Marshall JCM2000 Dsl board problems

[blindboybenton]

Good morning

I do repairs for a local guitar shop and they sometimes change the tubes and bias it themselves, so they don't have to pay me

They call me in to have a look at this DSL. after they fitted the tubes and biased it saying that it has got a loud noise when been on for 2 mins + only on clean. not there on od channels
without jack lead with guitar on quite. as soon as you connect up lead with guitar its noisy .
I aint talking a little bit .. most defently not right . normal noise levels on od channels

I got it back to workshop and found the marshall badged tubes ( Tung sol ) when running too hot 50ma + on the lowest setting
i changed them for TAD mashall tubes and its ok. . Bias is stable
This fault gets worse as the amp is on and getting hotter

checked and changed all pre amp tubes all voltages correct
not guitar lead or guitar


My question is knowing there is an issue with these board with the bias creeping up. All down to the type of material the board is made from i believe

Any other issues with other parts of the board bleed over voltages etc


This i why i hate doing jobs like this, as i would of tested it fully before lifting a screw driver

[Alex R]

New power board from Marshall is the only reliable answer. The old boards are a paler green and you can't see any weave under the surface, the newer ones don't fail and are a deeper slightly translucent green with visible weave under the green.

Take out the power valves, clip a vm to pin five , blow a hairdryer at the board in the power tube area and watch the bias voltage fall! Doesn't happen if the board is ok. This can be used to convince customers as the effect is very noticeable - heat based leakage from the B+ to the bias.

Some say grinding away chunks of the board around the pins concerned is a cure, that way madness (and no profit) lies as a porous board could go porous anywhere the B+ is too near the bias, there's more than one place. If in the trade in the UK it is well worth pursuing a trade relationship with Marshall if you don't already have one.

[blindboybenton]

this one does not have a bias failing or rising fault. that's steady.
But i am thinking that way. Marshall have confirmed its an old board version. the owner is in a rush and needs it for tomorrow
when i get it back im going to lift the board and check out the dc heaters as i know they play up, but that will effect the od channel and it isnt
effecting that

[AmpFix]

People always suggest replacing the board, but the board itself is rarely the problem. There is so much mis-information about these amps on the 'net, its unreal.

There are several parts that don't age well on these amps. There are a couple of things that can be done to clean things up:
1) The electrolytic caps that are part of the DC regulation and close to the heat generating power resistors should be changed. As they age, they leak. Also change the heater supply caps (at a minimum).
2) Change the regular resistors around the bias and heater supply to metal film
3) Check all ground connections and reflow suspect solder joints, especially on the rear board where all the jacks are.

Before you do all of that (which I think you should while its open), try to see whether the noise happens when you plug into the effects return jack. It does? That means the issue is in the power amp. If it doens't happen, the issue is in the preamp. I've seen odd noises happen for all kinds of reasons. The last time, it was -15vdc that failed (bad diode).

Good luck!

[Alex R]

Just to be clear, if the bias voltage falls when you heat the board, the board is the problem and needs replacing. If not, maybe not, but the old boards do get electrically porous and though I've never seen any there may be other problems associated with these substandard boards. The new board cures the heat-sensitive bias problem, I'm Marshall approved and I must have fitted twenty or thirty of them for that issue and none ever failed again.

Sorry if I misunderstood. So the problem is noise? What kind of noise?

[AmpFix]

Just to be clear, if the bias voltage falls when you heat the board, the board is the problem and needs replacing. If not, maybe not, but the old boards do get electrically porous and though I've never seen any there may be other problems associated with these substandard boards. The new board cures the heat-sensitive bias problem, I'm Marshall approved and I must have fitted twenty or thirty of them for that issue and none ever failed again.

No, not really. Heating any component will change its characteristics and the original resistors have been upgraded to metal film to get a better temperature coefficient. Examine the specs for any component: A quality part will have a temperature derating curve and temp coefficient value. I've seen perfectly good boards get replaced because the general attitude on forums is to just change the board. Sometimes, a bit of contamination on the surface of the PCB can lead to issues.

Look at the layout on these board - the electrolytics caps sit right next to heat generating devices. They will age and degrade and need to be replaced.

Regardless, this is not the problem that the OP is complaining about so why solve a problem that doesn't exist?

There are tons of original 1997, 1998 era boards that are gigging amps that have no issues at all, mine included. The internet has blown whatever issue there was on some amps out of proportion.

[blindboybenton]

I don't have the amp at present as customer wants it for saturday. when i get it back i will strip down and check the board out.. stay tuned folks

[Leo_Gnardo]

An article worthy of your attention:

The Marshall TSL122 TSL100 thermal bias drift repair page

I've had about a dozen of these drifty Marshalls to repair, this method has enabled me to fix all but one. And that one was the first, before I understood what the problem & solution were.

Of course it's bound to be a success replacing the entire board, a sure thing of course. If - IF - the amp's owner wants to spend all that loot.

FWIW I have heard noise in the amp output in these drifty Marshalls, as the output tubes' control grids are receiving current randomly leaked thru the board, what you would have to expect.

[drewl]

Same here.

I had one that was failing all the way back to the preamp, causing nasty hum.

I had isolated the output tube pins and traces and bias was stable but the board was breaking down in multiple places.