JCM600 fault

Parker

I bought a JCM600 combo in need of TLC. Plugged it in and all seems good (few scratchy pots, but otherwise fine). The only thing I have found is the clean volume has virtually zero effect until it's up to 8-10 and that then brings in a distortion in the background behind the clean sound. If I was a betting man, I'm guessing this maybe a Valve based issue?

ICBM

Given that it's a JCM600, all bets are off .

Try swapping the first and second preamp valves - if the fault stays the same, it's not a valve.

Among the possible faults on these which might cause that are bad valve sockets - they have a tendency to melt because they're plastic and the amp runs very hot, and if they do they shrink and can bend the valve pins or even crack the valve - a faulty jumper connector, or a failed treble cap in the tone stack (C3) - I've come across that more than once.

Parker

Thanks fella

ICBM

For what it's worth, this is what the valve sockets will look like if they're the cause - this is on a JTM60, but the JCM600 is the same - notice the sockets are discoloured and misshapen:



If so then you'll need to replace all of them with ceramic ones.

Also check there isn't a burn under the filament rectifier (far right in the pic), although that won't cause the fault yours has, it stops the amp working entirely if it fails or de-solders itself.

DJH83004

As per ICBM, my own personal experience is the number of ribbon cables so def worth an unplug / plug - not one of Jim's finest pieces of work I'm afraid, also relays in the mix, which can give issues, but best best is to swap or better still replace the preamp valves if you have any known good spares. 

ICBM

Forgot to say - if the sockets are as bad as the ones in that pic, *don't* put new valves in until they've been changed. The shrinkage on that one was so extreme that all the valve pins were bent sharply inwards, and you'll probably just crack the new valves as well.

Parker

Thanks guys - impeccable advice as usual! 

Parker

Cleaned all the pots, reconnected all the connectors and switched the preamp valves. Not sure which did it, but fully working now thank you! Sadly one of the knobs snapped off in transit so that's my next job - need to source a pot.

DJH83004

well done matey, saved a trip to your amp tech, and I have to say not a bad old amp once up and running.
 

ICBM

That's good news. Not all of the possible faults are as cheap as that to fix…

I be honest I don't agree with DJH, I've never liked them. Although to be fair, although they do have a multitude of faults, they can at least be repaired fairly easily, unlike some of the more modern ones.

DJH83004

Understand where ICBM is coming from, but my perspective is more from a repairer rather than user, and they can be sorted, even if if does mean swapping out the valve bases. Tone wise my guitar playing is limited mainly to pentatonic or blues scales - good enough to test an amp but not much more  

ICBM

DJH83004 said:

Understand where ICBM is coming from, but my perspective is more from a repairer rather than user

I bet they've provided you with more than a beer or two as well .

Parker

Can you advise the type of pot I need to order? The internet is far from clear and want to order at work today. 

ICBM

It should be a generic 16mm vertical PCB-mount pot with a split/splined shaft - I think! Pretty sure this series use split shafts - in whatever value it is.

Which pot is it? I have the schematic.

Parker

Its the gain treble. 

ICBM

Parker said:

Its the gain treble. 

B220K. If you want it to get less bright less quickly you could use a A220K instead.

This one is correct, although if you do a bit more digging you can probably find it in the UK for less.

http://www.tubeampdoctor.com/product_info.php?category_id=128&products_id=1591&mySID=egvokayz9NmmLqJwBEAew2

Parker

Thanks! 

jpfamps

I've got these in stock.

Send us an email.

Parker

Grr - so I've changed the Gain Treble pot without a second thought, put it all back together. Clean works fine but now the channel selection isn't working, no channel light and no gain. Any ideas guys? May get this in to a tech soon.

ICBM

Could be one or both of the T250mA fuses in the low-voltage supply circuit. With that not working the amp will default to the clean channel, no channel light, no reverb, no FX loop… so if any of those things work, it isn't that! But if they're all dead it probably is.

These aren't standard glass/cartridge fuses by the way, they're PCB-mounted soldered ones. They're two small black or brown cylinders on the front PCB next to the standby switch. They're a pain in the backside to replace because you have to take the board out.

DJH83004

First check - as per ICBM as always, but to get the front board out and change a pot is a fare old job on these, so double check all your interconnections.