Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Found the cause of no audio. RCA in the late 40's inserted a 5.1 ohm resistor in series with the audio discriminator (6AL5) filament. These often go bad. The 5.1 ohm resistor was open in this set and replacing it with two 10 ohm 1 watt metal film resistors in parallel found the audio.

The tuning was off frequency. Touched up channel 13 downward. Replaced the local oscillator 6J6 because ran out of tuning range on channel 13.

Also replaced the 5U4 rectifier. The rest of the tubes are as I found the set.

Total replacement parts count: 11 paper capacitors, 1 electrolytic (the one I inadvertently blew up with over voltage), one resistor and two tubes (5U4 and 6J6). RF/IF alignment still looks good: best point of video matches best audio. 10BP4 is a 1949 spare I had (original was bad). CRT tests low and is a little dim but it works.

Set ran continuously last night for a couple of hours with no major mishap. Will give it 10 hours today to see what happens. I am surprised at this little 73 year old survivor after sitting dormant for roughly 60 years.