Blackstar Artist 10 high voltage collapsing

[Stratfordade]

Most of those solder joints (at least in the attached pic) should be touched up.

Thank you but it’s a through-hole board (which I hate as desoldering difficult) and the joints on the other side which is main soldering side are all good. What you’re seeing is the solder that comes back through the plated hole.

[Mick Bailey]

Problem is they are hard to remove without special tools and my desoldering gun doesn't cut it!

I use a sharp pair of cutters to clip through the can as low down as I can get. The plastic base can then be removed, leaving the soldered stubs in the board, which are easily removed. Make sure to cut in line with the pads, then you're not squeezing them as the can cuts through. Alternatively, use Chipquik. I've used the clip method hundreds of times. Actually hundreds, no exaggeration - Peavey PA heads sometimes have a plague of failures.

[Stratfordade]

I use a sharp pair of cutters to clip through the can as low down as I can get. The plastic base can then be removed, leaving the soldered stubs in the board, which are easily removed. Make sure to cut in line with the pads, then you're not squeezing them as the can cuts through. Alternatively, use Chipquik. I've used the clip method hundreds of times. Actually hundreds, no exaggeration - Peavey PA heads sometimes have a plague of failures.

Thanks I’ll try this technique next time, though before reading this I’d got the cap off whole by melting at the pads while rocking the can side to side. Whole cap came away leaving the pads.

Ironically I then realised it was probably the 22nF metal film cap that was leaky, not the electrolytic which I think is too large (10uF) to be a decoupling cap but I’ll just replace both and see what happens to the grid voltage.

[Stratfordade]

Well I’ve lost count of how often I’ve removed and installed main PCB, which involves removing rear socket PCB and many spade connections (though I have sister amp to make sure all connections go back correctly).

Replaced suspect cap and I’m still seeing grid voltage at almost 300V when it’s zero on the other amp, and that’s with suspect cap disconnected! It should be a cinch as only a few components involved in grid circuit, but it’s one of those PCBs where the tracks sit under lacquer so are not obvious, and there’s no schematic to help with logical investigation.

I’m now just comparing resistance measurements in the problem area between the problem amp and its working ‘sister’, so may get lucky finding what’s causing that sky high grid voltage but don’t hold your breath!

[Stratfordade]

I might be onto something. There’s an N type power mosfet IRF830 involved in voltage switching in some way (the gate is driven from 0 to +16V on the ‘good’ amp when input jack inserted.

On the ‘bad’ amp all terminals are at ground regardless, and it’s a short across gate and source but measures open on the good one. Maybe that short is sending high voltage the wrong direction?

Have a new one to try.

[Stratfordade]

All fixed now. It was a combination of bad power MOSFET, and a cold solder joint. Very difficult with double-sided board to know what’s through plated and what’s not. Very hard board to investigate.