Restoration Commences on 1947 RCA Victor 721TCS
 

The aim of this thread is to revive a long domant 1940's RCA TV without the shotgun replacement of parts. I will only change what is absolutely necessary to get set running in excellent and reliable shape.

I acquired an RCA 721TCS 10" table model receiver in 2013. I was told by the seller that it had been his parents and the set had been relegated to the attic in around 1960 where it remained unpowered for 60 years. The seller said he hadn't dare plug it in.

Upon getting it home I immediately pulled the chassis to find an old mouse nest. There was a little urine damage on a removable plate shield behind the RF tuner. I removed it and with face mask and ntrile gloves, cleaned off the oxidized cadmium and rust and touched up the affected area of the plate with silver paint. Apart from that and the knawing of the insulation from a wire from the horizontal deflection coils to the horizontal centering pot, the set underneath was pristine. The set did not appear to have been serviced apart from the replacement of two 6SN7s, the 6BG6 and 12AU7. I then put the set away.

After 7 more years of sitting idle, I pulled the set out this week and have finally begun bringing it back to life. I want to follow a restorative process and will not shotgun replacement parts. So I will try not replace anything unless it is proven bad.

Yesterday I first began evaluating the electrolytic capacitors. I use a Sprague Tel Ohmike TO6 Capacitor Analyser which I picked up as trash some 20 years ago. The TO6, besides measuring capacitance and insulation resistance, has a dandy current limited power supply with ammeter for the reforming of electrolytic capacitor dielectric. If you examine the power supply of the attached schematic, you will find that RCA sets of the late 40's used a resistive divider to supply the voltages required in the set. The first thing I do to isolate the capacitors without having to physically remove them, is to lift the top end of the divider chain. This made available five electrolytics on the +225v rail to hook up my TO6 power supply. the negative of the TO6 goes to the -85v rail. The attach points are made readily available when you remove the divider panel as depicted in the photo.

I hooked up the TO6 last night and with 20mA applied, i read an initial voltage of 5volts. Almost a dead short! After about 5 minutes, with current maintained at 20mA, the voltage across the capacitors began to rise. The rule of thumb for reforming is to give 10 minutes plus 1 minute for each month the capacitor has remained dormant. Let's see... that would amount to 10 + (60 x 12) = 730 minutes or about 12 hours. So once I was comfortable that the voltage was rising and the current would not exceed 20mA, I left it cooking for the night.

Next morning, I was pleased to see that the voltage was at 300 but the current was about 10mA, a bit high! the capacitors were across the +225, -85 rails for a total of 310v and the capacitors appeared rated at 350v. So I cranked up the voltage until 350volts to see what would happen. I witnessed a slow and gradual climb and when i was across the room, I shortly heard a pop and what sounded like a pig squeal.

C160, the axial lead 6K6 audio output screen bypass began emitting a little gas and looked as if it was about to give out. I immediately turned off the DC supply and after discharging the capacitors, snipped the lead off the capacitor. In hindsight, I realized that this separately located axial lead capacitor was rated at only 300v and lifting the divider allowed the full 350v across it! No matter as it is easily replaced.

After clipping out the bad C160, I was pleasantly surprised to find the remaining 80, 40,40,30 uF capacitors yield a total leakage of 1.7mA. After another 8 hours at 350volts, the leakage has dropped to 1.5mA. So it looks like the capacitors have been reprieved. On next to the remaining three main electrolytics: C128C Vertical Out cathode bypass, C116A Boost B+, C116B +150v Filter and C128A chassis to -85v filter. The only other two electrolytics are the 1000uF and 250uF low voltage centering pot capacitors which if shorted will only prevent the centering pots from working. i will leave those for later as i suspect if they survived they will reform in normal operation.

More to come.....

Reformed the remaining electrolytic capacitors. I have run the electrolytics at full rated voltage for 24 hours and the leakage current is acceptable. Next to the paper capacitors.

I have lifted one end of three coupling capacitors. The leakage is pretty bad: the video amplifier coupling capacitor was 4 Mohms and will need to be replaced. The audio coupling capacitor is 40 Mohm and I think I will change that. Any capacitor with an appreciable DC voltage across and leakage should go. But there are a number of places where the leakage does not matter and I think I shall weigh on retaining a bunch.

As for the resistors, there is generally a pretty great leeway and even if out of tolerance, unless the resistance is critical, I will likely retain all the resistors.

I'm looking forward to following this.

