Ampeg Vt40 Trouble

[methodofcontrol]

this is an early 70's top mount version.
i bought this amp a few months ago in fair working order. it had a few minor
issues, but it works. so i finally got it on the bench to take care of the minor issues last night. my plan was to change the power cord, the power light, and the fuse holder and resolder any potential problem joints. simple enough, right?
ok well....the power cord that came w/ the amp when i bought it was already 3 pronged, it was just a cheesy grey cable so i changed it for an 18ga black one from the hardware store. no big deal. i grounded my cable to the center post on the power tranny standoff, whereas before the previous cable was poorly grounded by folding the wire over an inside washer for the standby switch.
the power light was a simple swap. old one out. new one in. it works now.
the fuse holder also should've been a cakewalk, i was only changing it because the one in there wasn't aesthetically correct. it just so happens the correct holder i was planning on installing was defective, i got it in. then the plastic snapped while i was screwing the fuse in. so i pulled the whole assembly and reinstalled the one that was in there prior.
i resoldered all of the pots, (sucked out old solder, melted fresh solder) and a few random wires that just didn't look that great. all of the resoldering was done on the pot board.
once i was finished i hooked up a jack to the speaker wires that are supposed to connect to the combo speakers. this way i could leave the chasis out and still connect an 8 ohm speaker load. the tubes were all out. i turned the power on and saw the power light working, so i turned the standby on and within a few seconds i heard a ticking type noise coming clearly from one of the external electrolytics. andi saw a small bit of smoke coming from approximately where the 12dw7 lies. (if you have a schematic, you'll see pin 1 of the 12dw7 connects directly to the can-cap in question. it's supposed to have 354v.)
i turned the standby off and the ticking stopped. i left the amp on for a moment so i could take a quick visual inspection and to smell if something had been burning. everything seemed fine, so i turned the lights off and turned the standby back on hoping to see some arcing or something so i would know if that was what was ticking. no luck, so i turned the amp off and walked away for about 2 hours.
i came back and figured i would try to give it another diagnosis. i wanted to see if the ticking was any better w/ the tubes in, so i cleaned all of the pins w/ deoxit and installed all of the tubes. i turned the amp on and everything seemed fine. i measured the voltage off pin 1 from the 12dw7 w/ the standby switch off and it only read 90vdc. i thought the standby switch would have to be on for it to read 354v so i let it warm up for about a minute then i flipped the standby switch on. as soon as the standby switch flipped there was a LOUD SNAP of blue light from right near the switch. but no fuses blew and no componants seem visually damaged, i probably left the amp on for 3 long seconds after the loud snap of electricity. i didn't want to risk shock from turning the amp off via the switches, so i unplugged it from the wall and came here to seek advice.
anyone have any idea what's going on?

[methodofcontrol]

just a thought....the neon pilot lights are not polarized, are they? i couldn't figure that out, but i tested to see if the light worked either way and it did so i figured it didn't matter. i hope that's not part of my problem. where the lamp hooks up seems to be an AC voltage, so i assume polarity is not an issue.

[Steve Conner]

It sounds like the electrolytic can cap has gone leaky. The smoke and later blue flash was probably a dropper resistor overheating and eventually blowing up because of the bad cap drawing too much current through it. I'd replace all the electrolytics and the dead resistor as well as any other dropper resistors that look burnt or measure wrong on an ohmmeter.

Neon lights are not polarised BTW.

[Amp Kat]

I guess it depends on the Neon used as to whether it's polarity sensitive but most are like NE 2 but much higher voltage. The schematic shows no polarity however and Stephen is right on with the voltage as it gets 120 for startup and the standby and power neon are the same. I do think your problem is the standby switch and would replace it asap.

[methodofcontrol]

thanks for the advice guys....but let's review for a moment:
should i:
A) replace all of the electrolytic caps (probably order through fliptops.net
unless someone knows a better place. there's no room iside the chasis to order the multisections individually and mount them on another board)
B) replace only the 'ticking' cap
c) replace the standby switch
D)replace can caps AND standby switch

also, as of yet i haven't found any burnt resistors. i will give another look. and to clarify, when i saw the blue flash and heard the loud SNAP...it was from the vicinity of the standby switch, but i can't say it was from the switch itself. i can test the switch for continuity w/ my meter but i'm not certain that will determine what the switch will do at voltage.