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i'm on a roll... the kgsshv


kevin gilmore

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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?

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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 by spritzer
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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?

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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 by Emooze
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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?

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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.

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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?

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