Newbie troubleshooting JCM 2000

[Dogs of Doom]

but, if that's as low as it can get, I'd bet that there's something else going on that needs addressed...

 

[dhahnke]

is that mV?

81mV / side should be ok...

Yes, my bad, 80.71mV DC, they are only off by about .03mV DC from each other.

 

[dhahnke]

but, if that's as low as it can get, I'd bet that there's something else going on that needs addressed...

The left side is the one that can't go lower, the right side can definitely go a smidge lower. When I checked the bias pots w/ a multi-meter their range was the same, so I'm doubtful it's the bias pots but the order I made just came in, so I can replace them if we think they may be problematic.

 

[Ximrats]

It might be wise to just take the bias circuitry off the main board and on to it's own dedicated little PCB now before you end up having more issues with the potential for the factory issue certain/most JCM2000s can suffer with which will, if left to carry on getting worse, will lead to blowing tubes, blowing fuses, and more importantly, blowing transformers. Repaired many a JCM2000 with this same issue and rebuilt my own I bought cheap because it had a blown transformer, the board was an utter mess from having a couple techs try and fix the issue without seemingly knowing what the problem was. It was an utter mess, so I rebuilt it and removed all of the bias circuitry and put it on to it's own board. Dremel'd out the pin on the power tubes to make sure the pin on the power tubes was well away from the PCB and installed my own little board and everything works perfectly (then, f course, I went on a modding spree and now it's heavily modified and sounds incredible, with a few different 'styles' of preamp to choose from, too.

Then I learned that they actually make a kit you can buy, it's pretty cheap, very simple to install if you follow the instructions which are detailed and easy to follow.

Here you go, it's Dr Tube's Stable Bias Mod. There's some information on the first link here about the issue with bias and JCM2000s if you're interested in the why's and the how's and what not and the second link is a link to order it. It's only € 20,00 for the full kit of parts and well worth buying and installing or having installed, even if for peace of mind. I would say there's a fairly good chance it'd help you with the issue you're having now, but of course carry on testing and observing and what not to try and properly diagnose the issue just in case there's something else going on.
It may not be the beginning of the bias drift issue, but for only € 20,00 considering a transformer is gonna cost a whole hell of a lot more, and replacing tubes, and constantly replacing fuses, and for peace of mind that it's going to be reliable and stay reliable...I'd say it's certainly worth doing, my man.

drtube.com/modification/jcm2000-stable-bias-mod/
drtube.com/product/dr-tube-jcm2000-stable-bias-kit-full-kit-of-parts/


I don't work for them or anything advertising, it's just that I've fixed bias issues on these amps so many times I've lost count and almost all of the time it ends up being related to this factory issue. I usually just make my own boards because I have parts and can whip up a PCB to add in faster than they could deliver it but for anyone else, even if you're not having any issues with bias or anything yet, I'd do it anyway just for that peace of mind.

I hope this helps.

 

[1dallek1]

Hey all, I apologize for yet another troubleshooting thread, but I honestly wasn't finding the information I was looking for.

Basically, I just got a JCM 2000 DSL100 (1999 build), blew an output tube (tube 2 when looking from the left) and ordered a new set to replace it.
- I started by making sure the speaker was still plugged in, no input is plugged in, and pulled all the output tubes.
- I verified pin 5 was stable (grounded to chassis, positive probe in pin 5) and saw steady numbers in each tube's pin 5 on standby (.28-.030) and steady voltages when warmed up (on for about an hour) both on standby and off standby (.016-.017).
- I put the new tubes in and tried reading the bias pins and the left side is sitting at .9mv and the right side is sitting at .6mv.
- When adjusting the bias screws nothing changes no matter how far i turn them
- I tried this with standby on and standby off, with standby off (amp live) and I heard a LOT of noise through the speaker with no input (volume at .5/10)

What am I doing wrong here, or is the amp hosed?


Edit: I tested pin 5 wrong and used VAC not VDC, I updated the results in a later post but will put them here as well.

