Acoustic 320 blows fuses. Expert advice needed..

[Jack W.]

Ok, I got it up on Variac 120VAC, AC current apron. 200mA. Vcc reads 97.5Vdc.
No TO3's installed yet. Poked around w/chop stick..all seems fine. No arc/ overcurrent.
With my limited SS experience...I notice on the output (no load) a DC element comes and goes...in other words the zero line on the scope will drop 1 or 2 volts the back to zero...30-40 secs or so later it may do it again or rise 1 or 2 volts...
The Vcc (Fluke 87) shows no change at 97.5Vdc when this happens..
Is this normal? I know dc on the output is fatal, so I'm leery of installing the output xsistors if this is still unstable.

[Amp Kat]

Sounds like a faulty cap giving it up when it gets hot or charged. You don't have to put all of the TO-3's back in to test it. One pair will do you just won't get full wattage.

[Jack W.]

Thanks Amp Kat, when I first started on this, I notice all original parts..so on this board I replaced all the electrolytics, I left all the ceramic discs though. I did replace the MF box caps. I also replaced the output cap with a Sprague. But with this done and the goofy intermittent dc fluctuations... you have me thinking of temps... I'll try again & try heating/cooling to see if it aggravates it before I run it w/two TO3's.

[Jack W.]

I did a heat/cool test on the PCB, no TO3's...didn't affect the odd DC bouncing. Put one Pair of TO3's in. Variac thru 60w : Used 8 ohm load & scoped.
Line @ 115Vac
Iac (line)= 140mA
Vcc= 80Vdc
Vdc out + = 40.6Vdc (plus output cap)
Vdc out - = 0.0Vdc (neg. output cap)
Ran 1khz sine to check output, saw expected heavy xover but symmetrical wave.
Didn't push it, still no more arcing. The bounce seems to have gone...or if it's there it's very, very minimal <.5vdc. So far so good. Will try to bring it up full pwr. Question now is...show I have a load on it at this point? If so should I use stated 2ohm load? I will use 2 ohms when all TO3's are put in prior to biasing.

[R.G.]

Think about what a hairline crack in a PCB trace or solder joint might do.

[Jack W.]

Thanks R.G. I will break out the high powered peepers. Do some continuity checks.
I appreciate it.

[Jack W.]

This is making me nuts...I ran it all day with one pair of TO3's in. Had signal w/crossover - bias set low. Had some dc bounce back. Replaced all .001 & .01 ceramic caps. Re sweat all solder joints.
Seems much more stable but still a bit of bouncing. <.3Vdc on scope @ 8 ohm load. Put two more TO3's in (another pair of MJ15003G - all 8 matched) here's what happened.
4 TO3's installed & tested for insulation.
Variac @ approx 90VAC
Pri current AC - approx. 145 mA
100w lamp
Vcc at supply cap = +75Vdc
8 Ohms load scoped
- - ARC ! BOOM! - medium arcing sound/click - Lamp started to glow! Shut down.
Checked the TO3's, all OK. Looked around, I don't see any flash, no burnt marks, no smells...
Slowly brought up to voltage - did it again, ARC/Lamp lit. Let is sit an hour. Slowly pwr up. Full power @120VAC 100w bulb - Won't arc ! Put signal thru it, xover large signal. I just don't trust this thing...
I still think it may be in the xformer(??) I'm going to take the bell off and take a quick look. I hear the arcing before the lamp lights.
Am I done here? One of the demon units sent to break me down???

[Jack W.]

After clearing my head & putting DDM meters at key points, it seemed to me that the arcing may be the result of the supply collapsing. The arching perhaps the output cap discharging at the output jacks across to the bell of the PT as the supply quickly swings south (?).
Anyway, here's some info for anyone else working on 320's. I find that each preamp PCB has a cap across the rails so I replaced all of them (50 & 100volt electrolytics.) Put all 8 TO3's in, bias low & powered up. No problems, no arcing, output scope stable. Xover wave form @ 10mV 1Khz input at combined preamp in. (back of amp)
Powered up again, set bias (current measured at primary, no signal, line voltage 120VAC, 2 ohm resistive load) to min required 45 watts as stated in service manual (i.e. 375mA). Amp seems to work fine, no fuzz, nice clean, loud signal.
Thanks again for all the help RG, AmpKat, gbono & others..
Your right..by dividing up each ckt and monitoring current & voltages with a bit of patience is the way to solve the toughest problems...
One odd thing I still need to figure out. When amp is turned on, there is a hum for about 30 secs. that disappears as the amp stabilizes. It's shown on the scope as the supply's sawtooth ripple. Not real loud but audible. I believe it's just the filter cap ramping up but shouldn't the amp just thump on like other SS amps?