Marhsall jcm2000 DSL-401 - footswitch won't switch between od1/od2 (front panel will)

[thehoj]

So I'm taking a look at a buddy's jcm2000 dsl-401 for the first time because when he plugs his footswitch in, he can switch between clean and od (always od2), but not between od1 and od2.

I've been working with this schematic in my attempts to resolve the issue:
http://www.drtube.com/schematics/mar...60-02-iss9.pdf

If I unplug the footswitch I can switch between all of the channels fine with the controls on the front.

I've attempted to fix this by running through a few things, and resoldering some stuff.

I opened up the footswitch and confirmed that the stomp switches themselves are fine, and they are indeed fine. So I desoldered and cut back the cable that was going into the footswitch in case there was a break in one of the wires coming into the footswitch, then I resoldered the wires back onto the switches. I also cut off the 1/4 TRS connector from the other end and soldered on a new one.
I tested for continuity between the wires inside the footswitch, and the connector end. It's all good, but still the footswitch will only switch between the clean and od2, not od1 and od2.

So I opened the amp up and spent the time to mark down which wires connect where (this amp is much more complicated then the amps I'm used to). I got the front panel off, got the tubes out and unscrewed the main PCB from the chassis.
I took a look at all of the solder joints that seemed to be pertinent, and resoldered them all. I also put a TRS cable into that jack and tested for continuity from the wire ends of the cable to the points on the PCB that the jack is soldered, it tested fine, so this seemed to indicated that the jack itself is okay.

I also resoldered anything else that looked iffy to me and put it all back together. Still the same issue.

Then I took a look at the little board which the channel switching buttons are attached to on the front panel, I noticed that one of the pins (base) on the transistor (T1, which is labelled as NTE 46) didn't seem to have a pad on the pcb to be soldered to, so it wasn't soldered in there properly. I took a look at the trace it was supposed to be soldered to and soldered a wire in place of the trace, so there's a wire going from the base of that transistor to R4 and R5 now, and still I seem to be having the same issue. I'm not 100% sure but I was kind of thinking that this transistor was a part of the switching between od1 and od2.

I've searched google over and over for different search criteria to see if anyone else has had this problem, but can't seem to find any information that helps me any further. Many people mention footswitch issues, but I'm fairly certain the switches and cabling in this footswitch are fine.

Am I looking at the wrong stuff? Does anyone have any suggestions for things that I could try to fix the problem?

Thanks.

[Mark Black]

Looks like plugging in a footswitch uses all the same internal switching circuit components, only substituting external switches for S2a & S4a. So it would seem the internal circuitry should be fine (assuming the schematic is correct for your revision amp).

Try inserting a TRS plug with nothing wired to it into the footswitch jack and see if you can get switching by shorting between the tip & sleeve and then between the ring & sleeve with a jumper wire.

[Enzo]

SInce the panel switches work, the circuit works. Plugging into the footswitch jack opens the normals in the jack, and the4 footswitch is now the path to ground. So then either the footswitch is not working itself, or the ground return of the footswitch jack is not there.

Is there continuity to ground from the sleeve contact in that jack?

And the footswitch itself? Rather than cutting off parts, just test it with a meter. The plug is a TRS, so connect an ohm meter to the sleeve of the plug, and see if you get continuity to the tip and ring contacts that can be turned off by the switches.

[thehoj]

Well yea, I guess I wasn't thorough enough. The cable must have a break in it somewhere, because it'll intermittently work.
So I replaced the cable and it switches channels perfectly fine now.. BUT, the clean channel no longer produces any sound.. I can get sound on the overdrive channels, but not the clean. As soon as I switch to OD1 or OD2 it works as expected, then I switch back to clean and absolutely nothing.
I tried reseating all of the preamp tubes, and tried a few new ones in the preamp even..
Any idea what could be causing this issue now?

[Enzo]

Any time you are working on something and all of a sudden there is a new problem, you can pretty much count on the fact it was something you did. You left a solder blob shorting something, you knocked a component lead off the board, you left a cable unplugged, who knows what.

I am not being unkind, it happens to all of us, myself included. The point is that unless it is a HUGE coincidence, the fact it was something I did is a large clue. I look at the area I was working, I look at any connections I broke to get at the boards. I make sure I didn;t misconnect any of the cables.


No way in the world I know what you did. The amp is sitting there, ultimately, and whether we ever figure out HOW it got screwed up, it sits there with a problem. SO approach the problem as if that is what it was brought in for in the first place. No clean signal.

Look on page 2 at the switching circuits. T1-T5 run the show. Now look at page 1. See VR1, I assume that is the clean volume. Halfway across the preamp. See the wiper, and how it is connected through R103 up to the grid pin 7 of V2? And just below and connected toR103 is T12. That JFET is a mute. If it is shorted, you get no clean. But it is a JFET, and JFETs are ON untill turned OFF by voltage at their gate. See its gate marked CLNG1? "Clean ground 1" That is the control signal for that JFET.

Back to page 2. Look at switching, out to the end of switching towards the right, see CLNG1? It is a voltage coming from D18, through T2, and T2 controlled by T3, which in turn is controlled by the FS and panel buttons. if any point in that breaks down, no clean because T12 can't turn OFF.

So: isolate the problem.

With the amp running, and VR1 up most of the way, measure resistance from the wiper of VR1 to ground. If you get something like 200 ohms, the JFET is ON (or shorted). It may be more convenient to take the reading at T12 or R103, doesn;t matter where. If you get several hundred k ohms, then the JFET is off and none of this that I described pertains.

But if the JFET is indeed on, then look at the gate, got voltage? Back at T2, the green LED3 on the panel gets its voltage from T2 just like D18 does. Does teh LED light up? If it does, then T2 and everyhting before it is working. T3 controls no0t only T2, but also the relay, so when working the panel switches, does that relay click off and on? If so, then T3 is working.

Thos amp is not as complicated as it looks. The whole preamp is V1 and V2. By the time you get to V3, you are at the FX loop return and the tone stack driver for OD, which is bypassed by relay for clean. I suppose C77, R106 or the relay could be bad, but I doubt it.

The signal from the input hits V1a then out C74. From there we branch to the clean and OD channels. OD is the branch down through C99 and into V1b for more gain. From C74 straight to the right is the clean, through the clean channel EQ,VR1, and into V2b, pin 7 through that mix resistor R103 we mentioned before. That's it, the whole clean channel. The OD channel went through some extra gain stages, but ultimately mixes into that same V2 pin 7 through the other mix resistor, R102. And from that point forward, we are back into circuits common to all channels. And your ODs are working.

SO if there is no clean, the problem lies between C74 and R103. Since you were working on the FS circuits, I tend to suspect the trouble is there, but the best thing to do is just troubleshoot the clean channel.

[vifer]

I purchased a DSL401 on the weekend and my f/s was exhibiting the same symptoms.
After some systematic continuity testing with the f/s cover open - Found the inner wires for the ring ( white wire) and GND ( black) were broken at the grommet. I was about to grab my tracer to find the cable break when I thought lets inspect cable and stress points. Those hard plastic grommets really clamp into the cable and are unforgiving with the slightest bit of stress. Such as if the cable is twisted. I had a spare cord grommet with stress relief tail ( spiral ) and replaced it. See how long that lasts.
Updated here for posterity in that it might help someone else.