This morning I began going thru and testing the paper caps. I thought I should test their behavior under DC voltage.

I first examined the 0.01ufd 400v coupling capacitor between the first audio and output stage. I used my old IM18 Heathkit VTVM to initially measure the dielectric resistance. It started slow and climbed to about 40Mohm. A new polypropylene cap would typically climb from near zero to infinity almost instantaneously.

I next applied a 350v DC and monitored current. Interestingly, the initial steady state current was about 1.5mA and slowly began climbing. The current climb began to accelerate and within one minute reached 15mA at which point I cut the voltage. Bear in mind that the 40uF caps reformed and descended to a current of 0.5mA for 24 hours under test.

A similar result with the 0.05uF 400v first to second stage coupling cap. The dielectric resistance was close to 80Mohm yet with 350 volts applied, the leakage current rose from 0.5mA to 4mA within one minute and was continuing to climb when I cut the voltage. I then applied only 20 volts to the third 0.05uF 400v coupling capacitor between the second detector and first video amplifier stage. It generally runs with only about 10volts DC across it. The leakage was under 100uA and did not change. So perhaps not under stress, it can remain.

The paper dielectric capacitors appear to be a serious threat if run with a high potential difference across them. It surprised me that a 0.01uF had considerably more leakage than 40uF electrolytic. Anyhow I shall proceed to change a slew of compromised paper capacitors today.

You'll probably end up replacing the electrolytic capacitors as well, before you are done.

Not necessarily. I have four pre-1950 RCA televisions and out of that bunch, there was one electrolytic fail. And it failed by opening and not shorting. I have a CTC5 color RCA with original electrolytics.

Paper dielectric capacitors are a real problem. Good quality Sprague and Mallory electrolytics starting from the late 1940's if properly handled will continue to function today in most cases if the reforming process is done with care. I am highlighting this project because the set sat unpowered for 60 years. If the set had been plugged in, the electrolytics would like have been irreparably damaged.

I spent the day replacing only the paper capacitors I feel would be under stress in circuit positions where subjected to high voltage. The electrolytics reformed nicely with each leaking under 1 mA at rated full voltage.

The paper capacitors, as expected all exhibited various degrees of leakage. my rational in retaining some is if the capacitors have under 1 Megohm in parallel with it. I performed futher tests on the paper capacitors which displayed a worrying thermal runaway failure mode. I placed 350 volts DC across a 0.05 uF 400v capacitor from the set. It showed just over 3mA leakage. However the leakage must have generated internal heat which increased the leakage current. within a minute or so, I witnessed the leakage current begin to accelerate and at 15mA, I removed the voltage. I let the capacitor cool and an hour later i repeated the test with leakage starting at 3mA and rising over time as before. I then reduced the voltage to 20volts and the leakage current was insignificant and remained steady.

I then tested the 0.1 uF 1000v capacitor which is used in the B+ Boost circuit as a filter. At 500VDC, it showed only 0.5mA. However I took a heat gun and raised the temperature of the capacitor only to the point of wax beginning to melt. The leakage increased with the heat. So I ended up replacing all the paper capacitors in the circuits with applied high voltage only except for one: at some point the 6BG6 Horiz Output screen bypass capacitor was replaced with a white bumblebee type. I applied 400v and heated it but the leakage remained at 0.1mA. Do not know what it is composed of or even if it was a paper diectric capacitor. I left it in.

I had a couple of old Black Beauties (0.25uF 400V and 0.1uF 1000V) which I substituted for the wax ones in the set. The Black Beauties under full (or near full in the case of the 1000v unit) showed little or no leakage even when heated.

I replaced the two 0.01uF capacitors from power line to chassis with two Orange Drops rated at 1600v.

I have included a picture of the underside so you may examine for yourself the capacitor replacement work I did today.

POWER UP.

Next came the moment of truth. I inserted the 10BP4, plugged in a cheater and switched it on. I had the chassis on its side to detect any wisps of smoke.

After switching on and waiting about 30 seconds, I heard the vertical sweep start. I advanced the brightness to rotate the ion trap and was rewarded with noise on the screen. I adjusted focus and haven't even touched the Vertical linearity or Vertical Height. I set my Sencore VA62 to and the 721TS to Channel 5 and was rewarded with a picture.

I immediately saw the horizontal hold began to drift and there was a 0.002uF on the Horizontal Frequency Control grid which I was earlier hesitant to replace. I replaced it and the horizontal frequency drift ceased.

