On 08/04/2020 22:24, Rob Jarratt via cctalk wrote: > I will look at all the suggestions, particularly of a failure on the > secondary side. Something must have burned up, because there was a distinct > burning smell after the initial failure, although I have never been able to > see any physical damage to anything, despite looking many times. > > > > But the thing that really puzzles me is that, after correcting the probes to > include the D19 anode, there doesn’t seem to be anything that would cause D19 > to trigger. Am I reading the trace wrong? >
The only paths to ground for VCC are C12, E3 and D19. The fact that VCC is stable for a period of time and then drops suddenly suggests it must be D19. It looks like there could be a pulse on D19 gate but it is in the order of 1-2 uS before VCC drops. There are more pulses prior to that but I wonder if some of that is just induced noise? It will be interesting to see what's happening on the secondary side, particularly with Q2. I guess you have already checked the obvious things like a short circuit on one of the outputs? Also worth checking the rectifier diodes D11, D12, D22 and D23 (I think). Matt