Voltage DC - Output tubes from left to right (when viewing the back of the amp)
1 2 3 4
-41.79 -40.70 -43.93 -44.55 Cold, Standby Off
-42.72 -42.73 -46.14 -46.13 Warm, Standby Off
-42.38 -42.39 -45.78 -45.77 Warm, Standby On

Hi what is the iss number on the board? is it less than 20? You might just want to buy a new board, they are reasonable. I had really old iss# board with the bias drift and the new board took care of all the issues of the earlier boards. So start with this simple thing and check the iss#. You need to pull the chassis out. where do you live, I am in Yonkers ny, I have a shop.
this is a good link

 

[Gunner64]

Agree with the above!

There's a ton of info on the forum about the dsl bias issue, the search function here is your friend. The Dr. Tube stable bias kit, yes, I mentioned that earlier in this thread, in that I used it with great results. I would install it, and new bias pots! I'm telling you these cheap f'in pots will give you fits.

Do what you want, but like many here I have been through all this with these amps more than once and I installed the Dr. Tube kit and the bias pots. Rock solid now. The tubes in my #1 have a range for the low -20's to the mid -90's per pair. -72 is ideal imo.

If you get the kit remember to get the hollow core drill bit.

 

[Dogs of Doom]

The left side is the one that can't go lower, the right side can definitely go a smidge lower. When I checked the bias pots w/ a multi-meter their range was the same, so I'm doubtful it's the bias pots but the order I made just came in, so I can replace them if we think they may be problematic.

what are the new tubes you installed?

JJ E34L ?

 

[1dallek1]

It might be wise to just take the bias circuitry off the main board and on to it's own dedicated little PCB now before you end up having more issues with the potential for the factory issue certain/most JCM2000s can suffer with which will, if left to carry on getting worse, will lead to blowing tubes, blowing fuses, and more importantly, blowing transformers. Repaired many a JCM2000 with this same issue and rebuilt my own I bought cheap because it had a blown transformer, the board was an utter mess from having a couple techs try and fix the issue without seemingly knowing what the problem was. It was an utter mess, so I rebuilt it and removed all of the bias circuitry and put it on to it's own board. Dremel'd out the pin on the power tubes to make sure the pin on the power tubes was well away from the PCB and installed my own little board and everything works perfectly (then, f course, I went on a modding spree and now it's heavily modified and sounds incredible, with a few different 'styles' of preamp to choose from, too.

Then I learned that they actually make a kit you can buy, it's pretty cheap, very simple to install if you follow the instructions which are detailed and easy to follow.

Here you go, it's Dr Tube's Stable Bias Mod. There's some information on the first link here about the issue with bias and JCM2000s if you're interested in the why's and the how's and what not and the second link is a link to order it. It's only € 20,00 for the full kit of parts and well worth buying and installing or having installed, even if for peace of mind. I would say there's a fairly good chance it'd help you with the issue you're having now, but of course carry on testing and observing and what not to try and properly diagnose the issue just in case there's something else going on.
It may not be the beginning of the bias drift issue, but for only € 20,00 considering a transformer is gonna cost a whole hell of a lot more, and replacing tubes, and constantly replacing fuses, and for peace of mind that it's going to be reliable and stay reliable...I'd say it's certainly worth doing, my man.

drtube.com/modification/jcm2000-stable-bias-mod/
drtube.com/product/dr-tube-jcm2000-stable-bias-kit-full-kit-of-parts/


I don't work for them or anything advertising, it's just that I've fixed bias issues on these amps so many times I've lost count and almost all of the time it ends up being related to this factory issue. I usually just make my own boards because I have parts and can whip up a PCB to add in faster than they could deliver it but for anyone else, even if you're not having any issues with bias or anything yet, I'd do it anyway just for that peace of mind.

I hope this helps.

do not do this, if you have a board iss# of less than 20 buy the marshall reissue board for this amp. There are mor issues than just the bias drift on the early boards. it makes sense money wise too.

