Prowler

Here's my problem: my Marshall JCM 2000 DSL 100w starts to sound harsh when I plug anything in to the effects loop. I've tried plugging a short good quality patch cable from fx send to fx return. When I take the patch cable off from fx return while i'm playing there's a big difference in sound. There's like too much treble and some more hum when the patch cable is plugged in.

Anybody have suggestions how to fix this problem? I really want to run my pedals into the loop again and now the loop is quite useless.

jcmjmp

Bring it to a tech. There might be something wrong with the loop it is sounds like crap with just a cable plugged in.

een7gdr

Prowler,

Are you playing the amp lowish volumes? If so then i suffered the same problem. Try putting the amp on the red channel, a short patch cable from fx send to return and then turn the volume to 0. You should hear a very tinny sound, this does not go away and sits on top of the normal signal as you turn it up.

I have developed a mod for this problem, and if it interests you please see my web page (gsharp.co.uk - Guitars/Amps) or the following thread DSL Effects loop help. Feedback needed from DSL/TSL users!

jcmjmp

een7gdr - You'll have to mention which version of the DSL circuit you're referring to for your mod AND the original poster should know which version he has before being able to determine whether a mod is applicable or not.

The Marshall DSL has had 7 different versions of its schematic and they don't have the same effect loop circuit.

When I look at the Issue 1 vs the Issue 7 schematic, I see a LOT of differences. There has been some cost cutting that happened over the years (typical of most companies) and some improvements (Grid stoppers).

A lot of the component numbers are different, some components are missing or added in 1 vs 7 issue designs.

een7gdr

Thanks jcmjmp,

I would also like to know how experienced the user is with regards to attempting to open up and modifying tube amps. If they are not qualified or have experience then any work is best undertaken by a good amp tech.

Prowler, how old is the amp? There will be a serial number on the rear indicating year of manufacture, also if you have been in the amp, PCB revision numbers will also be useful as jcmjmp mentions, they changed some component references through the years, although as far as i have been able to tell, the problem area the mod works with has not been affected layout-wise. If its a very recent amp i.e. in the last year or so any photos of the PCB with the tone controls on would be useful as they may again have changed things!

For your information, the mod was designed and tested on DSL 50 & 100 amps built from 1997-2000 and affects only the FX loop buffer. The original layout of the circuit picks up the thin, tinny signals from the tone circuit due to the placement of the components and the shared dual IC. This mod replicated the circuit but allows it to be re-positioned in the amp to reduce this pick up.

There have been at least 4 others on this forum who have used it so I shall confirm what year their amps were built in.

Prowler

Yes, I play at lowish volumes on the red channel and there is that tinny sound.

Prowler

The serial number says M-2003-23-1061-A

een7gdr

Yeah, it sounds like the same issue I had. My mod will correct this and as far as I can see, there are no differences in the FX loop buffers between different DSL issues, yours was built in 2003. If you are confident/qualified with working in tube amplifiers and know the risks then I can supply a mod for you to install, if not then an amp tech would have to do the mod for you. The install is pretty simple as you can see in the thread link i posted above but tube amps can be dangerous and no-one here would recommend opening one up if you dont know what you are doing. There are lethal voltages inside the amp even when disconnected from the mains.

The other option is to use the amp at louder volumes where you wont hear the problem but this may require a power attenuator if you have a volume limit for practising.

Good luck!

MaidenStrat02

My DSL50 was made in 2006 and I installed one of your kits and it worked for mine. Any idea what revision mine might be? I've been in it alot but wouldn't know if it had screen resistors, etc? I have done the C12 mod, F\X loop fix, Mercury Magnetics choke, PT and OT installs, along with completely removing the Reverb tank and circuit. Anymore insight into mine amp would be very helpful. Thanks.
P.S.: I don't mean to hijack your thread, sorry.

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een7gdr

You should be able to find the pcb revisions by looking at the main PCB designators on each of the boards. Mine had a number of revision numbers listed with one marked white so i can only assume thats the current revision when my board was made.

I think the majority of schematic changes were with the power amp section, changing the grid stoppers from 220k to 5k6 for instance. There are also a number of component designator changes as my power amp pcb doesnt match my schematics unfortunalty!

I took a load of photos inside the amp prior to any mods, just in case i had problems rebuilding the amp, ill check them later to find out my pcb revisions.

Once again, thanks for the endorsement!

jcmjmp

There's a board revision number i.e. JMC2-61-00 where the last 2 digits indicate the rev, in this case, rev "00". There's also an issue number for the schematic itself. On the issue number, the first visible number is the issue number i.e. If numbers 1 and 2 are blanked out in white, the issue number is 3