Marshall dsl 401 reverb

[Valvehead]

I have a dsl401 combo here, no reverb.

tank tests good

put in new cable

checked / repaired all solder joints

the 2 op amps in the reverb circuit have their correct voltage to them .

cleaned all pots and jacks

tried a cable in the rev fsw jack


any thoughts ? maybe the reverb pot ? (its one of those cheapo green plastic pots)


I had the amp on and bumped the tank---and I can hear the verb way, way in the background trying to come through.....

[soundguruman]

If all else seems normal, you could actually have a bad circuit track.
I mean wass hapenin? Do you have drive into the tank?
Do you have an O scope?
It might be time to get one.
Turn up the reverb control. Touch your finger to the reverb return cable tip.
you should hear noise, bzzzzzzz, when you touch just the return cable plug tip....
If you have no noise, your return amp could be fubar...
Is your tank hooked up backwards?
Is your reverb foot switch shorted to ground? Unplug that puppy...

[Valvehead]

No scope....

i dont have the fsw for it.

i did test the rev fsw jack--- with the hot contact open or closed it measures 3k to ground , the groung contact is ok.

when i touch the cable ends---no buzz. they arent backwards.

Theres a small pcb for the rca jacks with 2 " beads" on it. what are these and what do they do? how to test ? they look like diodes , L3 and L4 on the scem

[Jazz P Bass]

"L" is typically the label for inductor (a coil).

[soundguruman]

No scope....

i dont have the fsw for it.

i did test the rev fsw jack--- with the hot contact open or closed it measures 3k to ground , the groung contact is ok.

when i touch the cable ends---no buzz. they arent backwards.

Theres a small pcb for the rca jacks with 2 " beads" on it. what are these and what do they do? how to test ? they look like diodes , L3 and L4 on the scem

If you touched the cable ends, with reverb turned up, and no buzz, you have something wrong in the return circuit.
The return cable should buzz when you touch the tip.

[DrGonz78]

This would be really strange... I had similar problem as you are describing getting the reverb circuit working on this same amp.

Reverb return had a blown 12v zener diode open and it fixed the problem. Not saying that is what is going on here unless this failure is a common one. I don't think it is a common failure but I would guide to do a diode test on ZD5 anyway. Look for ZD5 on page 2 of the schematic around quadrants (B,4). Also, check resistor R135 and even take a good look at T15 just to be sure. All components located on the same section of the schematic. Just would be funny if this was the same problem but chances are it is not.

Also, could be some components to look at where the rev. return is found around (C,5) on page 1 of the schematic.

[Enzo]

Those L components that look like beads are... ferrite beads. They are a piece of wire with a wad of ferrite around the middle. They test like the piece of wire they are. Not likely bad.

If your return is not working, put a signal into the red jack. Is it at IC4 pin 5? Check JFET T8, is there voltage at its gate to turn it off? Is there low resistance from source to drain while powered?

[Valvehead]

I took some readings--

ZD5 on diode test , out of circuit tests .14v / .12v
across it in circuit , amp on , reading dc voltage i get 005mv and 031mv

r135 is ok

T15 at legs i get 32mv / 005mv / 005mv


signal into red jack --yes signal at ic4 pin 5

T8 i get low mv , .005mv at each leg , amp running


with amp on , at T8 i get these ohms at each leg = 50ohms / 76K / 89K

[Enzo]

If ZD5 is shorted, as it appears to be, then the voltage can not come up to turn off muting JFET T15, and so the reverb remains muted.

[Valvehead]

Damn you guys are good.......................

it was that zener ! Put in a new one = got verb .

THANKS

[DrGonz78]

Nice! Once in a blue moon advice like this works, but still not convinced it is a common failure or just some strange coincidence. Either way if it fixes the problem everyone is happy at the end of the day. Will watch this thread for future repairs of this nature on the 401 reverb circuit to see how common the zener failure might be, and what might be causing this failure. That is if it even proves to be a common one.

[Valvehead]

Wouldnt surprise me -- the jcm2000's seem to have common problems . Ive fixed dozens of them ! Now I know what to look for if verb is dead --its usually the little wires in the tank