V4 ONLY works on my bench

[nsubulysses]

I am experiencing such a strange phenomena.

I have a V4 that only works on my bench. On any other outlet it immediately blows the fuse when the standby is flipped. On my bench it works perfect, I've turned it off and on 100 times no problem. Puts out proper power and PS voltagees check normal. On another outlet it will blow the fuse every single time once the standby is flipped. It has done this in a practice space, home of the owner, and also on various other outlets that are in my building but not on my bench. I don't understand what's special about my bench. Even when I bypasss everything on my bench and plug directly into the wall it works fine.

I have run the amp through a bulb limiter to try to sort this and have found a few clues.

- The short comes after the PT, with both HV and filament secondary disconnected it does not light the bulb. Re-attach filament winding and no short. Re-attach HV winding and bulb lights brightly.
- When I turn on the power you can hear a buzz through the speakers. How is this possible if the standby is not engaged? In the past I have experienced an Ampeg V2 that did this and was mystified but it worked normally on my bench and other locations as well.
- When the short happens it ALWAYS blows diode D5. Occasionally it blows rectifier diodes too but not always. A lot of good 1N4007s have gone down in this battle
- I have re-capped the power supply but cannot find a discrepancy. Maybe I am missing something. STilll have never experienced something that only works on certain outlets and catastrophically fails on others.

I have checked the wiring against my V4 from the AC outlet up to the PT secondaries and it's exactly the same. Any thoughts? I'm at a loss for now.

[Alex R]

do you mean even with the chassis in the amp and the amp upright it works ok on your bench?

[52 Bill]

I would first check the wiring to your ac outlets.

[Leo_Gnardo]

Had a similar problem with a VT40. Turns out the reverb tank was shorting against some part of the circuit. Of course the amp always worked fine, with the cover off. Seeing arc marks on the reverb box I tracked down what it was hitting inside the amp with the cover on. Glad I found the problem & got 'er fixed - it's now owned by Earl Slick & he likes it a lot.

[g1]

I would first check the wiring to your ac outlets.

Agree, there has to be something different about your outlet than all the others.
And did you mean D6, the bias diode, or D5 the weird grounding diode?

[g1]

And one thing to look out for is replacement cap cans that are not insulated from chassis. The originals were insulated, if the replacements are not, instant short from the middle of the "totem pole".

[nsubulysses]

Thanks G1 I will check and recheck. I used F&T radials as replacements so I think I'm good on that front but definitely worth looking at. I'm dumbfounded, or maybe just dumb.....

I really wanna know what's so special about my bencH!

[nsubulysses]

Agree, there has to be something different about your outlet than all the others.
And did you mean D6, the bias diode, or D5 the weird grounding diode?

And yes thank you I misread the schema. it is grounding diode D5 that always blows.

[nsubulysses]

Alex R and Leo -- good thought on the cap replacement making contact with the reverb tank. I actually thought that was happening and re-positioned the caps because they were very close to the tank. Now they are completely clear of it. I thought I solved it when I did that. That was about the 3rd time I "solved" it.

I appreciate all your suggestions I need a fresh perspective

[TimmyP1955]

Test an extension cord, plug it into an outlet on which the amp failed, then run the cord to your bench. Measure from cord hot to bench hot, cord neutral to bench hot, cord ground to bench hot, etc. etc. to see what's up with the bench power. My guess is that the bench has an open ground, or reverse polarity with a bootleg ground (which I doubt, I think it would have bitten you at some point).

[Diablo]

My V4 also will buzz quietly through the speakers when on standby. It also does it with the output tubes pulled, so it has to be magnetic coupling between the PT and OT. When I flip the standby to power up the B+, the buzz goes away.

[nsubulysses]

Here are my wacky findings

Bench power
ground to hot -- 63V
ground to neutral -- 3V
hot to neutral -- 121V

Regular power
ground to hot -- 124V
ground to neutral -- 0V
hot to neutral --124V

Bench ground to normal ground tests OL. An aside question, when testing in continuity mode one DMM I have does not beep. The other one beeps for a second but it will not sustain it and ring out like when there is normal continuity. Both are Flukes. I bought em cheap but they're not cheapies. Why is this?

So i have a floating ground on my bench? Seems like if I had no ground my meter would just show random numbers when testing from ground to either prong.

Also I had a Band-Master plugged in on bench power and noticed that the ground switch will eliminate the buzz when flipped one direction. When plugging into normal power the ground switch does nothing. Operating as if it did not have a 3 prong on it, which it does.

[Enzo]

Continuity tests, like resistance checks can be confused by any residual voltage in the circuit.

Also on most FLukes I've used, the continuity test is also the diode test. A dead short between the probes results in a steady beep tone. Put the probes across a good diode junction or transistor junction, and it beeps once to indicate the junction voltage is within a reasonable range.

Rewire your bench, or at least fix the wiring.

[nsubulysses]

My bench is an isolation transformer??

[g1]

My bench is an isolation transformer??

No, it's a safety hazard . An isolation transformer isolates hot and neutral, you have only isolated ground.