Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Well it has another 5 hours which brings it up to a total of 30.

This evening I explored the slight smear. I first discounted RF/IF alignment and checked components is the video amplifier. The peaking coils are all good and the load resistors are within tolerance. I temporarily substituted a new 0.05 uF coupling capacitor between 2nd detector and video amplifier input and no change. I used my trusty grid dip meter to check peaking coil resonance. All appeared normal.

I decided to investigate the IF alignment using the grid dip meter Before even even touching any adjustment, injecting a modulated signal at the mixer in the tuner, I could check the sound trap and visually check the IF in a crude way. The traps were fine and you could listen as the dip matched the sound discriminator "zero" at 21.25 MHz. I did notice the video carrier appeared a tad high on the nyquist slope. It certainly helps to have no AGC as the test was quite simple. Typically for virtually all other sets with AGC, I would put a DC supply on the AGC line to prevent AGC interaction.

I ultimately found the last stagger tuned IF stage was tuned to 26 MHz. I tried swapping the 6AG5 in the last IF stage with a NOS RCA tube which appeared to reduce the video carrier on the nyquist slope. The multiburst 3.0MHz bar appeared more prominent and when connecting back the DVD player, the picture was noticably sharper.

I have an old but good AVO Mark III transconductance tester and I am inclined to test it tomorrow. I also find that the sync separation is not as good as I would anticipate: a normal contrast it is fine but the range is small before the picture bends. The 721TS uses the first iteration of the famous RCA 'Synchroguide' circuit but without the sine stabilization. So the performance I see may be normal for this pruned back design. But I will check the 6SN7 and the sync circuitry later tomorrow evening.