Hi Alan,
This is turning out to be a bit a saga, isn't it?
The one thing about that cct. that I'm not altogether sure about - and I think that this is the critical part of this whole saga - is what constitutes the feedback path? I can't see that it is comprised of the HV rect. diode V508 and thus round to the grid of the triode, via the RV500 pot. Seems to me that the feedback path is 5763 anode, primary of T500, C501 / R501 and back to the grid of the 5763, using the fact that the primary of T500 is centre-tapped to HT (a.c. ground) with each end of T500 thus being 180° out-of-phase with the other, the whole winding comprising the inductive part of a tuned cct., the 1 nF being the capacitor of that resonant cct. (Assuming that the osc. freq. is in the region of 15 kHz, that makes the effective inductance about 100 mH).
So, on that basis, since it isn't oscillating, the suspect items within that loop are the 1 nF, C501 & R501 - all of which I believe we have eliminated as the cause of 'no osc'. I also assume that C503 is also above suspicion: its role is quite critical in this cct..
The next likely cause then is excessive loading on the transformer secondary. So, disconnect all the loads on the secondaries at the transformer itself and see if the oscillation now starts. If it doesn't, then the next likely candidate has to be the transformer itself. Now if we do get to that stage, we haven't unequivocally proved that the transformer is indeed faulty, but there are ways of testing a suspect-faulty HV transformer - provided, of course, that it is removed from the item under test for an assessment. And if you do get to that point and are unsure of such a test procedure, give me a shout.
Al. / April 20 //
This is turning out to be a bit a saga, isn't it?
The one thing about that cct. that I'm not altogether sure about - and I think that this is the critical part of this whole saga - is what constitutes the feedback path? I can't see that it is comprised of the HV rect. diode V508 and thus round to the grid of the triode, via the RV500 pot. Seems to me that the feedback path is 5763 anode, primary of T500, C501 / R501 and back to the grid of the 5763, using the fact that the primary of T500 is centre-tapped to HT (a.c. ground) with each end of T500 thus being 180° out-of-phase with the other, the whole winding comprising the inductive part of a tuned cct., the 1 nF being the capacitor of that resonant cct. (Assuming that the osc. freq. is in the region of 15 kHz, that makes the effective inductance about 100 mH).
So, on that basis, since it isn't oscillating, the suspect items within that loop are the 1 nF, C501 & R501 - all of which I believe we have eliminated as the cause of 'no osc'. I also assume that C503 is also above suspicion: its role is quite critical in this cct..
The next likely cause then is excessive loading on the transformer secondary. So, disconnect all the loads on the secondaries at the transformer itself and see if the oscillation now starts. If it doesn't, then the next likely candidate has to be the transformer itself.
Now if we do get to that stage, we haven't unequivocally proved that the transformer is indeed faulty, but there are ways of testing a suspect-faulty HV transformer - provided, of course, that it is removed from the item under test for an assessment. And if you do get to that point and are unsure of such a test procedure, give me a shout.Al. / April 20 //
