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Hi Don,
Thanks for bringing us back to the original question and for mentioning one of the options I hadn't listed!? Turning the transformer OFF at a particular time relative to the AC voltage cycle might cause a voltage spike as the flux collapses faster than during a regular cycle!? If that's what causes this failure, how would a pause in STBY mode mitigate it?
Your point about low magnetic remanance interests me.? If low remanance means the amount of magnetization still present after removing voltage at the primary is low but persists indefinitely, then the timing for opening the switch might not affect inrush current next turn-on.? If low remanance means that the magnetization remains for a short time after power removal, then simply waiting a little while before turning the switch back on could avoid a heavy inrush current but probably not affect a turn-on HV spike at the secondary.
Here's another idea about the possibility of a turn-on timing causing a voltage spike at the secondary.? If the remanent magnetic field has one polarization and the voltage at the moment of turn-on moves it towards the other polarity, then there's even more room for a fast change in flux, and the accompanying voltage spike.
I think it's time to hook this transformer up to the storage 'scope and try a few things.
Contemplating that setup (how shall I simulate loads on the secondaries?) raises another idea.? When filament secondaries are connected to cold filaments, the heavy current draw on those windings might load down the rapidly rising flux at turn-on, reducing the turn-on voltage spike magnitude.? Turn-on immediately after turn-off means no cold filaments would be available to do this.
Cheers,
Halden VE7UTS