Marshall DSL

[Valvehead]

Im trying to figure out a jcm2000 , DSl 100. When i first fired it up 2 of the EL34's plates started to glow red and the amp started humming real loud. I shut it down and the replaced the 2 tubes. The problem went away .
I played through it real loud for 10 minutes. Works fine . ALL voltages and bias test ok. Screen resistors are fine.

I put the 2 suspect tubes back in. Played loudly for 15 minutes . Amp works perfect --no red plating .

The only thing i notice is the OT hums when in standby mode . When i go to Run mode, the OT hums fairly loud , doesnt seem normal to me. Not out of the speakers , but the trans itself.

When i remove the output tubes, the hum doesnt change from standby to run .

Is this normal for the JCM 2000 trans to be so loud ? I doubt it .

So, what do you think?

[Mars Amp Repair]

Well,
having both tubes redplating is unusual unless one was shorted in a way to cause the grid to go positive. If course that wouldn't explain what occured when you put the 'bad' tubes back in. Lightly tapping the power tubes with the handle of a screwdirver might show up an errant short in them.

You should look for something that would have affected the negative bias supply to the power tubes.

As for the humming...are you certain you're not just hearing the hum from the power transformer radiating through the chassis? would be weird for the bias to be normal & have the OT hum.

glen

[Valvehead]

oops--sorry i meant the POWER trans was making all the noise.


i tried tapping on the tubes with it running. Nothing, works fine now

Bias supply at the tubes looks fine. i think it was -.041mv

[Mars Amp Repair]

well,
I would suspect as long as the pt is not getting very hot over time, it's probably got loose laminations or just doesn't know the words ;-]

glen

[Mark Black]

Hate to keep repeating the same old line but I would seriously consider retouching the solders at all the controls, jacks, switches, multipins (especially), fuseholders, AC inlet, large and/or heat-producing components, and at any other point that looks a little "solder starved". On every PCB.

I've never worked on one of these models that didn't have a ring-cracked solder joint somewhere, and if it happens to be at the multipin interconnect from the main PCB to the bias control PCB that could cause the symptoms you report.

I used to think I could spot cracked solders with my naked eyes (now through reading glasses these days). Then I started using a 3x magnifier/lamp and realized I had probably been missing bad or marginal ones for years. Even with the 3x magnifier I occasionally see one that just looks "funny" and have to break out an 8x lens to find that there is indeed ugliness afoot. This proved especially the case with small multipin connector headers, of which the DSL's make much use.

After finding so many in the past on DSL/TSL amps I hardly bother looking for them anymore - I just nail every pin of all the aforementioned components, as prevention if nothing else.

There - I went and said it again (sorry).

[Valvehead]

Ok, ill break out my magnifying glasses and have a look .

thanks


i did find out that the tubes in it are 3 years old....

[Peter Clothier]

It is perfectly normal for the power transformer in a JCM2000 to hum loudly. That is just the way they are, no need to be overly concerned about it.
The stock valves supplied by Marshall are always an issue. So, if in doubt replace them for known good stock. Dry joints are a common fault as per comments above. I always replace the BR62 pre-amp bridge rectifier with a 10A equivelent regardles. The 1/4" Tab/Recepticle connectors often work loose and require retensioning.

Good luck.
Peter.

[Valvehead]

i did find 3 bad solder joints. I also saw about 10 questionable joints and re-flowed them.

Amp is still acting normal.....

for now....