Quote:
Originally Posted by Penthode
The reason you are losing sync is because when the brightness is turned up high or the contrast advanced, you are clipping the sync with the diode! I suspect the sync take off is at the plate of the video amplifier.
Compare your circuit with the RCA 630ts and note R148 which will alleviate the sync clip.
It is a mystery how it works all unless the diode was installed opposite to how you depict it. The CRT electron current flow in the original design is from is ground through the brightness control up thru R32 the 150K resistor to the cathode of the CRT. The diode depicted in your drawing would obstruct the beam current and there is no other path unless C19 the 0.05ufd coupling capacitor is very leaky!
|
Agree with the possibility of clipping the sync. Adding a series resistor as in the tube circuit would fix this.
Disagree with the "can't work" statement. The added diode is in the right direction for restoring DC at the CRT cathode. The sync tips are positive here, and C19 is charged by diode conduction during sync, then discharges through the CRT during active video. In the case of grid drive, since there is no grid current during active video (or at any time) to discharge the coupling capacitor, a resistor is added across the diode.
Investigation of the charge/discharge time constants with a scope by looking for tilt on the cathode waveform and sawtooth on the added 0.1 cap would help optimize the capacitor values.