Marshall DSL40C help please

[Randall]

Combo comes in with the complaint of sometimes when it is powered up there is no sound, and channel switching doesn't work. Power off/on corrects it. So far I have only been able to see it in this state once. It was after it was powered up with no problems, and left a few hours at idle. Then it was as described, no sound, no channel switching, and sure enough power off/on corrected it. Since then it has not done it again. All I have done is probe around and tap on relays, but it hasn't faultered.

So, in preperation for when I can catch it again, I could use some advice. I am looking for what might be in common with no output and no channel switching, At present I am looking at the big IC4 on the first page of the drawings. Looks to me like it receives 5v at pin 20 and distributes it to the various relay switching transistors depending on inputs. Since I see mute outputs as well as channel outputs I am inclined to start there. Easy enough to see if I have the 5v on pin 20, but what else should I be looking for? I have pulled the IC and reinstalled just to make sure it is connected and seated well.

DSL40-60-02-v02.pdf

[nosaj]

Check all your rails +12v -12v and a 5 v regulator , v+ and V- rails on page 2 .
Thats where i would start. Power off/on and it starts in my head says failing cap.
nosaj

[nevetslab]

I looked up the AT2313 IC that Marshall uses in that amp. It's a programmable IC and there's reports on the Marshall Forum that amps that use it have the same symptoms as you're describing. It's the source for controlling the relays. There sure are a lot of switches as well as relays thruout the signal path, so any one of those could interrupt the amps's function. Powering off and then on again restoring function sounds more like that part than a particular relay.

But, as you don't have a replacement for IC4 (I'm guessing), I'd go hunting with bright light and magnification to rule out solder joint issues. I did just recently have a DPDT 24V relay in a Fender Hot Rod Deluxe go intermittent, and found it by pressing on the PCB around that one relay...even after I had de-soldered and re-soldered the relay terminals. You might get lucky and it's one relay causing the grief. Relay 4 has that distinct pleasure of shorting out the send from the Preamp to the Power Amp stage. Looks like there's a jumper LK3 on the PCB that you could remove to take that muting relay function out of circuit.

I've never had one of these on my bench. On the report I found when I searched for AT2313 IC on the internet, if that part that Marshall has had programmed for them (or did internally)....if it goes away, then one would have to do some hard-wiring of functions those relays do thru the IC to get the amp functional.

Worst kind of failure to find.....works fine usually, then does what it's claimed to do. Could well be a suppy cap as nosaj suggests. Tedious task.

[Randall]

So far it has only gone into failure mode one time.I pulled the IC4 and re-inserted. Is it possible that this cleaned an iffy oxidized pin? I also have probed and tapped around all relays, pressed around on the board. Nothing seems to make any changes. I don't feel like spending the rest of my life on this. Maybe the answer is just to power off/on.

[nosaj]

When it went into failure mode that was the time to check your rails to see if one failed.
nosaj

I'd be wary about giving that answer to a customer. Some days you earn your bench fee other times it easy peasy and it all works out. Some are dogs .

[Randall]

"



"When it went into failure mode that was the time to check your rails to see if one failed."

Yes, of course. Doesn't do me much good if I can't get it to fail.