21-04-2013, 02:24 PM
So, more tests:-
It dawned on me that as the HT comes on with a bang due to the delay circuit there might be a brief burst of oscillation at that point. There isn't
Next I checked and changed C503. It measures 9.29n on my Cap Meter and >200M on my Megger - Ah, well. Of course changing it made no difference.
Finally I upped the Capacitor across the Transformer Primary to 10n - sorry Lawrence, I had to know. Lo, it oscillates, at about 300kHz, with about 180Vp-p at the 5763 Anode. That's not enough to make the Secondary circuits work - the 'old' circuit says 450Vp-p at 20kHz. So, is that simply a spurious effect of fitting a wrong Capacitor, or does it imply a Transformer problem? With 1n fitted there was oscillation on the 'scope but it's very high frequency and low amplitude. I'd put it down to general crud but I suppose it may not be. I'll put the 1n back and see what's what.
By the way, I did dig the corpse of the old Hunts out of the scrap bits pile and it does say .001.
Alan
It dawned on me that as the HT comes on with a bang due to the delay circuit there might be a brief burst of oscillation at that point. There isn't
Next I checked and changed C503. It measures 9.29n on my Cap Meter and >200M on my Megger - Ah, well. Of course changing it made no difference.
Finally I upped the Capacitor across the Transformer Primary to 10n - sorry Lawrence, I had to know. Lo, it oscillates, at about 300kHz, with about 180Vp-p at the 5763 Anode. That's not enough to make the Secondary circuits work - the 'old' circuit says 450Vp-p at 20kHz. So, is that simply a spurious effect of fitting a wrong Capacitor, or does it imply a Transformer problem? With 1n fitted there was oscillation on the 'scope but it's very high frequency and low amplitude. I'd put it down to general crud but I suppose it may not be. I'll put the 1n back and see what's what.
By the way, I did dig the corpse of the old Hunts out of the scrap bits pile and it does say .001.
Alan
By my arithmetic, that makes the total effective inductance 28 µH: seems far too small. Now add in all the 'invisible capacitance' associated with the physical manifestation of that design: makes the inductance even lower.
)
