Phil was only on about the voltage rating issue.
It was a 5% tolerance component. Maybe at 5% and with allowance for aging it will stay within the limits required by the design. But all bets would be off if I stuck in a 5% component of some other value.
If this had been the 200K component, (also a non-standard value), I'd happily have stuck in a 220K of 180K, and thought nothing of it. But it's a low value component connected to the feedback system. Who's to say what a 10% change to 5.6 ohms, let alone a 30% change to 4.7 ohms would do. Maybe nothing, but it's not worth the possible trouble to find out.
It's credible, given that there was no sign of distress, but it's odd that Q1 was taken out as well. I still can't see a mechanism for that, which is why I was somewhat surprised that the repair worked.
- but if they do fail for a 'real' reason, that failure is usually
Well, you wouldn't if they've been correctly specified. Phil was essentially arguing that I could substitute a component with a lower voltage rating.
As part of my research for this reply, I found a data sheet for a 0.22 ohm 0.25 W fusible, which can be reached from this page
It quotes a number of different voltage levels. In particular, it quotes a specific "Maximum withstand voltage after fusing." To my mind, in the particular circuit in question, that value would have to be 340V or greater.
It's not really the case that I was trying to match the exact characteristics. Indeed, I haven't, because a 4.7 ohm fusible requires more current to fail than a 6.2 ohm fusible of the same power rating.
Instead, my aim was to reproduce the original resistance, within the tolerance chosen by the designer, to minimise the chance of destabilising the circuit, while preserving a safe failure more should the transistor fail shorted. In this context, by safe I mean a failure mode that would have a fair chance of protecting the upstream components, thus leaving the board in a repairable state.
Sylvia.