Restoration Commences on 1947 RCA Victor 721TCS

[JohnCT]

Originally Posted by Penthode

The 721TS has been running for about an hour this morning. I am told the electrolytics will not last long term. I am curious what evidence is there of that?

If it's going to be a shelf queen and get run for 10 minutes several times a year, you may not have a problem.

I restored a 51 TV earlier this year and did it one part at a time just to bone up on tube troubleshooting. Once done, it was shotgunned (well, not fully done).

This TV has nine electro cans, and two were bad on first power up, one taking out the 5U4, the other taking out a 6X4. I pulled them and restuffed them, and soldiered on.

While working on the sound section, another can that had previously checked fine and had run cool for several hours decided to make itself known. A slight change in the picture followed by a hiss told me another can had enough. Where it was running cool for several hours beforehand, it now was *very* hot. The fact that it was also drooling like a 5 year old kid with a mouth full of Sour Gummy Worms was another clue..

So far, 4 cans restuffed and the other 5 will be done before final assembly. The paper caps that are running fine will also be replaced before final assembly and alignment.

John

[Penthode]

The 1947 RCA 721TS is still chugging along on it's original electrolytics. As I pointed out earlier, if the capacitor dielectrics are slowly and carefully reformed and the capacitors are afterward tested to confirm low leakage, they will last and last. This set had sat dormant for nearly 60 years in a house near Schenectady NY. So it must have been purchased to receive WRGB in those days.

It must be well over a thousand hours since I did the minimal rrevival, replacing only the paper capacitors which would likely fail. I left in the paper capacitors which are shunted by resistances 1Meg ohm or less.

The set is frequently run the set for hours at a time. had it on this evening when I recalled this thread.

I will check in the next year or so with another update.

[Penthode]

Originally Posted by Penthode

21 June, 2020

No sign of any problem with the original electrolytic capacitors since they were reformed. I shall intermittently keep the set on and keep an eye on it.

It has been more than two years running regularly. The original electrolytics remain fine.

I was told the original electrolytics would eventually fail. Any suggestions when this will be?

[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.

[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.

[Yamamaya42]

-gets out the popcorn, ready to watch the fireworks!

[Eric H]

I've restuffed a lot of late 40's capacitors, about 90% of them are dried up inside with the electrolyte turned to a white crystalline powder and the Aluminum foil is rotted and deteriorated. They often would still work to some extent, but no way were they okay.
Normally they would be damp to actually wet inside.

Low leakage is one part, but have you checked them for capacitance? I suspect the deteriorated ones have lost a lot.

[Yamamaya42]

Originally Posted by Eric H

I've restuffed a lot of late 40's capacitors, about 90% of them are dried up inside with the electrolyte turned to a white crystalline powder and the Aluminum foil is rotted and deteriorated. They often would still work to some extent, but no way were they okay.
Normally they would be damp to actually wet inside.

Low leakage is one part, but have you checked them for capacitance? I suspect the deteriorated ones have lost a lot.

oddly enough, the electrolytics on my 1949 FADA tested WAY higher than they were rated for when i replaced them, 2 to 4 times higher, at the time, I had no way to test for leakage, I just assumed they would all be bad, and replaced all caps, paper and electrolytic, most of them, if not all testing much higher than they should.

[Penthode]

Originally Posted by Yamamaya42

oddly enough, the electrolytics on my 1949 FADA tested WAY higher than they were rated for when i replaced them, 2 to 4 times higher, at the time, I had no way to test for leakage, I just assumed they would all be bad, and replaced all caps, paper and electrolytic, most of them, if not all testing much higher than they should.

I have seen this as well. My theory is that before reforming the dielectric, the remaining dielectric is thinner. Hence the capacitance rises. After reforming, the capacitance went down.

I didn't test before reforming in this instance. But after reforming the capacitance was within 20% on the units I tested. (I didn't bother to test ever unit as this would have required additional desoldering-resoldering.

But good observation. And the point I want to make is to try and conserve and maintain as much of the original early electronic circuitry as possible. This can be done with due caution. And remember, if I was doing this for anyone but myself, I would probably resort to changing all the electrolytic capacitors! From a personal side, I like maintaining as much originality as possible.

[Yamamaya42]

Originally Posted by Penthode

I have seen this as well. My theory is that before reforming the dielectric, the remaining dielectric is thinner. Hence the capacitance rises. After reforming, the capacitance went down.

I didn't test before reforming in this instance. But after reforming the capacitance was within 20% on the units I tested. (I didn't bother to test ever unit as this would have required additional desoldering-resoldering.

But good observation. And the point I want to make is to try and conserve and maintain as much of the original early electronic circuitry as possible. This can be done with due caution. And remember, if I was doing this for anyone but myself, I would probably resort to changing all the electrolytic capacitors! From a personal side, I like maintaining as much originality as possible.

I did restuff some of the original cans, and when I pulled the guts from them, they were as dry as the Sahara Desert inside...

[Penthode]

Originally Posted by Yamamaya42

I did restuff some of the original cans, and when I pulled the guts from them, they were as dry as the Sahara Desert inside...

That is why they are called "Dry Electrolytics"!!! Lol.