Find below a picture of the chassis underside and a picture of the image which immediately popped up. No audio yet and next is the 'soak test to see what will fail. Remember this has been a minimalist rejuvenation and I suspect until a prolonged stress something will fail. Let us see over the coming weeks. More to follow.....

Very interesting experiment! I can't wait to see what happens next.

Found the cause of no audio. RCA in the late 40's inserted a 5.1 ohm resistor in series with the audio discriminator (6AL5) filament. These often go bad. The 5.1 ohm resistor was open in this set and replacing it with two 10 ohm 1 watt metal film resistors in parallel found the audio.

The tuning was off frequency. Touched up channel 13 downward. Replaced the local oscillator 6J6 because ran out of tuning range on channel 13.

Also replaced the 5U4 rectifier. The rest of the tubes are as I found the set.

Total replacement parts count: 11 paper capacitors, 1 electrolytic (the one I inadvertently blew up with over voltage), one resistor and two tubes (5U4 and 6J6). RF/IF alignment still looks good: best point of video matches best audio. 10BP4 is a 1949 spare I had (original was bad). CRT tests low and is a little dim but it works.

Set ran continuously last night for a couple of hours with no major mishap. Will give it 10 hours today to see what happens. I am surprised at this little 73 year old survivor after sitting dormant for roughly 60 years.

The set has been running continuously for 6 hours. Been running a series of DVDs on the set. I have the 1933 film 42nd Street on it now.

Audio was low. I found the 1st audio IF socket dirty. Wiggling the tube in the socket brought the audio level right up. Plenty of audio now.

The picture is good along with good sound.

The electrolytics are fine. They are still cool and the set looks healthy.

Net parts replaced to bring them is set back to life are in the attached photo.

I plan to put a lot of hours on this set this week. Let us see if the electrolytics hold up.

Any idea why RCA put that 5.1 ohm resistor in the filament circuit of the discriminator tube?

not a clue, it is also there on the RCA 630ts (which is what my FADA is a clone of ) , but it's
2.2 ohms, and it also feeds the heater of the Horz sinc disc, also a 6AL5

Discussion of it here - http://www.foundcollection.com/2_81131c0ab8492ebb_1.htm

That is a good question. I thought about this from the different aspects of cathode temperature and space charge or possibly improving discriminator linearity.

I think the clue may be gathered from the Radio Designers Handbook by F. Langford-Smith Fourth Edition (1954) on page 1095 under FM Detectors. RCA was using the Foster-Seeley discriminator which according to Langford-Smith was subject to introducing troublesome heater-cathode hum with a miniature vacuum tube such as the 6AL5. The later Ratio Detector was not subject to the hum problem.

I believe that the introduction of the 5.1 ohms reduced the a.c. heater potential and lifted one leg of the filament from ground to help balance and reduce the hum potential between heater and cathode. Because the 6AL5 is a signal detector in this application, the reduced cathode temperature and resultant space charge reduction would not significantly affect its operation.

This is my opinion and I am open to other thoughts.

The RCA 721 TS ran 10 hours today with no problem. The 10BP4 is dim and has low emission, but produces a reasonable picture.

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.

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?

There is no sign of any heat or stress on the electrolytic capacitors after running about 14 hours in total. Any thoughts when they will fail?

The point of this exercise is to point out in a project such as this, that perhaps it is unwise to shotgun replace most of the electronic components. I am amazed reading here that even resistors that have marginally drifted out of tolerance are replaced. Paper capacitors indeed go leaky but even then, is it necessary to change all of them even if it makes no difference to circuit performance?

I have previously contested that the wholesale replacement of parts is unnecessary and will likely lead to considerable extra work and grief. It took me less than a day to get this set up and running.

The key here is to scrutinize the circuit in advance to identify components which will likely fail with catastrophic results eg up-in-flames, smoldering resistors or undo stress on the power transformer and rectifier. If concerned, you can always insert a slo=blow fuse in the B+ rail. More time should be spent paning before execution.

I contend the Sprague and Mallory branded electrolytics made from the late 1940's were very well manufactured and will operate satisfactorily and reliably even today. This is if and only if you reform the diectric properly and with a bit of care. In the last 50 years of repairing these old sets, I do not recall ever seeing a bad mica capacitor in an RCA set. I think more time spent understanding how the circuits work will result in less physical effort and result in a set that maintains more of its overall originality.

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

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.

