B100R pop on shutoff

[mtlbasslad]

Not going well here...

Disconnected 1 leg of C36 & C38, pulled C34 - still have a short on the neg rail...

Bridge next? sigh...

[Enzo]

Unless the TIP142s are where the TIP147s should be, they don't connect to V-. But if in doubt, just use your meter to see if any of the three legs of the 142s are grounded. If J5 and J21 are off, then there really is no path through the bridge to ground. However, it takes maybe ten seconds to check the bridge, and I like to preach this:

Never think up reasons not to check something.

As long as those three caps are unhooked at one end, do check the parts themselves for shortedness.

You have followed the trace from the negative corner of the bridge, past the three caps and on to the collector pins of the TIP147s? Does the trace ever hop over something through a wire jumper up topside? If so, make sure to trace along any such branchings.

For some reason, my factory file on this model lacks a layout page, but to find those caps or anything else connected to V-, I just look for component leads coming through from the parts side. I follow the trace until I see something soldered to it, I then flip it over to find out what that might be. If you encounter a wire jumper, unsolder one end and lift it, that isolates the section past the jumper. Now the short exists on one side or the other of the jumper, and is thus narrowed down as to location. Assuming you have such a jumper.

[g1]

Just how short is it? Ohms from V- to ground?

Edit: never mind, I see you stated "dead" short, so zero ohms.

[Enzo]

Actually, that is a good question. Is it really a dead short - zero ohms - or is it just a steady beep on the continuity check? I mean 50 ohms might as well be a dead short, but may tell us something different from zero ohms as to the source.

[mtlbasslad]

Well, cautiously encouraged here..

Digested good advice from you all, collectively you may have spent more time on this problem than I have. It's been a great learning curve, I'd love to give you all a hug - OK maybe just buy you all a beer or 2...

Pulled C34, C36 & C38 out (not just 1 leg) also the first jumper I found downstream from the bridge. turns out one of those little poly buggers (C36) shows a dead short. Seems to have a small black spot on it too, as in 'pfftt'. Not sure how I missed that earlier, blaming old eyes...

Now when I measure from J18 (my ground reference) to neg of bridge I get around 900 ohms. This would seem to be an improvement.

Now it's time to order a cap, & put the whole mess together again - hey, I'll get to try my new light bulb limiter YAY. (Still waiting for compliments on the industrial-chic build of that)

Stay tuned for an update - not breaking out the champagne yet (anyway I prefer Tequila)

Fingers crossed, thanks everybody.

Lorne & Joanie

[Enzo]

C36 shorted? OK, if the other two are not, put them back and see if the amp now works. It can get along without C36 until you have a chance to replace it. And that will let us find out if there are other issues that were hidden by the short.

[g1]

This shorted cap was just a side trip from repositioning the pilot light, so there were no other faults were there? I believe that cap shorted while you were looking for the -40 rail to tie the lamp to?

Nice job on the limiter lamp, you maybe do your own minor house wiring?

OK maybe just buy you all a beer or 2...

Like a friend of mine always used to say, "I can only stay for one or 2................ok, six".

Wait, no Tequila for us lowly plebes?

[mtlbasslad]

Well folks, this story has a happy ending (no, not that kind)

I had to ignore Enzo's good advice about testing before putting it back together - no bench power supply, not enough jumper cables, etc...

Managed to find a cap & some heat sink grease locally (well, 45 mins away) put it back together & plugged in (of course through the new light bulb limiter) & all is good!

So thanks again to you all, hats off to km6xz for nailing the original problem, Enzo, g1, JM Fahey & all of you for holding my hand all this time.

Bonus, the original problem - 'pop on shutoff' is gone. Interestingly, now on shutoff the pilot LED dies slowly instead of going off instantly.

And for g1 & everyone else - yes Tequila is offered along with the beer to all. Maybe even BBQ, weather permitting.

Many thanks to all

Lorne & Joanie

[Jazz P Bass]

That cap that finally shorted may have been the problem.

[mtlbasslad]

Indeed it was Jazz P Bass. Running fine now...

[Jazz P Bass]

Not too sure if that bad cap could have been found.

That is before it went.

Scarey.