Marshal DSL 401 help - low/no volume

[wagdog]

That's great. I think the V1b stage must have had vastly decreased gain, so in post # 1 when you bypassed around R104 & R93 to the V3A grid it boosted the signal enough to make it seem like it was working right.

Well, it was working. Didn't sound that good, but signal was making out of the preamp.

In retrospect, I learned a few things;

1. When you're having a bad day, stay away from amps. Heck, stay away from electricity in general!

2. When I saw the low voltage on the v2b plate, I should have resolved that first. I didn't think to measure the plate of v2b, which I should have. After I finally did that, it was pretty apparent there was DC voltage leaking onto it from the coupling cap. I was too focused on the effects loop (my fault for the bad measurements, see #1).

Anyway, what a nice sounding little amp this is. The clean channel is very responsive, almost shimmering. I'm glad to have it. Appreciate the help here, a lot.

Unfortunately, it now appears a new set of EL84s and 12ax7s have become unobtanium. I've got known good, but used/old tubes in it for now.

[g1]

Replaced C88, and the plate voltage on v2 jumped up to almost 200v - that's better! Put the old C88 on my vmm and it was about 8M of resistance, but was slowly dropping and never stopped dropping. I'm sure with 400v on it was letting DC through.

Clean channel now had plenty of volume, as does the OD1/OD2 channels. Shut the amp down, drained the caps (again), and soldered T12 back in.

Amp works great!

That's great. I think the V1b stage must have had vastly decreased gain, so in post # 1 when you bypassed around R104 & R93 to the V3A grid it boosted the signal enough to make it seem like it was working right.

[wagdog]

You can't replace a P-channel with an N-channel fet (or vice versa). Do you see any positive voltage at V2B grid?
You can remove T12 (or disconnect drain pin) as a test to see if V2b plate voltage corrects.

And this was the reply that got the amp fixed.

Removed T12 (could get to it from the front without removing the mainboard. No difference on the v2b plate.

Checked for DC voltage on the v2grid and found 4.25v. That doesn't belong there. C88 must be leaking DC voltage. Replaced C88, and the plate voltage on v2 jumped up to almost 200v - that's better! Put the old C88 on my vmm and it was about 8M of resistance, but was slowly dropping and never stopped dropping. I'm sure with 400v on it was letting DC through.

Clean channel now had plenty of volume, as does the OD1/OD2 channels. Shut the amp down, drained the caps (again), and soldered T12 back in.

Amp works great!

Thanks all for the help. Again, my apologies for the messed readings on IC1. No excuses, just a bad day - quit working on the amp for the day after that! Happy to get it sorted out.

[strato56]

I did notice when I plug directly into the return of the effects loop, I get sound when the clean channel is selected, but when I switch to OD1/OD2 I get nothing. Not sure why that is.

Keep in mind that the tonestack and volume pots are after the effect return for the OD-channel

[g1]

You can't replace a P-channel with an N-channel fet (or vice versa). Do you see any positive voltage at V2B grid?
You can remove T12 (or disconnect drain pin) as a test to see if V2b plate voltage corrects.

[wagdog]

Thinking out loud, could this be a problem with the footswitch jack/board? I know the TSL amps had issues with solder joint failures there.

I cleaned and resoldered the footswitch jack. Could still be an issue? I don't have a footswitch plugged in.

Disconnect one end of C89 in the cathode circuit of V1b and see if it affects the plate voltage.

Tried that. Even replaced C89. Checked the cathode resistor R124, 2.2k.

Thanks for the thoughts and replies.

Most of the JFETs in the amp are J174. I have about 50 J201 - are those a suitable replacement? I've looked at the data sheets and... I can't find a set that matches up closely enough for me to be sure of what I'm comparing.

[g1]

Disconnect one end of C89 in the cathode circuit of V1b and see if it affects the plate voltage.
edit: meant V2b, not V1

[glebert]

Thinking out loud, could this be a problem with the footswitch jack/board? I know the TSL amps had issues with solder joint failures there.

