Weird DSL +/- 15v supply problem.

[AmpFix]

2 Days ago, I sat down to start practicing. I fire up the DSL but to my surprise, no LEDs light up. Hmmm. I take it off standby and barely any sound is coming out. This has happened a handful of times since I've bought the amp in 1997 and usually, shutting the amp off and back on after a couple of seconds fixes the problem.

This time, turning the amp back on didn't help. I checked the rear board and the two fuses F1 & F2 were blown. I took out the fuses and rigged up a bunch of wire and fuses to see if the problem was just a fluke. The fuses blew right away.

1) I then disconected the rear board from all of its connections to other boards, leaving only the 22vac supply connected. I fire the amp and the fuses blow again. Damn! This is more serious than I thought! ... and I'm down 4 fuses.

2) I completely removed the board, checked out all of the components, diodes etc and everything checked out fine. Hmmmm. So what's going on?

3) I connect everything back together but leave the 22vac supply from the main board disconnected. Using an old 12vac wall wart adapter, I connect it to the board and voila! All the LEDs light up and the relays work. Good. Maybe there's a problem between the rear board and the tube board?

4) I connect an AC adapter to the main tube board and again, the LEDs light up, the fuses don't blow. Weird.

5) I re-connect the PT's 1st half of the 22vac tap back up and the fuses are OK, LEDs are lit up. I don't have enough fuses to try connecting both 22vac taps at the same time because I'm out of fuses but I'm wondering if that might have something to do with the problem I'm having. I tried powering the board with the other tap and that worked as well.

The only difference at this point is that I'm only using 1 half of the pt's 22vac taps. Could there be an internal short in the PT of something like that that would cause the fuses to blow? Could such an internal short be intermittent?

Help!

[bluesfreak]

Have you tried to measure Voltage supplied by the 22Vac Taps in respect to the CT and measured the resistance of each winding (PT unplugged)? This should give you an idea if there is something wrong on the PT but you can do as well the same measurement on the PCB, measure from the connectors 22Vac to the CT for both sides and check the rsistance. If there is a short you should be ablte to identify it without blowing another fuse....

[AmpFix]

I measured 22vac - 0 - 22vac right on the pt, on the rear jack board and also on the tube board. This is with the fuses blown, of course.

When I unplug the pt and measure DC resistance, I get 3.2Ω between the two 22vac taps and 1.6Ω from each 22vac tap to the center tap.

This has always looked right. What I don't understand is why this just started working again. I'll get some more fuses and try both taps connected to the amp again and see what happens. If it works, that means that there is an underlying problem that's just waiting to happen again.

The PT has been humming and the hum can be heard through the speakers, even in standby. When the amp is on, I get some hum on the lead channel as well. This is why I suspect the pt since everything else checked out ok.

[bluesfreak]

Maybe there has been a short under the PCB which you fixed when removing and reinstalling it? The PT hum is know to me on a lot of Marshalls, for me it is stray interference from the PT into the OT. I fixed it a couple of times by ensuring that the PT/OT was grounded perfectly, this means scraping the laquer from the area where the bolts are and use good star washers to tight the trannies down. they should have nearly zero ohm resistance to the chassis..

[AmpFix]

I reconnected both sides of the 22-0-22 vac supply to the rear board again and everything seems to have come up properly, without blowing fuses.

Not sure what happened but I suspect that maybe the rectifier diodes, or one of them has become either leaky or intermittent. I changed them all with some new ones (UF4007) and installed chassis mounted fuse holders instead of replacing the PCB mounted fuses.

I'll connect everything back up and do a sound test with results reported back.

[Don Moose]

I'm liking bluesfreak's idea - a bit of conductive debris rolling around inside the chassis and wreaking occasional havoc.

Standing the chassis on end and giving it a few good taps, then cleaning out whatever falls to the bottom - repeating for the other 3 sides - sounds like a worthwhile move.

[Mars Amp Repair]

FYI,
As far as the power xformer's humming coming through the speakers, I'd guess that the power tubes or others might be microphonic & picking up the vibration through the chassis. I know that's the least of your woes here, but thought I'd throw that in.

Your short problem might not be on the +-15V line. In the schemo I see where there is a little 1/2 wave supply that taps off of the same 22vac 2ndary through D-1,2, then the DC goes through R21 100ohm 1watt. There could be some issues out there, too. I'm not certain what that feeds, but might be a good place to look.

You could lift the R-1 to see if your short goes away (when it's nice enough to be there).

glen

[AmpFix]

FYI,
As far as the power xformer's humming coming through the speakers, I'd guess that the power tubes or others might be microphonic & picking up the vibration through the chassis. I know that's the least of your woes here, but thought I'd throw that in.

Your short problem might not be on the +-15V line. In the schemo I see where there is a little 1/2 wave supply that taps off of the same 22vac 2ndary through D-1,2, then the DC goes through R21 100ohm 1watt. There could be some issues out there, too. I'm not certain what that feeds, but might be a good place to look.

You could lift the R-1 to see if your short goes away (when it's nice enough to be there).

Yeah, I checked that 24vdc supply as well. That's used for the relays. It looks fine, just like the +/- 15v rails.

The hummy PT - I think that the PT is getting old and tired. I've done a LOT of shows with this thing and it might be time for a replacement. Its not a big deal and unrelated to this problem (a bad PT would have caused the mains fuse to blow, not the rear board fuses).

[Mars Amp Repair]

Yeah,
It's particularly tough when the short goes away when you're checking all that. If you can get the short to occur while it's connected, then you could disconnect things like that 100ohm resistor to the 24V relays as well as the +-15V supplies & see if you can get the short to occur.
when it doesn't, you at least know which path to take. glen

[AmpFix]

Yeah,
It's particularly tough when the short goes away when you're checking all that. If you can get the short to occur while it's connected, then you could disconnect things like that 100ohm resistor to the 24V relays as well as the +-15V supplies & see if you can get the short to occur.
when it doesn't, you at least know which path to take. glen

Good point.

[AmpFix]

I wired everything back together and no issues. Either there was conductive debris on the board or the diodes were intermittently shorting out. The chassis mounted fuses are real nice and it will be a lot easier to change 'em out if they blow.