George Cooper
New Member
JCM2000 / DSL401 Repair,
Last week I sorted out a very annoying intermittent fault on this amp.
The problem was that every so often the amp would go quiet, particularly on the overdrive mode. As is often the case with intermittent faults it would not show up for days on end, then it got worse until finally it was almost constant.
I did find this forum on line and checked the often quoted faults, the bridge rectifier solder joints were poor but not bad and the bridge checked out on the meter OK.
The foot switch connector jack socket was not right but replacement ( with one with gold plated contacts) made no difference.
So I found a circuit on line, sadly no voltages to help guide me however I noticed on the bench that two resistors were burning up slowly, R78 and R81. These resistors are in the power feed to V2 and V1 so something was clearly amiss after those resistors. Disconnecting R78 stopped it burning, and after re-connecting R78 I disconnected R81, still no burning. This pointed of course to only a hand full of components on the high voltage feed around V1, so I quickly tested with my meter on ohms some of the capacitors in circuit for speed, and got a particularly low reading with C58 and C64 (they are in parallel), removing them revealed that C64 (10nF @ 500V) was reading around 500 ohms which was not right, so this was replaced ( up rated to 1000V) and all now worked as it should. I replaced R77, R78 and R81 as the were all looking the worse fried due to this fault.
Following the comments about the bridge rectifier I decided that I would replace it with a higher spec one encased in metal, and mounted it an inch above the PCB to keep the heat away form the board.
Then following further reading I decided to reset the Bias voltage to 800mV.
Once reassembled the amp works and sounds great as well and hopefully will now be more reliable as well.
Hope this might be of some help to others.
Please be aware that if you are not comfortable working with high voltages then I would advise you to take this to an experienced technician, there are some shocking voltages in these amps.
Last week I sorted out a very annoying intermittent fault on this amp.
The problem was that every so often the amp would go quiet, particularly on the overdrive mode. As is often the case with intermittent faults it would not show up for days on end, then it got worse until finally it was almost constant.
I did find this forum on line and checked the often quoted faults, the bridge rectifier solder joints were poor but not bad and the bridge checked out on the meter OK.
The foot switch connector jack socket was not right but replacement ( with one with gold plated contacts) made no difference.
So I found a circuit on line, sadly no voltages to help guide me however I noticed on the bench that two resistors were burning up slowly, R78 and R81. These resistors are in the power feed to V2 and V1 so something was clearly amiss after those resistors. Disconnecting R78 stopped it burning, and after re-connecting R78 I disconnected R81, still no burning. This pointed of course to only a hand full of components on the high voltage feed around V1, so I quickly tested with my meter on ohms some of the capacitors in circuit for speed, and got a particularly low reading with C58 and C64 (they are in parallel), removing them revealed that C64 (10nF @ 500V) was reading around 500 ohms which was not right, so this was replaced ( up rated to 1000V) and all now worked as it should. I replaced R77, R78 and R81 as the were all looking the worse fried due to this fault.
Following the comments about the bridge rectifier I decided that I would replace it with a higher spec one encased in metal, and mounted it an inch above the PCB to keep the heat away form the board.
Then following further reading I decided to reset the Bias voltage to 800mV.
Once reassembled the amp works and sounds great as well and hopefully will now be more reliable as well.
Hope this might be of some help to others.
Please be aware that if you are not comfortable working with high voltages then I would advise you to take this to an experienced technician, there are some shocking voltages in these amps.