Marshall DSL

[Valvehead]

Anyone with the DSL 100 scem--can you go over it and tell me where the bias supply is/ comes from on there? Im finding it confusing, especially with all those damn CON connection points . The scem shows an un-used tap on the PT ?? but its used on the amp(3 wires coming from the winding under the tape)

Ive got one here thats got real low volume and the Power trans is humming pretty loud. Also one side on the bias test point is reading VERY high like 140-200 mv. Other side reads normal .

I did find a bad screen grid 1K/5w . Replaced that but thats not the only problem i guess.
All the tubes are ok.

[Amp Kat]

I think the original supply is on the main power supply bd and routed to the main tube board thru some voltage dropping resistors. Check the 1 ohm current sense resitors on the bad side and make sure it's not open. Also check the coupling cap on the grid coming off the PI as I've seen those bad too. Also check the trace from pin 3 to the other tube pin 3 and make sure that trace didn't open up.

[Valvehead]

all that stuff checks ok, thanks for the suggestions.


i did find something---on the little bias pot board. one of the legs of the bias pot ......... had NO SOLDER at all ........

flowed all those and now my bias looks very normal.



I did try this--preamp out of the marshall into my amp-----full volume-sounds great.


my amp with pre out to the marshall FX return, Low volume and a crazy distorted / crashing/ in/out sound.

problem is somewhere in this output section..........

double checked the tubes---tubes are not the problem

[Amp Kat]

Well if your getting about 80mv of bias on both sides and your plate voltage is ok around 500 V and the ground on pins 1 & 8 or at zero and the screen at pin 4 is around 495 then it should work. If the OT is bad could be a problem or a bad preamp tube or power tube and you checked those.. You may need a scope to find out where the signal is dropping out.

[Valvehead]

well--in this wacky amp-there is no pin 1 on the bottom of the board. ill have to take it apart for the 3rd time to see what/where its going

i do get the following :

pin 8 to ground is 49 ohms -all 4 tubes



pin 2 -39mv

pin 3 460v

pin 4 460v

pin 5 18mv

pin 6 66mv

pin 7 88mv

pin 8 -18mv

also--i dont have access to a scope.

[Valvehead]

ok , i tested the power tube sockets to ground--pin1 and 8 are fine with 1.2 ohms.


i went over the boards today with a magnifier. cant find a bad joint .

the amp seems to have its full 100w at LOW/ CLEAN signal ---as soon as i turn up the gain or volume and start pushing the signal, the amp cuts out to about 2 watts.

Now, remember , its definately AFTER the FX return in the output section.

While running the amp in its "cutting out" mode, guitar on , i have banged the amp around lightly, poked around everywhere inside with a sharpie , flexed the circuit boards , etc..
it wont come back up to 100w , so......i really dont think its a connection or bad joint.

i also checked the OT and it passes the "basic" tests .

starting to think its a component after the PI tube...

[Valvehead]

more testing-----------------


at idle with no input signal i get the following:

power tubes
pin 3 461v
pin 4 473

when i crank up the volume with no input signal, those stay the same .

when i plug in a guitar and get the amp to cut out and stay cut out , i get these:

pin 3 324v
pin 4 375v


is that normal ? or does that give a clue ?


i also tested before and after the .022 PI coupling caps going to the output tubes while doing this, no big change on voltage there.

[Alex R]

Check what's on the CT in cut-out condition. If it's normal B+ then I think it is likely to be the OT. These do often fail and I have seen at least one that just produced lousy sound and no output. It didn't move between conditions like this one though. Marshall replacement OTs are quite reasonable, if you're on their list.

[Alex R]

...and check the speaker jacks.

[Alex R]

Does the OT get hot in 'fail' condition?

[Valvehead]

when it cuts out the CT of the output trans measures 374vdc WHEN im playing thru it. at idle its 469v


also in cutout the b+ after the rectifiers is (playing thru it)411vdc , but at idle its 469vdc

neither trans gets hot

i will double check the jacks--but ive already re-flowed and cleaned them

[Valvehead]

where can i find a OT for this amp? ive searched the web an nobody sells the jcm2000 100w.


would jcm900 be the same one?

[Alex R]

Marshall keep stocks and will sell them to approved repairers and dealers. If you've found a JCM900 type you've found a reseller. Ask them whether it fits a JCM2000. It probably does, but check. I guess you don't have a known-good OT to clip in there? Even a 50w - you could pull two tubes.

It could be a problem in the OT, or even the PT, but we haven't isolated it yet. It might be worth putting an ammeter in the mains supply to see whether the current drain goes up when the voltage goes down. That would tend to confirm a soft short somewhere like the OT, or even a filter cap (any of them look bulgey/leaky? If the current is quite steady when the voltage drops I might tend to suspect a problem in the PT (though that doesn't eliminate shorted turns in the OT either, sigh... but it does at least eliminate a short to ground).

That humming - is it the PT only or through the speakers too?

What happens to the bias voltage on pin 5 when it's loud/quiet?

[Alex R]

The resellers tend to stock the TSL100 OT - ask first but that's very likely to be the same as the DSL.

[Enzo]

Marshall transformers tend to be 50 watt, 100 watt, and like that. As far as I know there are not special ones for the DSL100 and TSL 100. Part number is TXOP00001 for both.


By the way, try not calling the amp a JCM2000, that doesn't specify an amp model. The little DSL201 is a JCM2000 like the others.