Vox AC30CC2 - Potential short to ground with warm valves

[yakkmeister]

Hello!

I've just finished a (very) long-term repair on a mate's AC30CC2.
It was popping HT fuses like candy. Checked for typical standby switch issues.
Findings/repairs in order:

• 5AR4 valve appear to work fine
• Replaced with solid-state recto (T-SSR01) to be sure
• EL85 quartet had 1 member let the air in
• Replaced quartet with Sovtek EL85 set
• Found power caps (22uf/450v) were dead
• Replaced caps (22uf/500v)

I have a 150W incandescent hooked up as my current limiter (built since I got kissed by a Mig50's HT ... ).
All the above faults showed a very bright, immediate glow on the bulb.
Now I have different symptoms:

without valves at all:

• the bulb remains entirely dimmed
• no flash of any brightness upon power-on

with valves:
T-SSR01 only:

• no flash on start-up with either position of standby
• both power and standby lamps function, steady light

with or without preamp valves:

• small, dim flash on power-on, either position of standby switch
• light builds intensity slowly (presumably as valves are warming up)
• standby lamp pulses with trem (this is supposedly normal)

So, I would have suspected the power valves were dodgey, given this behaviour ... but they're essentially new and I'd consider them known-good.

Any ideas what I should be looking at next?
I have not taken any voltage readings (I think we can all appreciate that nothing done so-far is particularly tricky) but I'm happy to oblige.
Being in Aus, we're looking at 230 VAC @ 50 Hz.

[sumgai]

yakky,

I see no specifications anywhere for an EL85.  Did you perhaps mean an EL84?

In any event, I'd look very closely at the power tube sockets.  Some amps use second-rate sockets that can develop trace cracks.  These cracks then become pathways for crap, either electrical detritus or otherwise, and thence become short circuit pathways.  You almost can't see them with the unaided eye, and almost always a DMM will not detect them - the resistance involved needs a strong current to actually conduct, and a meter won't deliver that kind of juice. 

I assume that you've double-checked for bad solder joints, yes?   A joint that looks good on the outside can still present a high resistance to current, and thus may be a cause of too much current demand, and thence the blown fuses.

And lastly, the Power Transformer itself may have gone bye-bye.  Here again, a meter might read the windings as fine, but a large current demand may reveal a "flashover" short in said winding(s).  While it's more time, expense and trouble (manual effort) to exchange this beast, it may be necessary after you've tried all else.

HTH

sumgai

[yakkmeister]

Jan 6, 2018 12:30:15 GMT -5 sumgai said:

yakky,

I see no specifications anywhere for an EL85.  Did you perhaps mean an EL84?

You are correct! A brazen typo!

Jan 6, 2018 12:30:15 GMT -5 sumgai said:

In any event, I'd look very closely at the power tube sockets.  Some amps use second-rate sockets that can develop trace cracks.  These cracks then become pathways for crap, either electical detritus or otherwise, and thence become short circuit pathways.  You almost can't see them with the unaided eye, and almost always a DMM will not detect them - the resistance involved needs a strong current to actually conduct, and a meter won't deliver that kind of juice. 

Oh - I've read about this sort of thing. How would you test for it? Visually, with a magnifier (either a glass or a microscope device) or ... ?

Jan 6, 2018 12:30:15 GMT -5 sumgai said:

I assume that you've double-checked for bad solder joints, yes?   A joint that looks good on the outside can still present a high resistance to current, and thus may be a cause of too much current demand, and thence the blown fuses.

I haven't checked for resistance. They all look good but, as you said, doesn't mean they're not bad. I'll be going over everything again(again) once I've a plan of attack.
I do wonder if this isn't the most-likely scenario as there were some problematic areas on the PCB with old solder. Perhaps a full re-do on that will be in order.

Jan 6, 2018 12:30:15 GMT -5 sumgai said:

And lastly, the Power Transformer itself may have gone bye-bye.  Here again, a meter might read the windings as fine, but a large current demand may reveal a "flashover" short in said winding(s).  While it's more time, expense and trouble (manual effort) to exchange this beast, it may be necessary after you've tried all else.

