Buzz/click!-dead when switching off standby

[Exodus5]

I had planned on doing some mods to this Crate Blue Voodoo combo I have. I had a noise (turns out it was the reverb) but since it's nearly Christmas I went ahead and bought myself a pair of the Groove Tubes EL340-M (ie: Mullard copies). I installed them Saturday and played for about an hour on them with no problems. I took them out after recording a tone sample and put the 6l6's back in to record it with those tubes. I did that, and when I turned the amp off everything was fine.
Sunday I put the el34's back in and Turn on the amp (leaving it in standby). It was about 10 minutes before I could get back due to a phone call and when I flipped off the standby I got a pop/loud hum/house lights dimmed for a millisecond and then zap-the amp was dead. I found that the main fuse was blown. I went and got a few fuses and swapped the 6l6's back in to see if my power tubes were junk. I put in a new fuse and turned the amp on. All seemed good until I switched off the standby, at which time it hums just for a half-a-second and then blows the fuse. I took out all the tubes and got the same result. I disassembled the amp and found that one of the large capacitors (C53 if you have a schematic) seemed to be rounded/bulging on top. I can push it down like it's just a bent plastic cap and it pops back up. I found some docs that say it's either a cap or transformer. I'll have to disassemble to get to the other side of the board to check the cap, how do I verify the transformer is good?

[wizard333]

Shorted filter cap or shorted resistor across the 2 1 stage caps in series would be my first guess there.

[Exodus5]

Shorted filter cap or shorted resistor across the 2 1 stage caps in series would be my first guess there.

Thank you. C53 is one of those caps. I just need to take the board out, I guess. I hope I don't break anything else doing that.

[User Name]

was the "pop" from the speaker itself or from the cabinet?

was it the same from the hum?

have you removed the cap yet?

can you post a pic of the board with the suspect cap?

have you done any type of electronic repair before?

if you are limited to test equipment, one way you can test that transformer is by using a "dim bulb cluge".

i would suspect that a filter cap is shorted based on what you have stated. when the filter cap shorted, it pretty much caused the rectifier to start dumping straight to ground and there for caused the surge on the primary blowing the fuse.

[Steve Conner]

I had a similar problem with a tube amp recently, though it was one of my own homebuilds, not a Crate. It turned out to be a blown diode in the rectifier. Diodes don't fail often, but they go dead short when they do. Electrolytic caps are more likely to dry out or leak current, than become a complete short circuit.

If you can push down the bulging top of that capacitor, and it feels springy, it's no cause for concern. It is just what you said, a plastic cap, and there's a metal can underneath. If the guts were under pressure, then the metal can itself would bulge, and that would feel rock hard. Of course, the capacitor may still be faulty.

We call the "dim bulb cluge" the "light bulb limiter" here, just so you know if you try searching the forum for it.

[Exodus5]

was the "pop" from the speaker itself or from the cabinet?
It seemed to be a speaker pop but it was so loud it may have been electrical as well
was it the same from the hum?
I'm not sure what you're saying. I switched off standby and there was a loud speaker/maybe other pop! Then a loud hum for less than a second before complete power loss due to fuse burning out
have you removed the cap yet?
No I have not. I thought I had a local source but when I got home I realized their CAP was too tall.
can you post a pic of the board with the suspect cap?
After further checking it doesn't appear that the bulge is related. I need to check the cap which leads to the next two questions.
have you done any type of electronic repair before?
I studied it some in College, but never used what I learned and that was many many moons ago. I'm just getting back into it.
if you are limited to test equipment, one way you can test that transformer is by using a "dim bulb cluge".
I'llhave to look that up
i would suspect that a filter cap is shorted based on what you have stated. when the filter cap shorted, it pretty much caused the rectifier to start dumping straight to ground and there for caused the surge on the primary blowing the fuse.

[wizard333]

You can test the caps and resistors in the filter section with a VOM. MAKE SURE THEY ARE DRAINED! On *MOST* amps turning the amp off, unplugging it, and leaving the standby ON will drain them in a few minutes, but CHECK DC voltage at the positive side of the caps to ground to make SURE.

If the cap shows 0 resistance across it, its shorted. If the resistor shows either 0 or open, its bad, Usually if you lose a resistor there, the cap goes with it.

[Exodus5]

You can test the caps and resistors in the filter section with a VOM. MAKE SURE THEY ARE DRAINED! On *MOST* amps turning the amp off, unplugging it, and leaving the standby ON will drain them in a few minutes, but CHECK DC voltage at the positive side of the caps to ground to make SURE.

If the cap shows 0 resistance across it, its shorted. If the resistor shows either 0 or open, its bad, Usually if you lose a resistor there, the cap goes with it.

Thanks. Correct me if I'm wrong, but I still have to remove the board to access the legs of the capacitor, right?

Also, I realized I was using a 125v regular 3 amp fuse instead of the 250v 3amp slo-blo fuse. With the slo blo it makes a loud hum for a couple of seconds but still eventually pops the fuse.

