Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Originally Posted by kvflyer

Yes, it is interesting.

I know that it's a PITA but if there are not a lot of connections on the electrolytic capacitor, I disconnect every lead and test the capacitor for value first and leakage at the working voltage next. If it passes, I put everything back and then monitor the temperature. Sometimes you win, sometimes you lose.

When I bought this set about 6 years ago, it had sat in an attic for nearly 60 years unpowered. Apart from a mouse nest, it was a pretty clean and intact set. When I began the restoration, I decided to first evaluate the electrolytics. As it had not been powered on for many years, the filter capacitors were extremely leaky. I believe had it been powered as I found it, the capacitors would have been most likely destroyed and the rectifier destroyed.

I proceeded to reform the electrolytics by simply removing the 5U4 rectifier and with power off, connecting my Sprague TO6 supply across the B+ supply line. I limited the current to a maximum of 10mA and left the TO6 running all night. The voltage across the capacitors began to rise from less than 10 volts @ 10mA. I had taken into account any shunt resistance across the B supply and I recall removing some divider resistances.

The next morning the capacitor voltage had reached nearly 100 volts with minimal leakage. I raised the voltage and keeping maximum leakage less than 10mA. By the end of the day the total paralleled capacitor leakage was well under 1mA. I disconnected a few capacitors I could not readily access to ensure leakage was no more than 100uA at full rated voltage.

The point I am making here is that the capacitors will be destroyed unless a more gentle approach is made waking them up. I never use a variac to power up an old device. I will instead immediately look at the electrolytics and evaluate and see if they will reform. Then if the TV set dates prior to 1960, I will replace almost all the paper capacitors.

I have rebuilt a sizable number of electrolytics which will not reform. But with all 5 of my late 40's RCA Televisions, I have only replaced one chassis mount multiple section electrolytic. My CT100 had two bad chassis mount electrolytics and my CTC5 set has retained all the original electrolytics. Curiously, a 1961 CTC 11 RCA color I restored a decade ago required replacement of every chassis mount electrolytic. All but one the dielectric would not reform and the one that did suddenly failed in service. (The failure was not a short but went totally open.)

So there is no hard and fast rule. It is easy to reform the things and to test leakage and capacitance. And the experience I have had has been very good up to now.