I have running four other RCA sets from the late 40's. Two 8T243's (I sold one) an 8TC271 and a 9TC275. The 8T243s and the 9TC271 were dormant and never plugged in and I simply reformed the capacitors and use them regularly with no problem. I have had the 8T243 for 50 years this year (yep I was given it in 1970. I have used it for 50 years since on its original electrolytics. The 9TC275 on the other hand sat in a damp basement and the seller pugged it in until he said he saw smoke. I looked at the electrolytics and two cans were ruined and a third survived. I have a CTC5N color and it has all the original electrolytics. A CTC11 with high hours had bad electrolytics and i ended up replacing them all.

The 721TS I plan to keep and I want to retain as much of t original as possible. And my past reference is more than 50 years of restoring electronic equipment. I see that the Sprague and Mallory capacitors have an unduly bad reputation.

If the electrolytics lasts 100 hours and I do not abuse it I expect it will last the remainder of my lifetime. Would you agree? I would also venture it will outlast a modern replacement if I was to install one.

The 721TS has been running 20 hours now and still running fine...

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.

:tongue:

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

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.

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 have no way of knowing how long the caps might last or how often you will run it. It may go 100 hours, it may go 20, or it may go boom the next time you try it...

Anything I restore I do so with the expectation that the next guy won't have a lot to do if something inevitably fails - and that guy will be one of my two sons in most cases.

I have two items of significant age running original capacitors: one of my Fisher X-1000 integrated tube amps from 1962 has all it's original capacitors - my original X-1000 blew a can about 35 years ago. In any case, I don't let either Fisher run unless I'm in the same room with it.

The second is my 1929 RCA Radiola 62. This has no electrolytics at all and the paper capacitors are made with rice paper instead of rag. Radiola experts say don't change any caps in a 20s Radiola unless you run across one.

No chance. Go to DigiKey and order Panasonic low ESR, high pulse caps. These will outlive you and I combined.

I used to do monitor work for an arcade operator many years ago, and he would say that you're on bonus time...

Parting shot: when I restuffed the four cans on my TV, I did a postmortem on the innards and what I found is a lot of degradation in the foil and the paper as I unrolled it. It wasn't a matter of if it was a matter of when. The other five cans in my TV are still original and of the same manufacture, but I know what's inside and I know they need to be serviced before they burn out a part that could only be sourced from the nation of Unobtanium.

John

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.

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.

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 measured the capacitance and the capacitance has remained within 20%. Otherwise it would be replaced. Certainly the bad one that fail the reforming process appear deteriorated when opened.

But how do you define rotted aluminum with crystalline powder? I suppose we assume the crystalline is dried electrolyte? There is a chemical process within the can and I would expect the insides to appear messy. What is your evidence it is not supposed to look as it does? Or perhaps the question should be how does it differ from a newer capacitor?

I have also opened 1940's cans to restuff with new caps and found some moist and pliable. Would this mean that the capacitor had remained good? I am defining whether a capacitor is good or fails by capacitance, leakage and life test. Is yours the assumption if the capacitor is more than 30 years old, it must automatically be bad?

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...

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

these caps were not the "dry" type.
but became so over time.

"the electrolyte inside the capacitor slowly evaporates away over time,"

https://www.effectrode.com/knowledge...citor-rebuild/

Wrong. They're called dry Electrolytics as they are not a can of electrolyte fluid with a thin positive electrode in the center like the set Electrolytics used in 30s early 30s Philco cathedral radios.

If you tear open a good dry Electrolytic of any vintage at the very least there should be some electrolyte moisture on the foil/dielectric roll. If there isn't it's dead or about to be.

You can understand why I never made it as a stand up comedian. Nobody laughs at my jokes. :dunno:

Well it has another 5 hours which brings it up to a total of 30.

This evening I explored the slight smear. I first discounted RF/IF alignment and checked components is the video amplifier. The peaking coils are all good and the load resistors are within tolerance. I temporarily substituted a new 0.05 uF coupling capacitor between 2nd detector and video amplifier input and no change. I used my trusty grid dip meter to check peaking coil resonance. All appeared normal.

I decided to investigate the IF alignment using the grid dip meter Before even even touching any adjustment, injecting a modulated signal at the mixer in the tuner, I could check the sound trap and visually check the IF in a crude way. The traps were fine and you could listen as the dip matched the sound discriminator "zero" at 21.25 MHz. I did notice the video carrier appeared a tad high on the nyquist slope. It certainly helps to have no AGC as the test was quite simple. Typically for virtually all other sets with AGC, I would put a DC supply on the AGC line to prevent AGC interaction.

