Restoration Commences on 1947 RCA Victor 721TCS

[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.

[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.

[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.

[Penthode]

Originally Posted by Electronic M

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.

I am not saying I do not replace electrolytics if it is not warranted. When I reform a capacitor, I check the leakage and the final capacitance. If the leakage is low at fill rated voltage and the capacitance is as rated, then I will give it a pass. In this instance I will run the set 100 hours and again measure the capacitance and leakage.

And why do you suggest I am winning the lottery? I have repaired a number of sets and about 25% have retained the electrolytics if they behave. Okay the set remains on the bench for a couple of weeks while I give it a good run though. But I am perplexed why immediately the first thing suggested to do is to replace the electrolytics without even a rudimentary examination?

The set now has completed 25 hours. I am going to investigate the video amplifier this evening and run it a few more hours.

[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 replacing 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 caacitor 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 same note There appears to be 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.

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