 

[Gunner64]

The new boards have reports of issues too. They did nothing on the new boards to remedy the b+ and bias traces being less than 2mm apart, Marshall pretty much ignored that , probably because they would have had to re design the board to accommodate such a change.

I believe that had these traces been adequately spaced from the beginning we would not be talking about this issue now.

 

[1dallek1]

The new boards have reports of issues too. They did nothing on the new boards to remedy the b+ and bias traces being less than 2mm apart, Marshall pretty much ignored that , probably because they would have had to re design the board to accommodate such a change.

I believe that had these traces been adequately spaced from the beginning we would not be talking about this issue now.

I can't debate this? This is the first and last time marshal has ever issued a replacement like this, they were free way back. I have a network of other businesses and techs with standing that recommended the replacement. Finally the new board corrected the bias issues, it was running for days. This guy does not need confusion; if he has an early revision or ISS# he should replace the board. In fact he should take it to a reputable tech before he gets hurt.

 

[1dallek1]

I can't debate this? This is the first and last time marshal has ever issued a replacement like this, they were free way back. I have a network of other businesses and techs with standing that recommended the replacement. Finally the new board corrected the bias issues, it was running for days. This guy does not need confusion; if he has an early revision or ISS# he should replace the board. In fact he should take it to a reputable tech before he gets hurt.

I should have just said, there are allot of later rev JCM’s out there with proper revisions, the reissue is the latest I believe or right up there, they are doing fine? I should know I fix what is brought to me, never any returns. I have not heard any problems like that and they work, otherwise they would be back here and I would not have been told to just go with the new board for the drifting bias issues. Again those boards are a mess even if you fix the drift issue so it makes no sense to use a third party device.

 

[Leonard Neemoil]

From what I've read, the final fix is to replace the board AND install the Dr tube bias kit. If it were mine, that's exactly what I'd do. The bias kit is cheap and easy to install if you're already putting a new main board in. I've heard too much about problems after replacing only the main board.

 

[Gunner64]

That's your experience. Cant argue a mans experience. Thats Great.

I do know there are members here that have reported after replacing the board with an iss. 20 board the amp worked for a while, then started drifting again. I'm not making this shit up. Like the early pcb laden amps not all do it apparently, so just because you have never heard of the issue reappearing with the iss. 20 pcb, doesn't mean others haven't, and I think anyone replacing the pcb on their dsl shouldn't be aware of possible complications.

And I am not trying to confuse anyone.

And as far as finding a good tech, I told the op early in this thread that he should find a good tech. My first post , Post#5 if you would care to read it, as it appears you didn't read that the first time.

He was adamant on doing things himself.

 

[1dallek1]

That's your experience. Cant argue a mans experience. Thats Great.

I do know there are members here that have reported after replacing the board with an iss. 20 board the amp worked for a while, then started drifting again. I'm not making this shit up. Like the early pcb laden amps not all do it apparently, so just because you have never heard of the issue reappearing with the iss. 20 pcb, doesn't mean others haven't, and I think anyone replacing the pcb on their dsl shouldn't be aware of possible complications.

And I am not trying to confuse anyone.

And as far as finding a good tech, I told the op early in this thread that he should find a good tech. My first post , Post#5 if you would care to read it, as it appears you didn't read that the first time.

He was adamant on doing things himself.

What does drifting bias look like?

 

[dhahnke]

It might be wise to just take the bias circuitry off the main board and on to it's own dedicated little PCB now before you end up having more issues with the potential for the factory issue certain/most JCM2000s can suffer with which will, if left to carry on getting worse, will lead to blowing tubes, blowing fuses, and more importantly, blowing transformers. Repaired many a JCM2000 with this same issue and rebuilt my own I bought cheap because it had a blown transformer, the board was an utter mess from having a couple techs try and fix the issue without seemingly knowing what the problem was. It was an utter mess, so I rebuilt it and removed all of the bias circuitry and put it on to it's own board. Dremel'd out the pin on the power tubes to make sure the pin on the power tubes was well away from the PCB and installed my own little board and everything works perfectly (then, f course, I went on a modding spree and now it's heavily modified and sounds incredible, with a few different 'styles' of preamp to choose from, too.

