FIXED: My JCM2000 DSL401 amp just died...any ideas?

Voxman

My DSL401 was working perfectly.  But just switched it on and although power is on and all the lights are on, the valves aren't lit and no sound whatsoever - not even a hiss.  Checked the fuses - all fine.

Thoughts anyone?

hywelg

Cremation?

ICBM

If none of the valves are lit it will almost certainly be the internal PCB-mounted filament fuse. These never fail for anything but a 'nuisance blow' in my experience, and I don't have any problem increasing the fuse rating from the stock T6.3A to T10A, which will stop it ever going again unless there really is a serious short - highly unlikely. If you meter the start-up current you'll usually see it exceed 6.3A for well over 1 second (which is the usual time rating for a T fuse) until the valves warm up. This fault always occurs at power-up from cold and both the power light and the channel lights will still work.

If the power valves and the phase inverter are lit but the other three aren't, it's the filament bridge rectifier - this is the 'DSL401 fault' and is caused by it not being heatsinked, so over time it overheats and desolders its own pin connections. It can be fixed with a bolt-on heatsink so it won't happen again. This fault usually appears when the amp is hot, and can be intermittent at first.

Voxman

Thanks ICBM .  The bridge rectifier 'mod'was done some years ago.  No valves are lit so the internal fuse sounds like it might well be the culprit.  How do I physically get to it and where can I get hold of a T10A? 

ICBM

Voxman said:

Thanks ICBM .  The bridge rectifier 'mod'was done some years ago.  No valves are lit so the internal fuse sounds like it might well be the culprit.  How do I physically get to it and where can I get hold of a T10A? 

Take the chassis out - remove the top and middle back panels for the easiest access, disconnect the speaker and reverb tank, then undo the eight large bolts in the top and sides.

It's easiest with the amp lying face-down on a table with the handle facing you. When the chassis is loose you can lift it out holding the transformers - be careful it doesn't snag and tear the vinyl at the sides, it may be best to cut some cornflake packet into strips to slide down between the sides of the chassis and the cabinet first.

Lift the chassis out and lie it across the back of the cabinet with the valves in the space above the speaker. The fuse is near the third power valve socket from the right-hand end of the main PCB as you look at it with the knobs towards you.

You can get the fuses from Maplins if you can't find them online easily, but try Ebay first. You need a T10A 20mm fuse.

But check it first before ordering - if the fuse is *not* blown, it's likely to be either something trivial (a loose connector, probably) or more serious (blown power transformer winding - unlikely).

Voxman

Thanks very much ICBM - my goodness, they could have made it easier to get to!  But I'll have a go.

I want to make sure there's no current/voltage left to get a shock from - I know that can be kept there for some time - how do I make sure capacitors etc are drained?  I also read that, whilst you don't turn the power on obviously, you should keep the mains lead connected and plugged in to keep an earth and protect components against static charges etc...is that sensible advice or rubbish?  

ICBM

No, it's not really necessary and could be potentially dangerous. The danger with doing that is that you'll forget to turn off the mains or make some other mistake, and/or that you could then get a shock which travels via you to the chassis, if you're touching it. Better to disconnect from the mains entirely in my opinion.

Generally the components which are at risk from a static discharge are digital logic chips, and there are none of those in this amp.

There is also no risk of a shock from the caps in this amp if it's off and disconnected from the mains and has been for more than a minute or two - there is a built-in permanent drain resistor in the power supply. All amps should have this. It's especially necessary in amps with a filament fuse, since if it blows there is no way of discharging the caps via the valves.

Voxman

Thanks again ICBM - you're hired as my amp tech! 

Voxman

Pleased to report all is well & my DSL401 is fixed.  As ICBM thought, it was just a blown internal fuse.  As my amp tech guy is only 3.5 miles from me and was happy to only charge me £10 if it was just the internal fuse, I took it to him to do (just in case the problem was bigger) and with his power-screwdriver it took him 5 mins (me it would have taken about 10 times that).  Well worth it from my perspective.  

He fully concurred with ICBM that its a nuisance fuse & put in a T10A so theoretically it shouldn't go again.  He added that its totally daft Marshall didn't give easy access to this fuse, and you really shouldn't have to unscrew umpteen screws and drop the chassis just to get to the damn thing!  Why do amp designers make what should be simple, so awkward!?

Anyway, all done & fingers crossed I don't get any further problems.   

ICBM

Great!

Better than the TSL122 I have here anyway, which I had acquired last year for the parts value - over the holiday I thought I would have a go at actually finding out what was wrong with it, since it seemed a bit of a waste to scrap it, but I didn't want to spend the money on a new main board from Marshall. I thought I'd cracked it, but yesterday - luckily before I put it up for sale - it developed a different and completely unrelated fault.

So back to Plan A. At least the DSL401 is better than these...

Voxman

ICBM said:

Great!

Better than the TSL122 I have here anyway, which I had acquired last year for the parts value - over the holiday I thought I would have a go at actually finding out what was wrong with it, since it seemed a bit of a waste to scrap it, but I didn't want to spend the money on a new main board from Marshall. I thought I'd cracked it, but yesterday - luckily before I put it up for sale - it developed a different and completely unrelated fault.

So back to Plan A. At least the DSL401 is better than these...

Yeah, I've heard repeatedly from different sources that the TSL's can be fraught with problems.  I'm sure you'll suss it out though! 

ICBM

Voxman said:

Yeah, I've heard repeatedly from different sources that the TSL's can be fraught with problems.  I'm sure you'll suss it out though! 

I could just buy a new board from Marshall - that would almost certainly fix it all at once. But it's a lot of money, and I would still have to guarantee the amp to a buyer… and sadly, I'm not even confident I could then given the faults these amps have. Currently it's cost me less than the value of the parts, which I will almost certainly need at some point. At least if you take the hours of work out of the equation.

I had actually dismantled it last year, but I don't like to give up on things so when I had a couple of days free over the holiday I thought I would have a go at fixing it instead. It's still worthwhile finding out that your original instinct was correct though, it will save time in future .