[wagdog]

+15 on pin 5, I'd wager the IC bad, but the voltage comes from somewhere. If not inside the IC then external. SO where can it come from? Certainly not up from ground.. That pretty much leaves R67. You can get at R67 without pulling your board? I will assume you have the 15v on the lower end of that. SO what is the voltage on the top end of R67? If it is slightly higher, then the 15v is likely coming down from the R104 area. If the top end is somewhat lower than the bottom, that points to the IC being the source.

Now up at R104. I don't recall your voltages from earlier, but I'd expect no DC at R104/R93 But if there is +15 there too, it either comes from R104 via leaky C72, OR a faulty JFET T13 could be injecting it. If the gate shorts to the grain, the positive voltage CLNG1 signal could come through.

Welp, after replacing IC1 and C72, I realized that I shouldn't be working on amps today. Not only did I read mv as V this morning, but I also read the wrong pins on IC1 - there is -240mv on Pin 5 - I read it backwards.

I'm sorry for wasting your time with that goose chase. There is 16v on pin 8, not 5. 5 has a similar -mv as 6 and 7. I feel really dumb today.

Anyways, R67 was a bear to find as the numbers for some resistors is conveniently underneath them on the board, but I did find it. As expected, it has a minimal DC -mV on the top and bottom.

So I think I'm pretty much back at the beginning. I'm wondering why the plate of v2b (pin 6) has such a low voltage on it (90v-100v). I tried several different tubes and it varies a bit but not much. I know C72 is good.

I did notice when I plug directly into the return of the effects loop, I get sound when the clean channel is selected, but when I switch to OD1/OD2 I get nothing. Not sure why that is.

I'm wondering if T12, T13, or T14 is bad?

[Enzo]

+15 on pin 5, I'd wager the IC bad, but the voltage comes from somewhere. If not inside the IC then external. SO where can it come from? Certainly not up from ground.. That pretty much leaves R67. You can get at R67 without pulling your board? I will assume you have the 15v on the lower end of that. SO what is the voltage on the top end of R67? If it is slightly higher, then the 15v is likely coming down from the R104 area. If the top end is somewhat lower than the bottom, that points to the IC being the source.

Now up at R104. I don't recall your voltages from earlier, but I'd expect no DC at R104/R93 But if there is +15 there too, it either comes from R104 via leaky C72, OR a faulty JFET T13 could be injecting it. If the gate shorts to the grain, the positive voltage CLNG1 signal could come through.

[wagdog]

Have you tried a different tube there?

I have tried a few. The voltage varies, but not by much.

That would suggest that the 15V you measure at pin 5 IC1b is coming from IC1b itself and not from R67. Not a good sign for IC1.
Can you desolder pin5 and then measure voltages at R67.

Not without pulling the board and desoldering IC1 completely. I'm not against it, but it's not a quick task. I may as well just replace that IC if I'm going that route... which I may do if nothing else seems likely.

I think if there was really 15V at IC1b pin5, then pin7 would go to +15V also.
wagdog , can you double check that IC1 pin 5 voltage (and pin7)? If IC1b voltages previously listed (post #5) are correct, then you must replace IC1.

It changes a little from cycle on/off, this time around I got 15.8v on pin 5 and -470mv on pin 7. I'm also getting -16v on pin 4, if that helps.

[g1]

That would suggest that the 15V you measure at pin 5 IC1b is coming from IC1b itself and not from R67. Not a good sign for IC1.
Can you desolder pin5 and then measure voltages at R67.

I think if there was really 15V at IC1b pin5, then pin7 would go to +15V also.
wagdog , can you double check that IC1 pin 5 voltage (and pin7)? If IC1b voltages previously listed (post #5) are correct, then you must replace IC1.

[strato56]

That would suggest that the 15V you measure at pin 5 IC1b is coming from IC1b itself and not from R67. Not a good sign for IC1.
Can you desolder pin5 and then measure voltages at R67.

[glebert]

So there is 2.5x the current going through V2b as V2a according to the cathode voltage (assuming the cathode resistor values are not too far off). That would explain the low plate voltage at V2b. Not sure what could cause that, maybe oscillation? Have you tried a different tube there?

[wagdog]

What are the voltages at both sides from R104 ?

-450mv at R104/C72 and -470mv on the other side of R104.