Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Well it has been exactly two years and three months of regular operations. It was June 20, 2020 I first powered it up. It has been running fine ever since. And I am still waiting for those danged original 1947 vintage electrolytic capacitors to fail. There are five of them and they just won't go bad!

I am beside myself because I was sure they were going to fail by now.

Anyhow here is a photo I took a few minutes ago.

[kvflyer]

That's encouraging since I have an RCA 630TS that I purchased a few years ago. It is in great condition; the GE replacement CRT measures NOS. I guess I should bring it up slowly on my Sencore PR-57, monitoring the current and see what happens. I don't mind rebuilding the capacitors but if they hold good, why not???

Thanks for the encouraging update.

[Penthode]

Originally Posted by kvflyer

That's encouraging since I have an RCA 630TS that I purchased a few years ago. It is in great condition; the GE replacement CRT measures NOS. I guess I should bring it up slowly on my Sencore PR-57, monitoring the current and see what happens. I don't mind rebuilding the capacitors but if they hold good, why not???

Thanks for the encouraging update.

You could do it slowly that way. What I prefer is to put a current limited DC power supply across the electrolytics with power off. With the tubes not conducting and with a study of the schematic to see what bleeders are across the B plus, you can better gauge the condition of the electrolytics. In some cases I simply disconnect the electrolytics to better isolate to confirm the condition of the electrolytic.

I have restored set that must have been dormant for 50 years of more. My RCA 721TS the seller near Schenectady NY told me my 721TS was stuch in the attic in the early 60's and was not touched until they sold it to me in 2016. I cleaned out a mouse nest but didn't start to restore it until 2020. It absolutely had not been powered in the meantime so it likely sat unpowered for 60 years. Incredible when you think about it.

And that is what I think saved the electrolytics. I gently brought up the capacitors by limiting the current to no more that about 10mA per capacitor. If the set has sat for decades, the capacitors will be almost a dead short and it is almost certain the capacitors will be destroyed. Yhe key is to carefully monitor and ensure the current is strictly limited until the dielectric has properly reformed.

I would leave the DC supply on for the night and inch the voltage upwards while monitoring the current. When it reached close to the rated maximum voltage, the electrolytic, if healthy, it will draw only about 100uA. If the capacitors draw more than 1mA after reforming, then it must be restuffed.

To summarize, I do not like using a Variac to first power up. To me it is the lazy man approach to the problem and because you are more adequately monitoring the current, it is hit and miss. By using a high voltage DC supply with current and voltage monitoring, you can better decide whether the capacitor is a survivor or not.

Proof should be the 721TS which has lasted over two years on ALL the aluminum can capacitors. I am in the midst of restoring a CT100 color set (which when I have time again in a few weeks will get down to the final alignment) which I slowly reformed the capacitors with the DC supply. All have survived and work fine.

[kvflyer]

Originally Posted by Penthode

You could do it slowly that way. What I prefer is to put a current limited DC power supply across the electrolytics with power off. With the tubes not conducting and with a study of the schematic to see what bleeders are across the B plus, you can better gauge the condition of the electrolytics. In some cases I simply disconnect the electrolytics to better isolate to confirm the condition of the electrolytic....

I have a Heathkit High Voltage Power Supply, IP-17 which I have totally restored. It has voltage and current meters so it would be easy to do.

I have a working T100 that came up on all original electrolytic capacitors. But I have not turned it on for about 8-10 years though. It was rather easy to get going.

[Penthode]

Originally Posted by kvflyer

I have a Heathkit High Voltage Power Supply, IP-17 which I have totally restored. It has voltage and current meters so it would be easy to do.

I have a working T100 that came up on all original electrolytic capacitors. But I have not turned it on for about 8-10 years though. It was rather easy to get going.

8 to 10 years off, if the capacitors were previously good, I dont think will be a problem. I think it is a problem when it has been off for 40 or more years with a sudden power up..

If you use the DC HV power supply across the B plus line to reform the capacitors, I will be interested in your results.

[Penthode]

The set has now been running faultlessly since July 2020. That is two and a half years still running on the original electrolytics. Still no sign of failure.

[kvflyer]

Great to see this update and you have given me an incentive. I have an RCA 630TS that is in excellent physical condition and has all of the parts. The GE CRT, no doubt a replacement, tests NOS. I will start with replacing the paper capacitors in the sweep sections and audio and see how the electrolytic capacitors reform. Carefully, slowly! Thanks.