Then I learned that they actually make a kit you can buy, it's pretty cheap, very simple to install if you follow the instructions which are detailed and easy to follow.

Here you go, it's Dr Tube's Stable Bias Mod. There's some information on the first link here about the issue with bias and JCM2000s if you're interested in the why's and the how's and what not and the second link is a link to order it. It's only € 20,00 for the full kit of parts and well worth buying and installing or having installed, even if for peace of mind. I would say there's a fairly good chance it'd help you with the issue you're having now, but of course carry on testing and observing and what not to try and properly diagnose the issue just in case there's something else going on.
It may not be the beginning of the bias drift issue, but for only € 20,00 considering a transformer is gonna cost a whole hell of a lot more, and replacing tubes, and constantly replacing fuses, and for peace of mind that it's going to be reliable and stay reliable...I'd say it's certainly worth doing, my man.

drtube.com/modification/jcm2000-stable-bias-mod/
drtube.com/product/dr-tube-jcm2000-stable-bias-kit-full-kit-of-parts/


I don't work for them or anything advertising, it's just that I've fixed bias issues on these amps so many times I've lost count and almost all of the time it ends up being related to this factory issue. I usually just make my own boards because I have parts and can whip up a PCB to add in faster than they could deliver it but for anyone else, even if you're not having any issues with bias or anything yet, I'd do it anyway just for that peace of mind.

I hope this helps.

I was looking into doing this anyways to avoid future issues, I'm comfortable getting in and making the modifications. However, would it be more worthwhile getting a new board like others are stating?

 

[dhahnke]

So start with this simple thing and check the iss#.

do not do this, if you have a board iss# of less than 20 buy the marshall reissue board for this amp. There are mor issues than just the bias drift on the early boards. it makes sense money wise too.

Going by the video, it's issue number 5. Another user stated the lasted blocked out number is the issue number, in which case it's 4 (see the last pic on the second page).

What does drifting bias look like?

I haven't watched the bias actually drift, however, the initial issue was a tube popped and when checking the bias with new tubes (yes, not the best idea) I was reading around 300mv DC on the left side and couldn't go any lower. Now that I opened the amp up made sure connections were secure, checked resistors/etc, and cleaned the piss off the boards; the bias's move with each other as the amp warms up but the left side won't go below 80mV DC. I didn't check it the last time I play, but the next time I'll take readings over the course of an hour to see what's going on with each bias and if any movement is happening.

 

[dhahnke]

He was adamant on doing things himself.

That's true, I'm comfortable working with these things and try to be safe about it.

 

[dhahnke]

The new boards have reports of issues too. They did nothing on the new boards to remedy the b+ and bias traces being less than 2mm apart, Marshall pretty much ignored that , probably because they would have had to re design the board to accommodate such a change.

I believe that had these traces been adequately spaced from the beginning we would not be talking about this issue now.

Not trying to challenge you, but genuinely curious, what issues do the new boards have?
Also, I could only find a new board for the TSL, not the DSL. I'm not sure if you can use a TSL board w/ a DSL, but that'd be cool.

 

[1dallek1]

I was looking into doing this anyways to avoid future issues, I'm comfortable getting in and making the modifications. However, would it be more worthwhile getting a new board like others are stating?

You still have not said the revision of your board, the iss number; you may not have a board affected by bias drift? You are discussing a course of action before you know what is wrong. Buying a set of tubes does not guarantee anything, depending. I use an Amplitrex AT-1000 even that I am certain of the tubes I receive, I still have to see it myself. I also have a standing with my supplier over the years. And if you have an early board the bias issues are just part of the problems with those boards. That is why Marshall issued a whole new board instead of a fix. Regardless of the things I have heard here? I am suggesting you follow the proper steps. Anyway good luck

 

[dhahnke]

what are the new tubes you installed?

JJ E34L ?

I got two sets of Svetlana EL34's and still have one good set of Groove Tubes EL34 #4's