Restoration Commences on 1947 RCA Victor 721TCS

[Electronic M]

Originally Posted by Penthode

Miminal recap? I do not think it is as simple as that. It is a process of deciding to replace only what is necessary. There is a 0.25 uF paper capacitor in the cathode circuit of the 6BG6. It bypasses an 82 ohm cathode resistor. Even if the leakage is 5 Megohms, the shunt resistance is a small fraction of the capacitor leakage.

The capacitors I have changed I identified are subject to voltage breakdown. I have only changed the ones I see which pose a threat.

If I experience an alignment problem, I would investigate and if a straight forward voltage check would reveal the restor problem. And yes I would replace them. But I use these sets daily and I have no fear of the component I left remaining breaking down if the set runs on the bench for a couple of days without issue.

I have also seen electolytics fail over time. But did you reform them and check leakage before buttoning on the back cover? I am seeing the average recommended leakage expected on electrolytics of this vintage to keep the dielectric in place. I would have replaced them if the leakage was above my reference limit.

So this set is going to be a daily driver and i will not fear capacitor failure if it gets past 50 hours.

On the a continuing note, I have noticed the 721TS has a slight low frequency smear in the picture. I left one paper capacitor in place in the video amplifier: the 0.05 uF between the video second detector and the input of the first video amplifier stage. The DC voltage across it is only a couple of volts. It may have a high ESR which is affecting the picture. Then again it may be an open peaking coil or a drifted resistor. I put a multiburst from the Sencore VA-62 on the screen and a good clean 3.0 MHz response is seen, which is normal for a 3 stage IF set like this one and the tuning appears correct with no ringing and the video carrier appearing to drop on the nyquist slope and you reach optimum sound.

So the diagnosis continues. And the point of this post is to highlight the process of restoration rather than wholesale rebuilding.

My Heathkit C3 tests leakage at working voltage. I can also use it to to reform capacitors. I have done a number of sets where any cap that tested as leaking got changed and ones that were good went bad after extend use. Both sets I mentioned went through that and after weeks of use and dozzens of hours of running cool the lytics eventually decided to crap out.

I've found that in the time it takes to evaluate each cap I can recap a set several times over. And the time I waste on repeated repairs on my daily driver sets when I don't do a full recap is worth more to me than the cost of capacitors or maximal originality.

Each part has a bell curve of failure and on high production sets I'm sure some examples may still be usable with all original caps today and into the future...when you find one you are winning the lottery statistically speaking....my luck usually ain't that good and I don't like to gamble my scarse time on unreliable parts after doing similar experiments to yours.

Some makes and models are more picky about exact component value than others. I've seen it for myself plenty, and some who have restored a set like yours and a Muntz will complain that the Muntz will be picky about tubes and precise (not necessarily accurate values but precise) component values.