JCM 2000 Volume Drop

Handel85

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For some reason I cannot mag up your pictures, even so I suggest you reflow the preamp tube sockets.
We may be getting close to your problem.
I do not think you need the bias board mod, it is more likely to be one of your bias pots being faulty. Clean them both with contact cleaner and apply some Deoxit gold to lube and clean them, then retry. If it does not work they may need replacing.
sorry about that. I compressed the images so I could upload them direct to the forum instead of linking to them. should be easier to see now. I will clean the pots today and retry, thank you. you're talking about the two bias pots for the left and right pair of power tubes correct?
 

Handel85

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Update: Reflowed solder points that looked suspect and put the board back into the chassis, rebiased, and still getting low volume issues. As strange as it sounds I think the output transformer is on the fritz. It’s making a weird static / tinny sound when I play. It’s really hard to explain but I don’t think it’s the preamp or power tubes. Can anyone walk me through how to test the output and power transformers? Thank you
 

Handel85

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well, here's a new one to me. probably not to you guys. Everything has been perfectly fine since the new OT was replaced, but now this very high maintenance amp is making a loud static garbled sound in Channel B. volume is lower than bedroom levels and gain doesn't work. Channel A, both Clean and Crunch are perfectly fine. I REALLY hope this isn't another send my amp to the tech issue. I love the sound of this amp but man it's been quite a high maintenance cost hog. otherwise I truly do like it.

 

Tatzmann

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I wouldnt throw more money at it.
If you cant solve this yourself better
get rid of it.

Whats a tech hour nowadays? 100?
And since this is a 2000, many ordinary techs wont get this thing completely sorted out,
and some wont even work on it, and i can
fully understand that.

If you go to a tech ask him if he is
experienced with this Marshall model.

I'd start with the Bias Kit now, because
you have to undo many clip on connectors
for this and it might be a good time to
check all the solderjoints.

I had intermittent working Deep/Toneshift
on mine, it was a connector-solderjoint on
the righthand side of the front/potentiometer
pcb.

I had very low output. Solderjoint at the phaseinverter.

Had crackly noises. Solderjoint and accompanying burnt trace at v1 due to
arcing.

And of course the bias runaway.

I think you can fix it yourself with a little help
from the good guys here.
 

Handel85

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I've already been expressly denied permission by the wife to spend more money on it, so that is out of the question :D
I do feel competent enough to do basic solder repair on faulty solder joints if need be, etc. I just need to isolate the issue if possible.
after work I plan to take it apart and look over all the solder joints
 

Tatzmann

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R3 on your mainboard looks toasty.

Might be just the picture though. (looks
brown between the colorbands instead of blue)
 

Handel85

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wrapped up the work day and started troubleshooting things. Took out the chassis and snapped some pics for R3. also this wire was detached?
 

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Tatzmann

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R3 doesnt look good. Might be in spec still,
might change spec under working conditions.
Replace.

The green groundwire goes close to preamp tube v1 ( left upper corner on the mainboard on
your picture)

When i dissassembled mine i left this groundwire attached to the mainboard,
certainly to not stress the joints or it was
finnicky, cant remember.

Your last tech forgot to re-attach it.

Use other tech.:lol:

Ps: sorry bout the wife:p damn those shoes!
 

Handel85

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thanks @Tatzmann I took the chassis out and inspected everything and that resister isn't as bad as we thought. i think my pic makes it look worse than it is. I also reattached the ground wire. man that wire is a pain because the connecting terminal is stuck under one of the other boards and you can't reach under it to connect the wire. you are forced to take the board off to reattach it.

ok so the good news! it was a faulty preamp tube after a lot of trial and error with a few spares I had on hand. fingers crossed I don't have any more oddities out of this amp. I at least now know that if my distortion goes out or volume dips considerably, check my preamp tubes!
 

bicamen

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thanks @Tatzmann I took the chassis out and inspected everything and that resister isn't as bad as we thought. i think my pic makes it look worse than it is. I also reattached the ground wire. man that wire is a pain because the connecting terminal is stuck under one of the other boards and you can't reach under it to connect the wire. you are forced to take the board off to reattach it.

ok so the good news! it was a faulty preamp tube after a lot of trial and error with a few spares I had on hand. fingers crossed I don't have any more oddities out of this amp. I at least now know that if my distortion goes out or volume dips considerably, check my preamp tubes!
@Handel85
If you are still looking for the Dr Tube DSL bias kit, let me know. I have a brand new, unopened one available and willing to sell for cheap. Thanks!
 

Handel85

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@Handel85
If you are still looking for the Dr Tube DSL bias kit, let me know. I have a brand new, unopened one available and willing to sell for cheap. Thanks!
I am definitely interested @bicamen can you hold on to it for me? I need at least a month to recoup some cash after spending what I had on the JCM2000 repairs, the new transformer, etc. but yes I'd definitely like to buy it from you. shoot me a PM with cost details, etc when you get a chance and thanks!
 

bicamen

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I am definitely interested @bicamen can you hold on to it for me? I need at least a month to recoup some cash after spending what I had on the JCM2000 repairs, the new transformer, etc. but yes I'd definitely like to buy it from you. shoot me a PM with cost details, etc when you get a chance and thanks!
Yes. No problem. As soon as I figure out how to send the PM I will send the info lol.
 

Handel85

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I think you should be able to click the envelope near the top right corner of the website and click “start new conversation”
 

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