Repair of a DSL401

George Cooper

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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.
 

FreshOats

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Thank you, already finding this helpful. My JCM2000 DSL401 started intermittently dropping in sound. It just fades out and then after a min or so fades back in. I recently replaced the power tubes, which 2 looked as if they were done, and the pre-amp tubes just in case. But it still cuts out. I had the bias set to 880 mV based on some calculations, but I'm going to try dropping that to 800, since it seems to be running really hot. Also going to check for any shitty soldering connections.
 

George Cooper

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Thank you, already finding this helpful. My JCM2000 DSL401 started intermittently dropping in sound. It just fades out and then after a min or so fades back in. I recently replaced the power tubes, which 2 looked as if they were done, and the pre-amp tubes just in case. But it still cuts out. I had the bias set to 880 mV based on some calculations, but I'm going to try dropping that to 800, since it seems to be running really hot. Also going to check for any shitty soldering connections.
If your amplifier is fading out and back in again that points to a heater supply problem, look at the tubes and see which (if any) are not glowing due to lack of heater supply. I think the most likely issue would be with the large bridge rectifiers solder joints, if this is the case I would suggest replacing it with higher rated one and mounting it off the PCB.
 

Roach

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Thank you, already finding this helpful. My JCM2000 DSL401 started intermittently dropping in sound. It just fades out and then after a min or so fades back in. I recently replaced the power tubes, which 2 looked as if they were done, and the pre-amp tubes just in case. But it still cuts out. I had the bias set to 880 mV based on some calculations, but I'm going to try dropping that to 800, since it seems to be running really hot. Also going to check for any shitty soldering connections.

Hey I had this happen to me as well and spent too much money with a bs audio company in san mateo, before taking it to a very cool guy that checked my amp for nothing. He found the problem was not actually a BR, at all.
The problem with my dsl401 was one preamp tube, (#2), was reading as bad, because the tube was not staying tightly inserted to the socket.

OK , So it seems elementary, but keep in mind a high priced audio company didn't find this problem, ( though they gave me a new preamp tube and said mine was bad), then another tech, who owned an amp company, both, missed the issue, (1st look.)
So it couldn't hurt to verify all your preamp tubes are secure.

He cleaned it all up and found i didn't need a new preamp tube, both my new one and the one that got replaced were fine, just was not securely connected. He also did the BR mod for me. But it made no difference as to the fading/surging intermitted volume described. That went away when all the preamp tubes were secured.
 

Roach

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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.

Yes thank you. Very good info
 

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