I will have to pass it on if it gets to the point where the transformer is likely ... this amp is on it's second transformer already and neither I or the owner have the cash for a spare. They're significantly harder to come by here in the upside-down.

Thanks for the info, Sumgai! It looks, for the most part, that it's probably still within my skill-level to sort out.

[sumgai]

yakky,

If you know which end of the soldering iron not to grab with your fingers, then you're probably capable of finding and fixing 95% of all amp problems. Not to be mistaken for some kinda braggart, I have had my fair share of unsolvable problems over the course of neary 60 years, so I share your frustrations.

And you're also correct in that running over the "worth fixing" budget is valid only for sentimental reasons.  But if this is already on its second PT, then why was it replaced the first time?   I wonder, did someone just shotgun the thing, replacing parts left and right and hoping for the best?  Is the original problem actually still present, and taking its toll on the PT, or on other parts, yet again?

Checking for carbon traces can be tough.  My first method is with a jeweler's loupe, a magnifier that fits around the eye.  It's held in place by squinting, but if you're wearing glasses like me, then that's a bit difficult - I just hold it by hand, and use an Orange Stick to poke and prod and move things out of the way whilst sleuthing for clues.  (You shouldn't have both hands inside that chassis anyways, right?!)  Look inside the socket at the pins themselves, and make sure they haven't gone gunnysack on you, too.  Oh, and provide plenty of light, these things can hide in the dark real easy.

The only other way that I'm sure will work is to simply replace the sockets out of hand.  Yes, that's guessing at its finest, but sometimes you've gotta go with your gut instinct. (But keep it under your hat if that didn't solve the problem.)

Re-heating solder joints on a PCB is a must, don't even think of skipping that step.  Wave solder machines are notorious for getting out of adjustment in some fashion, and Whammo!, the board comes out the exit end in not-quite-ready-for-prime-time condition.  In fact, try this first, before anything else.  You never know.....

HTH

sumgai

[yakkmeister]

Sorry this has taken soooo long to reply!
Work work work ... you know how it be!


So it's mostly anxiety stopping me from getting this repair done. I'm going to go over those tracks again, as you suggested, and do a triple check. if it still ends up borked ... my mate's going to spend some money on a MrFixer

[sumgai]

I seem to have missed that your latest post was weeks ago - Gaahhh!


One more thing to confuse you... Somewhere around 14 or 15 years ago, give or take a decade, I had an FSR come in with similar problems. After going through things as described above, it turned out that the reverb driver transformer was bad, shorting only when warmed up... and that took longer than we'd expect, on the order of 10 minutes or so.

How did I find it? By getting desperate, and pulling a trick out of my tookbag usually reserved for transistorized stuff... cooling spray. This is not something I'd ordinarily recommend for tube amps, because it can damage tubes if you get too close and spray them while they're hot. Quick temp changes like that can be hazardous to your wallet. If you do use the spray, try to put in older tubes so that if they go gunnysack, you won't curse quite so much.

Spray components one at a time, and see where the problem suddenly "goes away". In the case of shorts and such, that's not so easy, but it can be done, if you pay attention and exercise patience. Perhaps I was lucky, perhaps it was good sense after all the "normal" stuff had been done, I dunno. Your mileage may vary.


a) Re-solder everything first;
b) Look closely at tube sockets for carbon tracks;
c) Use a freeze-spray to cool down suspect components;
d) Advise the owner of cost-to-repair versus resale-value. Sentimental value optional.

That's my take, hope it helps.




sumgai

[yakkmeister]

Thanks for the extra tips, Sumgai!

I do have an update and a funny story, though ...

I made some repairs as I was planning to do but, as feared, the dim glow remained.
So I told my friend I was stumped and I had failed ... boo!

While I was waiting for him to come get it, I found a step-by-step guide on amp troubleshooting - I figured it was worth a go.
I went through the guide 2 or 3 times, stopping at the same point each time ... the guide was suggesting that the bulb *should* glow, but just a little.
In the guide, he was using a much lower wattage bulb than I. This lead me to believe that the glow I was seeing, while kinda-bright-I-guess in intensity, was a very very small percentage of the full brightness.
This would mean the the repairs I had done were 100% sufficient in the first place.

They were.
He's got the amp back and he's very happy.