Marshall DSL100 nasty noise and fluctuating plate v problem

[mresistor]

Hi guys..   I've got a DSL100 here that emits a nasty noise after it is taken out of standby..   I have been trying to find the problem without much success, so I thought maybe some of you have more experience with these amps.   When taken out of standby after about 20 seconds the amp emits cracking distortion type sound out of the speaker.  It also happens at any time during the time the amp is on..   it comes and goes..  it sounds like an arc to me,  I have monitored the bias voltage and the plate voltage and the plate voltage drops around 20 volts when the sound occurs and it also drops the bias voltage correspondingly.  I have had a supposed  Marshall tech tell me that the main board is bad. I did a continuity check on pins 4 and 5 of the power tube sockets and after removing the PI coupling caps and C33 lytic cap the meter slowly climbs its way up to infinity. He seems to think the problem is in the area on the board where the bias ckt is. I suppose there could be some insulation breakdown on the board with higher voltage than a meter puts out..  I am reluctant to just order a 60 -02 main board without knowing for sure if it will fix this problem. They are expensive.
I went over the board with a magnifying glass and reflowed alot of solder joints..   the power tube solder joints had been reflowed sometime in the past as there is a lot of flux residue left over there .  I inspected them too and they look good. I have also checked all the jumper wire connections and spade terminal connections for tightness


Here's the schematic links 
https://drtube.com/library/schematics/69-marshall-schemas#JCM2000-DSL


here is 60-02 issue 7 which is in this amp  https://drtube.com/schematics/marshall/JCM2-60-02%20(2001)%20iss7.pdf

[mresistor]

I played the amp for about 20 minutes and it has been on (out of stdby) for an hour now

plate V 477    and steady
grid v -37.5 and steady

impressions..  this amp doesn't seem to have much bass responce and it doesn't impress me as being loud..  full out on 10 not what I would expect from a 100 watt amp.


No nasty sound now for an hour.. so it's only happening cold at startup..   I don't have any freeze spray .. 

[shooter]

I don't have any freeze spray .. 

canned air, keyboard spray, works in a pinch, shake it, turn it upside down, walla shock those parts 

[mikeymike]

I've heard this series of amplifier had flawed boards from the factory. You should check youtube for the issue. The solder joints aren't the issue per-se, it's more the boards themselves are flawed. Possibly conductive? Not sure exactly what the issue was but it's moderately documented on youtube at least. I might be talking about the earlier rendition of this series called the JCM2000 not the DSL100's they're known as now. Not sure if the board issue follows both series or just the prior one.

I seem to also recall that one of the problems people had were an unstable bias.

[mresistor]

Thanks Mikey..  it is an early 2000s JCM2000 DSL100..   and I've watched Guitarologists vids and others on you tube..   it could very well be the board is conductive,, but the bias seems stable,  which has been a major issue on bad boards..  there are a plethora of capacitors on the board.. as well as semicondutors and an optocoupler.   I would hate to buy a new $250 board only to find the problem still exists.. 

[mresistor]

This is what the guy on the Marshall forum has been telling me to get  -  or a new board..   but I have not noticed the bias to be unstable in this amp.. 


https://drtube.com/en/modifications/jcm2000-stable-bias-mod

[Ritchie200]

Just for giggles plug a jumper into your send and return jacks.  If it suddenly works and is louder they may need a cleaning and retention.  I always say do the easy stuff first.

Jim

[mresistor]

Thanks Ritchie..  have done that ..     no improvement..   also sprayed them with deoxit early on.

[glass54]

Hi mresistor
There was a problem with the bias pcb and I do have an article on it for TSL/DSL and its not Dr Tube (Its over 4Mbytes !). Basically you rebuild the bias onto the main PCB. See attached photo.
If you like I can do a PM for you as I cant find it on the net.
Kind regards
Mirek

[mresistor]

Thanks Mirek..  I saw that info that is in this link  http://www.lynx.net/~jc/TSL122.html?fbclid=IwAR02fkd6gtbkw7OoaPBIrurHQ7s-YqXjJdxMoSJk1HcBarDfxvPTnMAnHUk


I also found info in that link that suggested replacing the 220K grid stoppers with 5K6 and upgrading the two 220K 10K and 33K bias resistors to 1 watt metal film, as the originals are all carbon film and were prone to decreasing in value when heated up. I think 1/2 watt metal film would suffice.. and the holes in the board are not very big and it would be bad to damage the plating. I replaced the grid stoppers and I think the amp sounds better.


The bias v on this amp is -40v. With no power tubes installed and power turned on the voltage drops to -39. With the tubes in and powered on the bias drops to -35-36v.   This also seems to be a problem.
The new issue of the board is issue 20 I think and it has been recommended to me to replace the old one.  I assume there are other upgrades on the board besides better substrate. The FR3 on the original boards is pretty thin.

[mresistor]

Ok update today..    I had ordered an effects return jack from British Audio in Nashville  because it was acting wonky  and thought it might be the source of the crackling, but was unsure.. because it would stop after a a short time.    The new one came in today and I replaced it and no more crackling..  what a relief. 


The bias seems to be holding steady  so I'm going to call this one fixed.  Thanks to all that participated..   much appreciated.