[Exodus5]

I had a similar problem with a tube amp recently, though it was one of my own homebuilds, not a Crate. It turned out to be a blown diode in the rectifier. Diodes don't fail often, but they go dead short when they do. Electrolytic caps are more likely to dry out or leak current, than become a complete short circuit.

If you can push down the bulging top of that capacitor, and it feels springy, it's no cause for concern. It is just what you said, a plastic cap, and there's a metal can underneath. If the guts were under pressure, then the metal can itself would bulge, and that would feel rock hard. Of course, the capacitor may still be faulty.

We call the "dim bulb cluge" the "light bulb limiter" here, just so you know if you try searching the forum for it.

Thanks for the info. I checked all the caps none are shorted. I'm also checking resistors, but I'm starting to worry on this...
I still need to look into this light bulb limiter thing...

[Steve Conner]

Well, I'd check the diodes before the resistors. I don't have the schematic, but there should be four in the power supply, and maybe another two, four or six near the power tubes. 1N4007, 1N5408 or whatever. Desolder one end of each diode out of the board, and use the diode check function on your multimeter.

I also had a fancy Focusrite mic pre blow out on me with similar symptoms, it hummed loudly and popped its mains fuse. In this case the problem was obvious, because the shorted diode incinerated itself and left a blackened crater in the board.

The light bulb limiter is just a light bulb connected in series with one of the incoming AC lines to your equipment. If there's a short circuit in there, then instead of blowing a fuse or burning out components, the lightbulb just comes on near full brightness. Once you've fixed the problem, the bulb will just light dimly, and you can risk applying the line voltage directly.

[Exodus5]

Well, I'd check the diodes before the resistors. I don't have the schematic, but there should be four in the power supply, and maybe another two, four or six near the power tubes. 1N4007, 1N5408 or whatever. Desolder one end of each diode out of the board, and use the diode check function on your multimeter.

I also had a fancy Focusrite mic pre blow out on me with similar symptoms, it hummed loudly and popped its mains fuse. In this case the problem was obvious, because the shorted diode incinerated itself and left a blackened crater in the board.

The light bulb limiter is just a light bulb connected in series with one of the incoming AC lines to your equipment. If there's a short circuit in there, then instead of blowing a fuse or burning out components, the lightbulb just comes on near full brightness. Once you've fixed the problem, the bulb will just light dimly, and you can risk applying the line voltage directly.

Thanks. I actually noticed that group of diodes and went to check them with the meter. I don't know if I've EVER checked a diode since I learned to do it in college and I checked them still in the board. I guess I'll have to desolder some things. I've already got replacement filter caps coming since I've got it disassembled.

[User Name]

on the diodes,

black lead on the banded side, (cathode), you should see a value of about .7 on the meter in the diode function. if you see .0, then the diode is bad.

[Exodus5]

on the diodes,

black lead on the banded side, (cathode), you should see a value of about .7 on the meter in the diode function. if you see .0, then the diode is bad.

Thanks for the help guys!
I checked the diodes like you said and three came up with .618 while one gave about .653. I guess they're good. I didn't check the other diodes (I ran home on my lunch hour). Now I'm determined to find this. I found a page that told how to check the transformers, but their wire description didn't seem to fit what is in my amp. I didn't find anything meeting ground, "so I got that goin for me..which is nice".
Nothing appears burnt. No bulging caps or anything. I did notice that when checking resistance of some caps they immediately went above 15K which means they're good (right?) but some were extremely slow to count up. I'm guessing this is just normal?
I think I checked every resistor and all were exactly as they should be. I still need to check the other diodes on the board.
I forgot to add, I noticed marks/indicators on the board with a sharpie that lead me to believe I am not the first person to service this amp. It looks like the fuse holder and maybe one resistor might have been repaired/replaced.

[Exodus5]

Update:
I checked the capacitors and found c9 and c10 (at the MID control) virtually dead. I found several resistors that were no longer near their rating (some almost half). I also found that both diodes on one side of the power tube sub-board are dead. I have found no smoking gun yet. I still haven't been able to properly troubleshoot the transformers except to say none of the leads appear to go to ground.
One question for now that I think I already know the answer to...This board has many WIMA MKS "box-style" film capacitors. I think they're metal film. In my preparations for modding this amp I ordered several Sprauge Orange-Drop caps and one of them is the exact rating to replace C9 (one of the aforementioned box caps). Is this OK to do or should I just replace it with a like part? My investigation seems to indicate it is OK, but I want some more experienced opinions.

[Steve Conner]

Watch out! You can't always check resistors and capacitors in circuit, they may read lower than their actual value because of other stuff in parallel with them.

You said you found two dead diodes on the power amp sub-board. Did they fail short? If so, there's your problem. They'll short out the main B+ and blow the fuse.

Replace them and I bet the amp will be fine.