I ultimately found the last stagger tuned IF stage tuned to 26 MHz may be pushing up the frequency response around the video carrier. I tried swapping the 6AG5 in the last IF stage with a NOS RCA tube which appeared to reduce the video carrier on the nyquist slope. The multiburst 3.0MHz bar appeared more prominent and when connecting back the DVD player, the picture was noticably sharper.

I have an old but good AVO Mark III transconductance tester and I am inclined to test the 6AG5 tomorrow. I also found that the sync separation is not as good as I would anticipate: at normal contrast, sync separation is fine but the contrast adjustment range is narrow before the picture bends. The 721TS uses the first iteration of the famous RCA 'Synchroguide' circuit but without the sine stabilization. So the performance I see may be normal for this pruned back design. But I will check the 6SN7 and the sync circuitry later tomorrow evening.

Well it has another 5 hours which brings it up to a total of 30.

This evening I explored the slight smear. I first discounted RF/IF alignment and checked components is the video amplifier. The peaking coils are all good and the load resistors are within tolerance. I temporarily substituted a new 0.05 uF coupling capacitor between 2nd detector and video amplifier input and no change. I used my trusty grid dip meter to check peaking coil resonance. All appeared normal.

I decided to investigate the IF alignment using the grid dip meter Before even even touching any adjustment, injecting a modulated signal at the mixer in the tuner, I could check the sound trap and visually check the IF in a crude way. The traps were fine and you could listen as the dip matched the sound discriminator "zero" at 21.25 MHz. I did notice the video carrier appeared a tad high on the nyquist slope. It certainly helps to have no AGC as the test was quite simple. Typically for virtually all other sets with AGC, I would put a DC supply on the AGC line to prevent AGC interaction.

I ultimately found the last stagger tuned IF stage was tuned to 26 MHz. I tried swapping the 6AG5 in the last IF stage with a NOS RCA tube which appeared to reduce the video carrier on the nyquist slope. The multiburst 3.0MHz bar appeared more prominent and when connecting back the DVD player, the picture was noticably sharper.

I have an old but good AVO Mark III transconductance tester and I am inclined to test it tomorrow. I also find that the sync separation is not as good as I would anticipate: a normal contrast it is fine but the range is small before the picture bends. The 721TS uses the first iteration of the famous RCA 'Synchroguide' circuit but without the sine stabilization. So the performance I see may be normal for this pruned back design. But I will check the 6SN7 and the sync circuitry later tomorrow evening.

Here is a shot of the grid dip meter coupled to the tuner mixer tube for the frequency response check. I actually rested the coil on the tube as I swung the calibrated dial. Accuracy is reasonable and the trap null was indicated on the dial at 21.25 MHz. I also see the original green sealing lacquer still on the adjustments which suggests it has never been touched.

Running a so far an hour this morning checking the picture and sound quality. Sound is excellent with no buzz. Picture sharper since the 3rd IF tube change yesterday. Haven't touched the alignment: still has the factory settings.

Perhaps I should take things less seriously and have a chuckle.

I've been banging my head on the wall with an early tube MPX decoder in a radio that will recognize the 19KHz from a generator but not a station...I have to complete this and some design improvements to the set in a few days. The pressure is making me a bit edgy.

That said your experience leads me to have completely different feelings about vintage capacitors than you, and whats left of my understanding of college engineering reliability statistics tells me I shouldn't change my mind.

Well I am a professional Electronics Engineer. I know what I have read and been told. In a professional sense, keeping old capacitors like this is wrong. But heck this is a hobby about extreme vintage televisions. The owners of this set put it away in 1960 and forgot about it because by 1960 it was if not obsolete, quaintly old fashioned and dated.

For historical reasons, I want to keep as much originality to it consistent with it working reasonably reliably. Perhaps even the 60 years it sat unused help preserve the electrolytics? Who knows? All I can say is that the capacitors show the correct leakage and capacity and the set will operate for a dozen hours at a time without getting warm.

As for the other components, so long a the picture and sound remain excellent as the set currently does, what is the issue

From purely a hobby perspective and as I wish to keep this set and because I am technically inclined, I am very happy that I have such a good working set with much of its originality maintained
I am going to keep the set running and will update this column as the set approaches 100 hours on time.

Meanwhile I gave an old 630TS someone tried to repair and gave up on. I am going to give it a go and try to clean up the wiring.