Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

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.....

[Penthode]

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.

[vts1134]

I'm looking forward to following this.

[Penthode]

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.

[Tim Tress]

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

[Penthode]

Originally Posted by Tim Tress

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.

[Penthode]

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.....

[AlanInSitges]

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

[Penthode]

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.

[Penthode]

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.

[Tom9589]

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

[Yamamaya42]

Originally Posted by Tom9589

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

[old_coot88]

Originally Posted by Tom9589

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

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

[Penthode]

Originally Posted by Tom9589

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

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.

[Penthode]

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.