[Penthode]

I have an 721TS update. It's not about the filter capacitors this time. The set has about 350 hours since my minimalist restoration and the filter capacitors are still fine).

This is about a nagging sync problem. I am surprised how well this little set works despite it being a pared down design. The sync separator is very basic: a sync amplifier fed from the video amplifier (12AU7) plate to a 1/2 6SN7 Sync Amplifier followed by a 6AL5 sync clamp to1/2 6SN7 sync separator. The set is pre-AGC with the contrast adjustment controlling gain in the RF/IF section of the receiver.

The problem stems from a relatively narrow adjustment range affecting horizontal sync. This set features the earliest version of RCA's classic a later widely licenced Synchroguide Horizontal AFC without the stabilization coil. I checked pin 6 (cathode follower sync feed) as I varied the contrast control and found that the sync separation diminished either side or a very narrow range of contrast adjustment. In other words, the sync separation was not very good.

This is not suprising considering the limting factor of cost in the design. The sync clamp or limiter works well as it keeps the sync tip at at fixed DC level. I tested and swapped the 6SN7 with no improvement. And the capacitors all tested good resistor values good.

I was tempted to swap the low amplification factor (20) 6SN7 with a (70) 6SL7. But this would require further redesign as the amplifier gain would be too great.

Picture 1 is the Sync Section of the 721TS with the modification applied.
Picture 2 Is the 621TS Sync Separator included for comparison.
Picture 3 is the separated horizontal sync.
Picture 4 is a picture off air of TVO this evening via an RCA branded set top box. The set is running behind where I am writing this.

I ultimately considered altering the cut off of the sync separator portion of the 6SN7: by applying a pull up resistor of 510kohm as depicted, the clipping action would remain wider with video gain changes. The result is that the waveform depicted was maintained at a somewhat wider range of contrast adjustment. This will be displayed in the video as less video pulling of the horizontal sweep as the contrast control is adjusted.

The sync separator cathode after adding the resistor, showed less ingress of video (in the range depicted with the white arrow) over a wider range of contrast control adjustment.

I would be curious how other with the 721TS finds this problem with contrast adjustment. I have included the circuit of the 621TS which is very similar to the overall design to the 721TS. The 621TS uses the earlier 6H6 as opposed to the 6AL5 for the sync clamp. And the 621TS is still pre-Synchroguide and would be curious how the contrast adjustment affects it.

I have added the resistor and am happy with the overall result.

Below:
Picture 1 is the 721TS sync separator diagram with the modification made.
Picture 2 is the 621TS sync separator included for comparison.
Picture 3 is the 721TS sync separator output horizontal sync waveform with the range depicted where you will see video ingress when the contrast control is adjusted.
Picture 4 is the set sitting behind me as I write. It is receiving this evening TVO (TV Ontario) via an RCA Branded set top box.