Buzz/click!-dead when switching off standby

[Exodus5]

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.

Well we're halfway there. I replaced the two bad diodes and reassembled everything. Powered her up and when I switched off standby...nothing. I changed cables/guitars, checked my volume settings but nothing. When I turn the volume way up I do get an awful gurgly-sounding hiss. On the gain channel I can very faintly hear the guitar if I turn it way up but it's mostly drowned out by the hissing noise. I wonder what else I've broken. I swapped out to 3 different sets of power tubes, so I think that rules them out for now. I DID change the filter caps since I was already in there. When I get home this afternoon I plan to put the original caps back in to see if that could be it. Am I on the right track?

[wizard333]

You need a capacitance meter (not a multi meter with capacitance setting......they are usually wildly inaccurate) to check the caps, and you really cant check many caps/resistors in the circuit. You can generally get an idea if they are a dead short though, but checking for specified value is very problematic.

When you flip standby and you get a ffffZZZT *POP* then a blown fuse, the cause is always the same: Something is shorted, almost always to ground. The mission now is to find out what.

Check your schematic and see whether the first stage filtering is before or after the standby. If its after, you know its not the first stage filtering; if it were, you'd get the death pop right after turning power on. Sometimes the schems arent accurate; drain everything and check it manually for continuity and see where the standby is in the system.

Good things: Its not your PT (at least, its not shorting out), its not your heaters/heater circuit, and depending on the amp, it may not be your first stage filtering or the related resistors. If its not that, its not your rectifier either, because a standby after the first stage filtering is also after the rectifier.

Bad things: It something else and you dont know what.

I replacing the diodes stopped the immediate death but leaves you with no output, there is something else wrong. Are you sure you put the diodes in the right direction? Check plate voltages (power and pre-amp), check screen voltages at pin 4 of your power tubes. Are those things reasonable? Check idle current. You can show nice plate voltages and 0 idle current if your screen grid resistors are dead. If all that looks good, start looking for shorts or lack of continuity elsewhere.

Try to isolate it by stage. Does your amp have an effect loop (if memory serves, a BV does have one). Plug into the return of the effects loop. Same symptom? If so then your problem is after that, i.e. very close to the end of the circuit. If plugging into the loop return gives you a nice loud guitar signal, then your problem is before that. Is your problem on both channels? If so then it occurs after the channels come back together or before they split. Does manipulating channel volumes or tone controls affect it? If so its before that. If not, its after. You can use things like that to isolate the problem before you start fooling with the meter or measureing values, and it can help you focus in on where to look.

[wizard333]

PS: Blue Voodoos have notoriously crappy soldering, which often causes all knds of problems.

One thing I'd do while I had that amp apart was re-solder every single point by hand. You'll be glad you did.

[Exodus5]

PS: Blue Voodoos have notoriously crappy soldering, which often causes all knds of problems.

One thing I'd do while I had that amp apart was re-solder every single point by hand. You'll be glad you did.

The sad thing is I think their soldering looks great compared to my work
Actually, I wanted to ask what you guys usually use for this purpose since I've never worked a PC board before. I'm using some tiny 60/40 I got at Radio Shack.
I can verify with no doubt that I've installed the diodes back in the correct alignment although not quite as perfectly centered as the originals. I found all the schematics on this thread:
http://music-electronics-forum.com/t672/
The bad diodes were D3 and D4 on the first schematic page (they're located 1/2 way down the page) and the one Cap I removed to test was C9 (located on the 3rd schematic page, all courtesy of Enzo) which is part of the MID circuit on the clean channel. I also replaced (refering back to schematic 3 on the aforementioned thread) C52 and C53 with brand new caps of the same ratings. I've been bothering Steve Conner (much thanks) on PM and he mentioned the wire connections from the transformers, which I was sure I had right (labeled them, took pictures) but will verify again tonight.
I have to admit I don't know exactly how or where to check plate voltages safely. My meter is rated for 600V DC. Is that enough? I did connect to the return and I do get good sound from it (not too loud and kinda sounds like I have gain on it). At this point I get a loud buzzing when I turn up volume (this affects both channels) and if I put the amp back on standby the noise continues for maybe 6 seconds. I'm thinking that has to be Cap related. Also, when the buzzing is going on if I touch the front panel the buzzing volume drops a tiny bit. I swear to everyone here I didn't touch anything else except replacing filter caps (I've put in the original 100uF's due to improper voltage rating but left in the 47uF's since they are new and shouldn't be the problem), removing one cap to check it (C9) and replacing those bad diodes on the power tube board. That means that my problem now is either a wrong wire connection, a bad solder on C9, bad solder/cap on those two filter caps (C52 and C53) or the diode shorting out has damaged something else. That last one is the one I fear, of course, because the caps were brand new and my soldering (though not as good as probably 90% of the guys here) actually came out suprisingly well. It isn't the physical tubes themselves because I have 4 sets of power tubes and two sets of preamp tubes and I've tried every combination to no avail.