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Hi all, this is my first post on this forum, I usully frequent the marshallamps forum but they don't seem to have the in-depth knowedge that this forum provides,but I have been reading quite a lot recently.
I have a tsl 122 from 2002 in bits at the moment. this amp has the issue 5 main board and has a very loud hum (50htz measured with cromatic tuner) and the bias drift problem (150 mv on test point at rear).
the hum is strange in that it is worst on the clean channel and not so bad on lead nd crunch, in my 4210 the hums and buzzes get worse with the extra gain.
This hum is present with all the gain and volume pots on 0 as soon as a lead is plugged in to the front of the amp, the amp does pass and amplify the guitar signal.
I have read varoius threads about the bias problem on this amp and have also changed many of the components in the bias circuit, at one point i thought that I had found the source of the problem when c36 and c37 reacted badly to a hairdrier when out of the circuit, but the replacement caps did nothing to correct the problem.
I feel that the only solution to this will be a new main board as reported by many of the posters on here.
This still leaves me with the hum, I am not a tech but am a qualified electrician and have some understanding of electronics, I am now even managing to get my head round valve technology, a little, but by taking out the pi valve the hum stops so i presume that the hum comes from the preamp and so may be corrected by the new main board, but the fact that the hum is worse on the clean channel makes me think that the problem could be on the clean channel pcb or even worse an earthing issuesomewhere miles away.
one other place the hum is bad is if I plug the guitar into the effect return
which I think would normaly point to power amp section, but the effect loop does not seem to be so simple on the drawings as on my 4210.
any help with this would be greatly excepted
ps sorry to come straight in with a request for help.
I have a tsl 122 from 2002 in bits at the moment. this amp has the issue 5 main board and has a very loud hum (50htz measured with cromatic tuner) and the bias drift problem (150 mv on test point at rear).
the hum is strange in that it is worst on the clean channel and not so bad on lead nd crunch, in my 4210 the hums and buzzes get worse with the extra gain.
This hum is present with all the gain and volume pots on 0 as soon as a lead is plugged in to the front of the amp, the amp does pass and amplify the guitar signal.
I have read varoius threads about the bias problem on this amp and have also changed many of the components in the bias circuit, at one point i thought that I had found the source of the problem when c36 and c37 reacted badly to a hairdrier when out of the circuit, but the replacement caps did nothing to correct the problem.
I feel that the only solution to this will be a new main board as reported by many of the posters on here.
This still leaves me with the hum, I am not a tech but am a qualified electrician and have some understanding of electronics, I am now even managing to get my head round valve technology, a little, but by taking out the pi valve the hum stops so i presume that the hum comes from the preamp and so may be corrected by the new main board, but the fact that the hum is worse on the clean channel makes me think that the problem could be on the clean channel pcb or even worse an earthing issuesomewhere miles away.
one other place the hum is bad is if I plug the guitar into the effect return
which I think would normaly point to power amp section, but the effect loop does not seem to be so simple on the drawings as on my 4210.
any help with this would be greatly excepted
ps sorry to come straight in with a request for help.
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