Marshall DSL401 - no FX loop

[Don]

I've got a 2001 DSL401 that I bought as a basket case. I reassembled the amp with a new reverb tank, bridge rectifier, tubes, bias and it sounds great and has been reliable at rehearsals except that the FX loop does not work at all. The reverb is a bit weak as well, but I compared it to a friend's DSL401 and it sounds pretty similar. His FX loop functions, though.

It does not matter what effect I plug in or even if I jumper the send and return jacks, the loop does nothing. I am getting +23.6v at R135, -15.3 at D10, +15.3 at D9 -15.3 at IC4 pin 4, +15.3 at IC4 pin 8 and +23.7v at R13.

Before I bought the amp, the previous owner replaced all of the potentiometer with correct parts (that was done well, I checked all values and solder joints) and some traces on the PCB were repaired in the past- none related to the FX loop.
The jacks look like the solder joints are good and appear to be functioning.

Thanks in advance for your help.

[Yelly]

- did you forget FX Mix on the front?
- did you try guitar directly into Return, FX mix up, any sound?
- remember, you canīt jump send/return jacks without opening the returnjack, or it would be shorted. Next, itīs a parallel loop, so might be difficult hearing much change jumping it.
That said, everything is mute controlled by Pchannel FETs, could be one like T7 thatīs shorted. That model has a bit odd switching going on. Also, IC4 is the reverb return buffer, try look at IC1 for the fx loop.

[Don]

Thanks Yelly.

I've cranked the FX Mix on the front and am considering the quality of that pot. It is new, of the correct type and value and appears to be installed well.

I have plugged straight into the return with the mix up with no results.

I jumpered the send return jacks by plugging a cord into them.

Overall, I expect very little from this FX loop- I've used my friend's and it wasn't something I'd use with pedals.

I'll look at IC1 and T7. I included info on IC4 because the reverb and FX loop in this amp seem to have a relationship.

[g1]

Also T16 which can do the same as T7. You can just remove them as a test.
Can you try the send into another amp? It probably works as it is very close to the reverb send which seems to be working.
If you can verify send is working, then you can concentrate on the return side.

Edit: Does the board have a revision number so we can find the proper schematic?
Also, did you mean +15V at pin 8 of the IC's?

[Don]

Thanks g-one.

I just connected the Marshall's FX loop send to my Mesa Studio .22's FX loop return and it worked fine- the Marshall's pre-amp played through the Mesa's power amp.

**Then I connected the Mesa's FX loop send to the Marshall's FX loop return and that worked fine as well. Yet effects do nothing in the Marshall's loop.

The board has DL40-60-00, ISS (black box where 6 would be), 7, 8, 9, 10 on it.

Yes. +15v at pin 8 on IC4. I just corrected that in my original post. Thanks.

**This statement is incorrect.

[g1]

When you had the 22's send connected to the dsl return, did the fx mix pot work?
Can you try again with your guitar plugged straight into the return?
If the return works for the 22, it should work for the guitar.

[Don]

Hmm...

Either I was mistaken (or it intermittently worked) when I said it worked when I plugged the Mesa's send into the Marshall's return because it does not work now, no matter where the Marshall's FX Mix post is set.

The Marshall's send works well into the Mesa's return, though.

Sorry for the confusion!

[Jazz P Bass]

Resolder the jack?

[Don]

Thanks Jazz P Bass, I'm going to take a closer look at the jack and mix pot.

[Jazz P Bass]

Just resolder them.

End of any question about the integrity.

[Don]

I removed T16 and the FX loop works fine. Thanks!

[Jazz P Bass]

Are you going to replace it?

[Don]

I'm not going to replace it. The amp functions fine without it. Some people remove T15 and T16 in an attempt to lessen the latency in channel switching. I don't believe it makes a difference. T15 is connected to the reverb return. T16 is connected to the FX loop return.

[Jazz P Bass]

Hmmm.
I think of it as being an 'anti pop' mute.

Note: Future Post:
Hi, I have a DSL-401 that pops when I toggle the FX.

[Don]

To tell you the truth, I don't use the FX loop anyway. If I hear a pop or other noises, I'll know what to replace.