin chokes with like 8uf of filter capacitance), caps are super cheap. especially for a pcb amp like this with radial electro caps, you'll spend more on shipping from mouser (five bucks probably) than you will on the actual capacitors. just be mindful of lead spacing and capacitor diameter. this is just on the dc heaters or was the main bridge rectifier replaced?
I replaced the main bridge rectifier.
 

KaptinKangaru

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Recitifer is basically just a set of 4 diodes in one component. Test as you would any diodes (your component tester or multimeter may have a diode testing option, otherwise resistance should be inifinite in one direction, near zero in the other). I assume you know how a diode works, right?



Caps is a case of visual inspection first. Bulged, leaking, leaked = failed, dead, gone. If they check out visually, discharge them (you know how to discharge caps safely with a high wattage resistor, right?) and test them with an ESR meter out of circuit. If they test within ~10% of the rating, chances are they're probably fine. Any deviation greater than that, replace them.

EDIT: I feel obliged to remind you, considering that you're asking these questions, that those capacitors can hold a nasty charge that can give you a good jolt or even burns. You've been warned. :)

Lol I know all too well about the dangerous voltage in amps.

I'll test the old BR when I get home this evening and report back
 

PelliX

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Lol I know all too well about the dangerous voltage in amps.

I'll test the old BR when I get home this evening and report back

Righto.

owever, in your amp the hum changes in intensity when you probe the board. Ground loops can also cause at 120Hz hum and if there's a bad ground connection that would make sense if you're pressing on it.

@KraftyBob also made a good point here, I think. Worth investigating, certainly.
 

tmingle

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I replaced all the Electrolytics, BR & checked all the 4007 diodes on mine. I have attached a list of Mouser part#s for the Caps, BR & diodes. The classic failure on this amp(And quite possibly the failure mode here) is the failure of the solder joints on the BR. IIRC, the bridge rectifier is undersized & gets very hot. When it heats up, the connections are intermittent & the sound cuts out. BR1 supplies DC power to the filaments for V1-V3. The hum may be caused by reduced filament voltage to the preamp tubes.

I am convinced that, over time the excess heat damages the PCB in these amps beyond reasonable repair.
 

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KaptinKangaru

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Sorry for dropping the ball guys, I just picked up a Peavey Invective MH for a steal last night, it's going to be hard to even think about the marshall. I'm probably going to use the empty combo with the V30 for the Peavey, and shelf the marshall chassis for "one day". That is unless someone in jersey wants this thing for 250, a lot I know, but I just want my money back, and it does at least have a Vintage 30 in it.
 

Norfolk Martin

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I'm not aiming this as the OP, and am sympathetic to his problem, but, coming from a family of techs, I am a little disturbed by the modern view that that, if you can solder, you can fault you own tube amp.

Unless you get lucky, attempting to troubleshoot and repair a circuit with no understanding of how it works, or what voltages and circuit behaviors are normal can make things worse. "Tutorials" concentrating on changing components until the problem goes away not only waste a lot of of the owner's money, but also introduce the possibility of creating new faults every time you change a component. I'm particularly concerned by advice like " first change all the tubes" without any attempt to determine if a tube is faulty, or even what stage the fault is in.

I still do a bit of bench work in my spare time, and the biggest problems are always amps that the owner has already tried to work on. Normally I know that, as it is currently wired, the amp used to work. I don't know that if someone has already tried to fix it.
 

duffhuff

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Sorry for dropping the ball guys, I just picked up a Peavey Invective MH for a steal last night, it's going to be hard to even think about the marshall. I'm probably going to use the empty combo with the V30 for the Peavey, and shelf the marshall chassis for "one day". That is unless someone in jersey wants this thing for 250, a lot I know, but I just want my money back, and it does at least have a Vintage 30 in it.

Just seen this thread and looked at the Youtube short attached.
When you're pressing the board right by the rectifier the hum quietens down. In the googledoc image you posted of the underside of the board, it can be seen that neither of the two filament supply filter caps have much (if any) solder on their pins. You need to re-flow these for a start I think.
 

shabli64

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Hi.
It is necessary to well solder the legs of the capacitors C73, C78.

222.JPG
 

Emtbreid

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Sorry for dropping the ball guys, I just picked up a Peavey Invective MH for a steal last night, it's going to be hard to even think about the marshall. I'm probably going to use the empty combo with the V30 for the Peavey, and shelf the marshall chassis for "one day". That is unless someone in jersey wants this thing for 250, a lot I know, but I just want my money back, and it does at least have a Vintage 30 in it.
Hey there, see you’ve found the place. I tinkered and troubleshooted my 401 for months on end, and in the end did exactly as you did…. Bought another amp. (A peavey at that just like you!)
A big part of that decision was based on the fact that I always needed some kind of pedal to get it to sound the way I wanted. Fast forward a year later, and I came across a 78 Marshall JMP 2104 2x12 combo and never looked back. Whereas I needed some kind of pedal to make my 401 sound good, I’m now happy plugging straight into the JMP. The JMP era Marshall’s are also pcb amps starting in 1973 onwards but the way they are laid out and built are far superior to the more modern ones. If you ever decide to give Marshall a chance again I’d recommend looking for a late 70’s JMP master volume…. They are life changing.
 
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KaptinKangaru

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Ok folks, I'm back. Got the wrong kind of Positive news, so I'm stuck in the house.

I've got the board back out, and I'm going over the joints.

In the meantime, I tested the old bridge rectifier, it tested open between positive and AC (Cross checked normal behavior against 2 brand new ones, and the one I replaced on the board to be sure).

That being said, what possible cascade of failures am I now seeking?
 

KaptinKangaru

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I went over a few dodgy looking solder joints, and tried it out. It still has a small amount of hum, though much much less, I could live with it. However, moving the chassis around/tapping around it still changes the hum for the worse. I don't want to put it back into the cabinet and find it hums loudly again. I'm about ready to toss this thing.
 

tmingle

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Make sure you check the joints on the small filter caps on the B+ bus. A couple were loose on mine the last time I fixed it. If you have a scope, check the DC filament supply to ensure pure DC to the filaments on V1, V2 & V3.
IIRC, most of the caps in this amp were specified right at or near there maximum voltage.
There are plenty of reasons why Marshall does not support this amp. Mine did not even last 6 months before I started having issues.
 

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