Marshall JCM 900 Crackling & Drop Outs

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blue_thunder

Member
Joined
Jun 11, 2009
Messages
11
I've got a JCM 900 head that has developed a crackling sound and drop outs when I adjust the master volume or reverb knobs. It is happening on both channels, but all of the other knobs are quiet as can be. I've tried spraying contact cleaner into the pots and working them back and forth, but it didn't seem to help any. I ran a cable from the effects send to another amp and there was no crackling or drop outs when I adjusted any of the knobs (of course this bypassed the master volume & reverb so those knobs did absolutely nothing). If I run the cable from one of the preamp outs, the noise is there when I turn the master controls. I tried swapping the tubes, but the noise was still there. I rewetted the solder joints on the knobs, but that didn't help either. I cleaned the jacks & pententiometer for the effects loop, but the noise is still there. I looked for loose solder joints, but found none. Can anyone tell me what else could be causing this problem?
 
should find the circuit and have a look , some have a master volume that works
in the bias circuit of the output tubes [ i believe like the dsl 410  jcm 2000 ]
and if bad causes crackling and loss of level
 
Hi,
did you try to clean dust in ALL sockets in the back with air spray? I had similar problem on my JCM900 and other Marshalls and there were only dust in the speaker's sockets that dropped the sound randomly. You MUST open the head and clean all contacts inside...
Hope this help
electrichild
 
I believe you can find the schematic at www.schematicheaven.com
..check the opamp closest to the master volume for DC on the output 
..I dont remember the number on the IC but it's a special opamp with two opamps in one pakage sharing a common output and select pin decides wich gets used..5502 I think it was called..
j

 
blue_thunder said:
I've got a JCM 900 head that has developed a crackling sound and drop outs when I adjust the master volume or reverb knobs. It is happening on both channels, but all of the other knobs are quiet as can be. I've tried spraying contact cleaner into the pots and working them back and forth, but it didn't seem to help any. I ran a cable from the effects send to another amp and there was no crackling or drop outs when I adjusted any of the knobs (of course this bypassed the master volume & reverb so those knobs did absolutely nothing). If I run the cable from one of the preamp outs, the noise is there when I turn the master controls. I tried swapping the tubes, but the noise was still there. I rewetted the solder joints on the knobs, but that didn't help either. I cleaned the jacks & pententiometer for the effects loop, but the noise is still there. I looked for loose solder joints, but found none. Can anyone tell me what else could be causing this problem?
Click to expand...

You may be getting DC voltage on your pots. This will result in a scratchy/crackly sound as you adjust them. I've never heard of drop-outs from this, but the other symptoms fit. Check to see if the capacitors in front of the pots are shorted to DC or otherwise not working correctly. I believe you'll have to unsolder one end and check the capacitance that way.

Mason
 
It's possible the pots (when being passed DC) could pass DC onto the grid of the following stage and shift the bias out of working range I suppose.
 
I had a similar problem on my JCM900 for the master volume... Marshall uses bad quality pots and mine was fucked. I solved the problem simply by changing the pot for a new (bad quality too  ;D)... I never heard it anymore...
 
I fired up my Crate VC5310 yesterday and it did the same thing. If I pound my hand on the top of it, it  sounds right for a minute and then goes back to the same thing. All of the tubes were replaced not too long ago. I know it has a pcb and I'm not too excited about opening the thing up.

Matt
 
FWIW ,  my Marshall dsl has the master volume bias pot
[ i better look at the schematic again  , i think it's that way ]
and changing that fixed the exact same problem
[ i first tried resoldering it , but no luck ]
 

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