Marshall DSL 401 hum

[axg20202]

I'm trying to repair this for a friend. It is a later model with cream coloured single board and bias adjuster on the board (no externally accessible adjuster). Amp seems to run OK-ish, although I don't think it is as loud as it should be, but there is a significant hum even with nothing connected at the input jack as soon as the amp warms up. I suspect the hum is in the output stage as going in via the fx return doesn't eliminate it. Nothing on the board is blown or looks stressed. I have already reflowed solder on all tube sockets. Nothing looks amiss and all tubes light up. Swapping to new tubes had no effect (and I used a Q.max punch to open up the chassis holes to allow it to take regular sized EL84 tubes :-) ). One small fault I have found is that the master volume knob is very crackly and noisy even after a thorough clean with contact cleaner (all pots now cleaned). Could a failure at the master volume pot cause a constant hum? What else are the usual suspects on this amp?

[The Dude]

Output tube substitution would be a quick/easy start. After that, check B+ for ripple. You may have a bad filter cap or one with cracked/cold solder joints.

[oc disorder]

Hi axg20202 and welcome to the forum.
As the design has the master volume after the phase invertor in this case the bias supply for the EL84 output tubes has to
pass through the master volume.Any damage to that pot could upset the bias getting to these two pairs.
Was thinking you may have had a difficult time cleaning those pots. If the master is still noisy I was going to suggest removing it
and slightly prying it apart to get some Deoxit or cleaner into it.
Then I thought for all that trouble which may not cure it, you might as well replace the pot particularly while it's still available for purchase!
You could check the DC volts at the grid (pin 2) of the opposite pairs to see if there is any major difference but if the pot is crackling for audio perhaps replacement is the best thing.

I noted you tried substituting other o/p tubes with no effect also The Dude's suggestion checking for ripple on the HT or B+ is an excellent place to start.

Attaching some documents .. will probably need bias adjustment after Master vol replacement.

Here's some other discussions re the DSL 401

http://music-electronics-forum.com/t9476/

marshall dsl 401 - AMPAGE Forum

Link to Tube Amp Dr Marshall® Dual-Pot, 2x A200K, PC Mount, D-Shaft - Marshall style and

(PDF & duplicate jpg of 401 bias adjust)

[The Dude]

Oops! Apologies. I missed the part about you already swapping tubes. And I forgot,......Welcome to the place!

[axg20202]

Thanks both, appreciate the help. I'll keep poking around and report back.

[axg20202]

Well, I was about to pull the trigger on a parts order and thought, no, let's have another go at it! I was not convinced that a faulty pot or two could cause the hum and it turns out I was right. I decided to check every solder pad with a magnifying glass. The heat from the rectifier had caused it to desolder itself from the board very slightly. Not even enough to notice with the naked eye. Reflowed them and it is sorted for now, but I will ask my friend if he wants to spend some cash and have me fix the issue properly with an upgraded rectifier and heat sink/fan, and fix the pots while I'm at it. What a terrible design. This is why I would never spend serious money on a tube amp built with PCBs - cheaply made expensive crap that doesn't last. Mr Marshall would be turning in his grave. Or maybe not as he made a boat load! By the way, I live 100 yards from where he had his first shop in West London. Times have changed.