21-04-2013, 08:22 PM
So, back on the job after too much Sunday dinner 
First off fire it up and check the frequency etc. Yes, it really is 300kHz and there is a signal on the 5763 Grid, so there isn't an O/C between the Transformer and the Grid.
Better go back to the 1n and find out what's happening. I touched the 10n and it fell off at one end - dry joint
The 300kHz is actually the frequency with no Capacitor at all. With 10n there's a horrible sqegging waveform at about 25kHz - not good.
So, back to 1n, a new one just to be on the safe side. That produced a fairly low level oscillation which I'd previously dismissed as garage crud.
Now I was back to something approaching square one I disconnected the Resistor between the Transformer Secondary and V505 etc. Bingo, a beautiful 400v+ Sin wave on the Anode of the 5763. The period is about 17uS - 59kHz. Well, there's no particular reason to believe that this version runs at the same frequency as the 'old' one, and it is a Ferrite Transformer.
Measured with the AVO there's -800v on A10 and A6, and -610v on A7. It's not clear that RV500 makes any difference, yet.
With the lights off it's possible to see the Rectifier Heaters. V505 is OK, V504 is glowing blue with no sign of a Heater and V503 appears to be completey dead. The AVO says there is continuity between the Heater wires - disconnected - but no glow.
Why a couple of duff Rectifiers should kill the Oscilator I don't know.
So, I'll order a couple of Rectifiers if I can track some down.
Now I need some beer
Alan
First off fire it up and check the frequency etc. Yes, it really is 300kHz and there is a signal on the 5763 Grid, so there isn't an O/C between the Transformer and the Grid.
Better go back to the 1n and find out what's happening. I touched the 10n and it fell off at one end - dry joint
The 300kHz is actually the frequency with no Capacitor at all. With 10n there's a horrible sqegging waveform at about 25kHz - not good.
So, back to 1n, a new one just to be on the safe side. That produced a fairly low level oscillation which I'd previously dismissed as garage crud.
Now I was back to something approaching square one I disconnected the Resistor between the Transformer Secondary and V505 etc. Bingo, a beautiful 400v+ Sin wave on the Anode of the 5763. The period is about 17uS - 59kHz. Well, there's no particular reason to believe that this version runs at the same frequency as the 'old' one, and it is a Ferrite Transformer.
Measured with the AVO there's -800v on A10 and A6, and -610v on A7. It's not clear that RV500 makes any difference, yet.
With the lights off it's possible to see the Rectifier Heaters. V505 is OK, V504 is glowing blue with no sign of a Heater and V503 appears to be completey dead. The AVO says there is continuity between the Heater wires - disconnected - but no glow.
Why a couple of duff Rectifiers should kill the Oscilator I don't know.
So, I'll order a couple of Rectifiers if I can track some down.
Now I need some beer
Alan
