Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Originally Posted by Electronic M

You can get lucky and have a set come to life on a minimal recap , and you can get really lucky and have it last, but experience with that approach tells me it's a waste of effort.

I have a 62 Zenith 29JC20 (their first consumer color set) that I tested every lytic for leakage on my Heathkit C3 in and initially changed only 2-4 sections. All the cans ran cool for a few weeks then over the course of a month they started dying at a section or 2 a week after 3-4 rounds of this I got mad and changed the remaining ~3 sections.
My CTC4 also illustrates this... When I got it about 8 years ago I changed all caps, but the doubler lytics (which tested fine and ran cool) ran it for a few months, concluded it had alignment issues and shelves it in a move till covid came knocking... I variaced it and then found changing all the off tolerance resistors fixed the alignment issues I was experiencing. When I finally had it done as I was putting the back on the screen image shrunk and the sides started undulating....you guessed it one of the doublers started leaking.

The minimalist approach is fine for learning, and for sets destined to become low use shelf queens, but daily drivers are too troublesome with original caps...I've learned that the hard way enough to not be convinced otherwise.

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.