Marshall JCM 2000 TSL 122

[goldtop5]

I have a Marshall JCM 2000 TSL 122 that's having trouble with the bias setting. It keeps climbing higher and higher, way above the specified values (90mV). Any clues or hints?

[Jazz P Bass]

Can you identify the PC board part number?

It may be that series of 'crappy' boards.

That's what they do, the bias climbs with heat.

Not that that Is it.
Monitor the bias voltage & see if it is changing.

[The Dude]

Here's a whole page on the subject, although we don't know if it applies to your particular amp.

The Marshall TSL122 JCM2000 Repair/Mods Page

[goldtop5]

Here is a number I found on the board: JCM2-60-00-60

[goldtop5]

I replaced the 1k, 5w and 5.6k resistors. Still the same problem. Could this possibly be a defective pcb?

[goldtop5]

I just got off the phone with US Music Corp. The tech mentioned that the older versions for the TSL are defective and need to be replaced with a newer version.

[nevetslab]

The JCM2000 amp, whether it's the TSL122 or the other variants that use that main PCB assembly with the little 2-pot bias board & the 3-pin header for convenient monitoring of the effective plate current thru each half of the power tube stage.....they have the nasty habit over time to develop solder fractures. You can't remove the little board without first removing the rear panel PCB, then the main PCB (mark your plug-on cables first). That's usually what's responsible for wandering bias levels.

I've found solder fractures on the pot leads, the 7-pin header on the small board, as well as on the main PCB. I've also had to clean the small pots on the bias board (a little dab of Caig DeOxit seeped into the pot to get onto the carbon track & exercise the pot a bunch) while the board is out of the chassis. I know it's a pain to get at the board, but well worth the effort. I've had the same wandering bias problem on a number of the amps here in our rental dept, so I'm finally getting on to the preventative maintenance cycle to get at all the remaining amps that are in the inventory and haven't been into my shop.

Once the pots are clean (or replaced), and you now have restored solid solder connections thru-out the bias path (de-solder first, then re-solder; don't just add solder), that bias voltage will remain stable (assuming you already have the bias supply steady).

While you have the rear panel PCB out....take a close look at all the phone jacks, impedance select switch, etc. You'll no doubt find solder fractures up and down the board. Might as well remove the AC Mains PCB as well, as you're bound to find solder fractures on the IEC 320 Mains connector connections and the fuse holders.

[Jazz P Bass]

Good points.

Keeping in mind if it is the 'bad' board, the best thing to do is simply replace it.
While Marshall has them available.

[jvm]

I've done two of these. New board in both cases.

[Alex R]

http://music-electronics-forum.com/t36875/#post350582

[Audiotexan]

Here's a whole page on the subject, although we don't know if it applies to your particular amp.

The Marshall TSL122 JCM2000 Repair/Mods Page

Thanks for sharing this!

[davetutt]

Rev 5 boards have duff pcb material and you need to remove the entire pcb around pins 4 and 5 of all the EL34 tube bases and hard wire back at least 2-3cm if not more. I strip off the pcb tracks back to the origins at the biasing source and the output from the drive coupling caps and fit 32/0.2 insulated wire instead at the top / valve base side of the pcb. Also change all the bias resistors for 1-2 watt metal film resistors including the 10k feed resistor down in the far corner by the diodes as it is very leaky and will stop the bias being anything other than a thermal nightmare!

[goldtop5]

Thanks for the advice guys! I've tried everything I could to solve this issue. I've replaced every possible suspected component and still the problem persisted. So we ordered a new pcb, installed it....problem solved!!! I was told by us music corp that the older pcb ver had problems.

[davetutt]

Hope this helps others who stumble into the TSL100 /122 trap. If you don't want to pay for a new board then once you know what to do its not too difficult. Ultimately the issue causes mains transformer failure. It also causes melt down in the EL34 valves. I have measured full HT potential on the bias pins (pin 5) which should obviously be a negative potential not at 460 volts positive. Initially the failure will shorten the output valve life. I have found that about 6 gigs is all you get from a new output set which if you are Slash probably isn't too much of a concern but for most pub players the cost is just too much.