Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Originally Posted by Kevin Kuehn

How are the CRT and receiving tubes faring after 1000 hours? Just putting this into perspective, we've put 1000 hours on a riding lawnmower over a period of 8 years and consider that it's fully depreciated. Likewise it's a remarkable amount of TV time and quite a testament to late 40's electronic technology. In so many ways this has been one remarkable TV.

That is a good question. I tested the tubes on my AVO MkIII tranconductance tester when I was resusitating this set. The set has required a touch up of vertical height and linearity. Curiously I had to reduce width slightly. As far as RF sensitivity or the quality of video and audio, no noticeable degradation.

A few carbon resistors discovered drifted when initial restoration and troubleshooting were replaced. I only tested and changed resistors which obviously would affect performance. An example was insufficient horizontal drive due to the oscillator plate supply resistor drifting upward. I intentionally replaced the resistors with period new old stock carbon resistors.

Recall I only changed the paper capacitors for which leakage would cause bias problems. That is if leakage was acceptable in the circuit, I left it in. That accounts for about a quarter of the original paper caps remaining. All of the original electrolytic chassis mounted cans after 1000 hours remain healthy which is remarkable after 75 years.

The CRT was a late 50's replacement shortly before the set was put into storage and so the emission remains good. Typical of late 40's CRTs, the focus isn't perfect across the entire CRT but the set does provide a decent picture.

In summary, I think the greatest concern for reliability back then and now is the paper capacitors. In order to make a reliable yet as original as possible set, a lot of thought was given to the restoration strategy. This included understanding how the circuits functioned and anticipating problems. Certainly during the initial extensive bench test monitoring a close eye was kept on the original electrolytics for failure. After 1000 hours they are still fine.

It is a daily runner: I had the set on today for about 5 hours. It will probably be on for about 10 hours this weekend.