20-04-2013, 09:02 PM
More Tests:-
First off I disconnected the wire to A7. Everything was much the same. In retrospect it's a pretty dumb move as it effectively puts +150v on 12AU7b grid wrt the Kathode. That turns the Grid/Kathode into a Diode, and that's where the -150 on the Grid comes from.
Next I tried it with the 12AU7 removed. Again it doesn't make a great deal of difference to the 5763 voltages, and it still didn't oscillate.
Finally I decided I'd change C501 and C518 - no Al, I don't know what C518 does. Once again it made no difference. The two Capacitors I removed measure OK and are >200M on my BM100 Megger.
I was standing there scratching various peripheral bits and pondering what to do next when I noticed that R501, which lurks virtually under the Transformer, hadn't been changed. Indeed, I hadn't ticked it off on my BoM. Changing it was a pig, and it made not one iota of difference.
So, what next?
I'm nervous about the 1n across the Transformer. Although I'm certain I read the original Hunts OK it seems a desperately small value. I could slip in a 10n just to see what happens.
I don't like the conditions on the 12AU7, although they seem reasonable if the circuit isn't oscillating, and hence should be the normal startup conditions. I could use a Bench PSU suitably arranged to turn the 12AU7b off(ish).
I'm desperately trying not to have to attack the Transformer, but .......
Alan
First off I disconnected the wire to A7. Everything was much the same. In retrospect it's a pretty dumb move as it effectively puts +150v on 12AU7b grid wrt the Kathode. That turns the Grid/Kathode into a Diode, and that's where the -150 on the Grid comes from.
Next I tried it with the 12AU7 removed. Again it doesn't make a great deal of difference to the 5763 voltages, and it still didn't oscillate.
Finally I decided I'd change C501 and C518 - no Al, I don't know what C518 does. Once again it made no difference. The two Capacitors I removed measure OK and are >200M on my BM100 Megger.
I was standing there scratching various peripheral bits and pondering what to do next when I noticed that R501, which lurks virtually under the Transformer, hadn't been changed. Indeed, I hadn't ticked it off on my BoM. Changing it was a pig, and it made not one iota of difference.
So, what next?
I'm nervous about the 1n across the Transformer. Although I'm certain I read the original Hunts OK it seems a desperately small value. I could slip in a 10n just to see what happens.
I don't like the conditions on the 12AU7, although they seem reasonable if the circuit isn't oscillating, and hence should be the normal startup conditions. I could use a Bench PSU suitably arranged to turn the 12AU7b off(ish).
I'm desperately trying not to have to attack the Transformer, but .......
Alan
