DSL4oc problems

[RWood]

A local music store that sends work across my bench has sent me 3 DSL40s this month. All have had the same symptoms. They all 3 claim to play great for 15 minutes, then start screaming. My first thought was bad solder joint, but couldn't find any. All 3 had 1 power tube shorting, with signs of obvious red plating. After rolling a known good tube, the bias on the shorting tubes position were 70-80ma with 42 or so on the good tubes on all 3 amps.
After fresh set of matched power tubes and balanced 42 ma biasing, they were left idling for 2 hours with 5 minutes of playing every 15 minutes or so with no problems.
The unbalanced bias has me puzzled. All resistors and bias pots test fairly well. Surely Marshall wouldn't send them out so badly biased like that?
Each amp had been sold for about 3 months so under warranty, but they wanted to know if it were a simple quick fix to keep customers happy.

[frankdrebin]

I know for sure DSL50 and 100 have a bias problem due to heating and electronic leaking between boards,after 30 minutes in general.
That was in first generation.
Best thing to solve is rebuilding the bias circuit separately.

[Stevem]

Pull out all the outputs and check for stray resistance from the grid and the other tube pins as the board may be conducting / bleeding voltage.

[Littlewyan]

The conductive board issue I believe affected all DSL and TSL amps. It was only first generation but first gen lasted quite a few years!

[RWood]

The 1 that played well after retube/rebias finally trashed again. Starts out 32ma bias and finally started rising slowly until I turned it off at 75ma.
after cooling down it is back to low 30s.
All 3 have same issue and are less than 4 months old, so they'll have to go back to Marshall tech...but I want to satisfy my own curiosity first.
I was thinking oscillation but conductive board sounds likely also.
I will check board and scope it tomorrow.

It is a shame that a great brand like Marshall has put out so much trash lately. These 3 are the only ones that shop sold and all 3 are back for repair.
I also have 5 AVT150 heads with the bad TDA7293s that will soon become JTM45s and 1987s.
Sounds like Marshall is going the same direction as Weber.

[Littlewyan]

Were they old stock? As thats definitely the issue you got with the 1st Amps. You can buy new boards from Hotrox for them. Marshall denied they had an issue for years.

[lovetone]

The DSL40C is current production, there is no schematic available on the Marshall Image bank because these are a new model.

I had an issue with one of these, not the output stage but in the preamp it turned out to be a short circuit capacitor across the rail to ground. This was a .001 through hole device but vey small in size.

When you have the fault monitor what the -ve dc is doing also the screen you should see these move.

The problem now is that a lot of these amplifiers are made in Korea in the same factory that makes Blackstar. The quality of some components is not great.

If I get a chance I'll call the service department and ask if they have had any problems.

Geoff

[lovetone]

I called Marshall who have no known faults so far with this amplifier, they said to check that the -ve bias volts is getting to the tube base.

It could be it passes through a plated through hole which has failed.

The schematic is not on the system.

Geoff

[RockinRocket]

Pm me your email if you need the DSL 40c schematic