You got a part number for that blue-ish film cap in the PSU, Birgir?

Not really as I bought just over one thousand of them off ebay some years back and they were custom made for some company by S+M. 

So based on my playing around with LTSpice and some measurements, one of the 680u caps has been shorted out. The voltages across the two 500k resistors aren't stable. I pulled up the one cap I suspected was gone and lo and behold, the top of the solder pad was gone and there was some thermal damage to the cap bottom. The issue I have now is, the top solder pad is gone and that's where the trace connects. Is there an easy fix to this or do I need a new board?

Just connect a wire to the nearest part on the same trace.  Does the same thing... 

 

Edit:  One thing, try to use just enough solder when soldering the caps and not drown the joint in solder.  Just enough to bridge it and there shouldn't be any issues. 

Edited April 27, 201313 yr by spritzer

So based on my playing around with LTSpice and some measurements, one of the 680u caps has been shorted out. The voltages across the two 500k resistors aren't stable. I pulled up the one cap I suspected was gone and lo and behold, the top of the solder pad was gone and there was some thermal damage to the cap bottom. The issue I have now is, the top solder pad is gone and that's where the trace connects. Is there an easy fix to this or do I need a new board?

 

how did this happen?

I'm not sure, I thought I made sure to have enough space between board and caps but apparently not. I'm worried that the root cause is something else and magic smoke will appear when I fix this issue.

You can add insulation to the board with tape or something like that.  Nail polish works very well to to mask exposed traces. 

why have i never had this problem!  making me paranoid

Never had it either, even on PSU's I fucked up in the design phase and had to fix, exposing the ground next to the pads.  That was at 700V... 

I really don't think there is a problem with the boards. My second build is going very well and without any trouble ( fingers crossed) but I am very OCD and triple check ( or more) every step and every solder.

sometimes... shit happens...

and that is why I paid attention to your posts and use lots of kapton tape. Better safe than sorry 

Just hate troubleshooting, harder than stuffing.

Edited April 28, 201313 yr by eggil

why have i never had this problem!  making me paranoid

Different board house?

So I fixed the cap issue with some 600V wire to the 500k resistor in parallel. Not very pretty and apparently not the end. Now I'm blowing fuses just about every time I power-up so I must have fried something somewhere else without making it obvious what it was. Reading back through some posts, it's also possible that a 2A fast fuse just isn't enough even with one thermistor. I'm going to get some bigger fuses and see what happens.

 

Update: the 3A fuses go out in spectacular fashion almost immediately. What's the best way to tell if I've blown something? Measure resistance across the various transistors?

Edited April 28, 201313 yr by Emooze

2A fuse works for 240V but is definately too too small for 120V. To charge the same caps, the amp draws twice the current from 120V. I think even the 3.15A fuse wouldn't survive without at least one thermister.

Come to think of it... The transformer is only 2*450V@ 125mA, so it is outputting 3~4 times the rated VA at the start up.... Possibly bad for the transformer?

I use 2AT (slow blow) on 230V but I never tried anything else.  No thermistors either. 

 

How are you using just one thermistor on 117V?  I'd put one on each leg just to be safe.  Using one on 230V works as it's in series with the windings. 

I've just got the one in series after the fuse and before it goes to both primary windings. It hadn't occurred to me to use one for each winding. I can try that, I just need to order more.

That's how Pass does in on the F amps.  Makes sense if neutral and live were swapped. 

I wired two in series right after the line before the primary.

One thermistor definately doesn't work with 2A and two thermistors in series is pushing it with 2A, so I figured that one thermistor with 3A is pushing it as well.

Anyway, turning on the amp with only one thermistor still dims the light very noticeably while two is much better.

... it's also possible that a 2A fast fuse just isn't enough even with one thermistor...

 

 

Have you tried "slow-blow"? Maybe the fast blow could be the issue?

That's how Pass does in on the F amps.  Makes sense if neutral and live were swapped. 

On your voltage there is only one used, if you look carefully, placed /between/ the two primary windings. The other is a ground loop breaker.

I know, see my post above. 

So I fixed the cap issue with some 600V wire to the 500k resistor in parallel. Not very pretty and apparently not the end. Now I'm blowing fuses just about every time I power-up so I must have fried something somewhere else without making it obvious what it was. Reading back through some posts, it's also possible that a 2A fast fuse just isn't enough even with one thermistor. I'm going to get some bigger fuses and see what happens.

 

Update: the 3A fuses go out in spectacular fashion almost immediately. What's the best way to tell if I've blown something? Measure resistance across the various transistors?

 

You didn't connect the earths between the secondary windings on the poweer supply board to the chassis, did you?

Good point. 

No, I'm not even in a chassis yet. This is on a wood table.