JCM 2000 bias pins maxing at 15mV

[TimmyP1955]

I think that the second one down is the bias pot you need. Marshall Amp Parts - Marshall Pots

[DrGonz78]

I think that the second one down is the bias pot you need. Marshall Amp Parts - Marshall Pots

Not so sure if that would fit the JCM (TSL/DSL 100) series bias boards. The types that are in there are more like these ones...
0.15w Linear Horizontal Variable Resistor - Preset - Trimmer (5 Pack) 100R to 1M | eBay

[TimmyP1955]

I believe you are correct.

I think the best thing would be to trash the dual pot crap and make it into a 'normal' single pot bias circuit that most amps get along with just fine. I'd prefer to use a "real" pot, but where to put it?

[DrGonz78]

I believe you are correct.

I think the best thing would be to trash the dual pot crap and make it into a 'normal' single pot bias circuit that most amps get along with just fine. I'd prefer to use a "real" pot, but where to put it?

There is this example here that does just that.
The Marshall TSL122 JCM2000 Repair/Mods Page

[Djizix]

Also, it is wise to check all the output speaker jacks to see if any are worn out and open. The jacks are supposed to be closed circuit when no plug is inserted. I have one here that one of the 4 ohm jacks tip is open and will not mechanically close any more. That part needs to be investigated either way if you buy a new board or not since your still going to be using the output board in the amp.

So, was this a general suggestion, or an implication that my syptoms are related? The new PCB is in and hooked up and I'm getting the exact same syptoms. My next two ventures are the bias pots, which tested correctly, and the output jacks. I am now in the process of testing the jacks. The two contacts are reading differently on all 3 output jacks, I'm just not sure what it means. The jacks have two contact points. They're all testing for continuity when nothing inserted. The 16 ohm jack is testing no continuity when inserted on both contacts. The middle 4/8 ohm jack is testing continuity on both contacts when inserted. The end 4/8 ohm jack is testing continuity on the first contact, and no continuity on the second contact.

[DrGonz78]

So, was this a general suggestion, or an implication that my syptoms are related? The new PCB is in and hooked up and I'm getting the exact same syptoms. My next two ventures are the bias pots, which tested correctly, and the output jacks. I am now in the process of testing the jacks. The two contacts are reading differently on all 3 output jacks, I'm just not sure what it means. The jacks have two contact points. They're all testing for continuity when nothing inserted. The 16 ohm jack is testing no continuity when inserted on both contacts. The middle 4/8 ohm jack is testing continuity on both contacts when inserted. The end 4/8 ohm jack is testing continuity on the first contact, and no continuity on the second contact.

It was a general suggestion and something to inspect while your in there. As you insert a jack into the 16 ohm jack it will automatically prevent the 4 and 8 ohm jacks from functioning. On my amp I found the 8 ohm jack to be faulty on the tip switching mechanism, which meant it was not grounded out. I was also not sure if that would have a negative effect, but perhaps, as Leo pointed out, the 16 ohm jack cuts out the 4 and 8 ohm jacks. So it might not have mattered.

They're all testing for continuity when nothing inserted. The 16 ohm jack is testing no continuity when inserted on both contacts. The middle 4/8 ohm jack is testing continuity on both contacts when inserted. The end 4/8 ohm jack is testing continuity on the first contact, and no continuity on the second contact.

That sounds correct as I read it. When testing the 4 and 8 ohms switching contacts you have to alternate the 4-8 ohm switch too as I recall.

Have you tested voltage at pin 5 on all the output tubes? We need to test that to see what bias voltage you are getting. Seems like one side is getting very low bias voltage and the other very high, presumably. The high bias voltage will yield a cool bias on the tubes and the low bias voltage will make it hot. I really wish we would have gotten those measurements before board replacement. It's all a learning process for all of us.

[Djizix]

Also tested pin 5. On the left pair I was able to get my bias voltage at -35v. This is the side that is biasing at 130mV. Yes I know the pins are backwards. The right side pair was at -48v. This side would only bias at 8mV. So I have no idea where tis leads me.

You must have missed this. I'm having trouble remembering exactly how I had the bias pots set, but I think I remember the lowest I could get the right side was -45v so there wasn't much adjustment. This was the method of taking one tube out at a time and checking the empty slot. I think I remember someone saying that would work. I haven't tried measuring with the new board because it seems at eye value that its doing the same thing as the last board and I'm worried of damaging other components.

From what I've read the new PCB is a peace of mind item and thats half of my intention, so its not a huge deal that it didn't magically fix my problem. I had enough knowledge to replace what I knew needed fixed (melted components lol). Now I need real help diagnosing this problem, and I noticed noone chimed in when I said I was from southern Illinois so I'm truthfully not sure what all my options are here.

[g1]

Remove power tubes.
Adjust bias pots so pin5 on all power tube sockets reads -48V.
Install power tubes and check bias. Report results.

[Djizix]

Remove power tubes.
Adjust bias pots so pin5 on all power tube sockets reads -48V.
Install power tubes and check bias. Report results.

So we're back to square one. I attempted to get the bias voltage to -48v on both pair. The left pair has no adjustment at all and stays at -52v, and the right pair I can adjust from -25v to -50v. So I got them as close to the same as possible and now my bias readings are back to around 15 mV. This definitely makes me feel better about not frying something, but the amp will not sound anywhere near worth playing.

Also, I should note that my preamp tubes are not all glowing at the same brightness. On the left two, V1 and V2 I believe, the glow is strong and I can see them glow from a distance. V3 and V4, however, are barely glowing, and I have to get up close and get the right angle to see them glowing. If that is at all related.

[g1]

This is good. It seems to indicate one of the bias pots is open or has a bad connection. As far as the circuit is concerned it is as if the bias control is stuck on full negative voltage.

edit: by good, I mean much better than if one side was overconducting and overheating.

[Djizix]

I'll get some new pots ordered and installed and see where that leads me then. Do you have any advice about the way my preamp tubes are acting though? I obviously need to replace them all, but that's a venture I'd like to save for another day if I don't HAVE to replace them now.

And I should also ask, when replacing these pots, so far I've only been able to find 22k pots that won't match my pin layout, is it possible to hardwire a different type in with say, 20g wire and not affect the performance of the pot?

EDIT: New development...I plugged an input in and I'm getting no signal, whereas before I was able to get a signal with the low bias settings.

[Leo_Gnardo]

And I should also ask, when replacing these pots, so far I've only been able to find 22k pots that won't match my pin layout, is it possible to hardwire a different type in with say, 20g wire and not affect the performance of the pot?

Don't rule out using 20K or 25K pots, much more common. The Bourns series I pointed out to you at Mouser has several different pin layout selections. Though they're in the catalog, not every type is in stock, you may have to hunt 'em down at Newark, DigiKey or elsewhere. Here's a data sheet which may help you select the appropriate Bourns model to fit your circuit board. And in a pinch, you can bend up some wires and make just about any trimpot work.

http://www.mouser.com/ds/2/54/386-776606.pdf