Marshall DSL100 Humming

[rf7]

I've got this DSL100 that I'm working on, and it's humming pretty loudly. I've pulled the PI tube and it still hums, so it's not a preamp issue. I've tried new power tubes and it still hums.

I already found 3 things wrong with it that I fixed. One of them was that Resistor R76 actually got so hot that one side of it had fallen out of the pcb. The amp was bought new in January, could it be bad filter caps?

What else should I check?

[Enzo]

If it is new and less than a year old, it is under warranty, why not let a service center handle it?

Doubtful a filter cap has failed so quick, though anything is possible. I'd be more thinking of a broken trace, broken solder connection, missing solder connection, bias WAAAY out of balance, things like that.

Do the two biases adjust out the same? If one adjusts but has to be WAY over to one side, I'd be concerned.

[Amp Kat]

I agree that bias would be the first suspect and check those 1 ohm current sense resistors from Cathode to ground.

[rf7]

The bias adjusted fine for both sides of the amp. And the 1 ohm resistors seem to be fine, about 1.1 ohms.

The local warranty service takes about 6 weeks to turn an amp around, so I get to work on it instead.

[rf7]

Check this out, I opened another one up and R76 had fallen completely out of the PCB. It melted onto a tie-wrap and some wires:


You can see the space where it came from below, right next to the brown resistor:


And R74 looked like it was getting hot enough to tilt out of position. Oh, and C46 is a dead short, that's the culprit on this one!

[rf7]

Do the two biases adjust out the same? If one adjusts but has to be WAY over to one side, I'd be concerned.

What does it mean if the biases don't adjust out the same? It seems to work fine for 2 tubes, but when I go to 4 tubes, one side pulls half the bias current of the other. All the voltages look right.

[Enzo]

Only one thing will melt those resistor off the board - too much current through them. And that has to come through the tube screen. A tube dies and shorts out the bias for that side, and its neghbor tube also draws excess current because the bais was shorted out by the baad neighbor.

A missing screen resistor will leave a tube not conducting. So if one side biases up at half the other side, it means one tube is not conducting. CHeck voltage on pins 3 and 4 of the tubes. ALL FOUR tubes must have B+ on both plate and screen. Missing screen B+ means open screen resistor most of the time.

Replace C46 with a 1kV type. C46 is a common failure.

[rf7]

Yes, you are correct, C46 was shorted out. I noticed on tube V8 only, that pins 3 and 4 had the same voltage. Usually pin 4 is just a touch lower than pin 3. Now the PI is humming, so I'm looking into that. It's not the tube, I've put a few good ones in there, and it's still humming. More fun!

[rf7]

The plot thickens: I've found 2 sources of the hum. One is that there is a ground differential between the OT and the chassis. I measure 2mV there and if I jumper it to the chassis, the hum goes away. The other source of hum is caused by depressing the Deep switch, and it's about 10 times worse than the OT ground loop hum.

I tightened the OT mounting screws and I have replaced the 4558 IC that is attached to the Deep switch circuit, SW5b on the front panel pcb, but neither of those helped at all.

I was going to add a ground wire to the OT ground to fix that issue. And I've got more digging into the Deep circuit to do.

[Enzo]

The deep switch boosts the low end response of the amp. Hum is in the low end register, so pressing deep will increase the hum amplified.

I think they are in the far corner from the power transformer, but on the main circuit board. Or on this one are they near the input jack? There are two largish diodes, a 10 ohm resistor, and a cap, all in parallel. D1, D2, R1, C1. Are they open?

The chassis is not the circuit ground in this amp. Grounds aer on the board, and this little group of parts ultimately references the circuit ground to chassis. So there shuold be abut 10 ohms between chassis and circuit ground. When measuring B+ or something, it doesn't matter, but when going for small hum or signal levels, it does.

[rf7]

I think they are in the far corner from the power transformer, but on the main circuit board. Or on this one are they near the input jack? There are two largish diodes, a 10 ohm resistor, and a cap, all in parallel. D1, D2, R1, C1. Are they open?

No, they measure a short circuit. R1 was missing, so I installed a 10 ohm resistor.

The chassis is not the circuit ground in this amp. Grounds are on the board, and this little group of parts ultimately references the circuit ground to chassis. So there should be about 10 ohms between chassis and circuit ground. When measuring B+ or something, it doesn't matter, but when going for small hum or signal levels, it does.

That's true of the old pcb for this amp, but on this pcb (JCM2-60-02) there is a spade connector at W14 which ties the main board's ground to the chassis. So, in the current version of this amp, the chassis is circuit ground.

Nonetheless, I disconnected W14, and measured 10 ohms between chassis and circuit ground. The amp still hummed in that configuration,so I re-connected it.

[Enzo]

Oh well.

[bajaman]

I have a Marshall DSL 100 head in the workshop - it too is giving me grief - bad hum, runaway bias etc. The board is the 02 variant BUT it had the 220k control grid resistors fitted as shown on the 00 issue 1 schematic????
It had C46 fitted. By the way when I said it was the 02 variant, I was referringf to the silkscreen resistor numbers etc., - the board has the silkscreen 00 designation on it - what a weird amp Marshall made here!!
I have worked on a number of Marshall amps over the last 20 + years but have to say this one is a total piece of shit from a construction and design perspective. It uses a ridiculous biasing sub board with tiny pots connected by a seven way plug in (at both ends) ribbon cable with pressed wire connections similar to a telephone connector - an accident waiting to happen!!!
The circuit board is double sided - I did as suggested - resoldered all components, then used my ohm meter tocheck continuity - guess what - even AFTER resoldering, one of the contol grid resistors was not connected to pin 5 on one of the output tubes, so I resoldered all the valve socket pins on the top side of the board as well!!!! - that guaranteed conectivity. I have soldered and hard wired the bias pot sub board - goodbye flakey plug in connectors!! , but still have bad hum issues with this piece of crap.
Honestly, I think Marshall lost the plot after the JCM 800 series amplifiers - Soldano built what Marshall should have released for the 900 series - these DSL amps are a nightmare birds nest of plug in cables dry joints - what were they on when they conceived these amps???!!!
The owner bought this amp new, and has had continual problems with runaway bias and overheating etc. 6 months ago I had to get the power transformer rewound and the heat at that time destroyed the output tubes and warped the plastic ventilation grills on the top of the cabinet!!!
He has a 50 watt JCM 800 which has never given him ANY trouble - a fine amplifier, and recently I resurrected an old Fender Twin Reverb - new caps, plate loads, tubes etc., which he has fallen in love with. He just wants me to get this DSL crapper going so he can sell it. Failing this he suggested that I throw it off the end of the pier!!!!!
PS: check out those board mounted main filter capacitors - total el cheapo!

[rf7]

Oh well.

Enzo, thanks for your help, I really appreciate it.

I have a Marshall DSL 100 head in the workshop - it too is giving me grief - bad hum, runaway bias etc. He just wants me to get this DSL crapper going so he can sell it. Failing this he suggested that I throw it off the end of the pier!!!!!
PS: check out those board mounted main filter capacitors - total el cheapo!

Believe me, I know how you feel.

[Amp Kat]

Enzo, thanks for your help, I really appreciate it.



Believe me, I know how you feel.

Glad you got it going dude ! fwiw we see probably more failures with those amps than any other on here because of those reasons and the bad part is it was one of their best selling amps so theres tons of them out there. Your right though they are an accident waiting to happen.