Restoration Commences on 1947 RCA Victor 721TCS

[Electronic M]

Originally Posted by Penthode

I have been silent the last couple of days. I estimate the set has 75 hours and the electrolytics are fine.

I take exception to the comment Tom made. If the capacitor has sat unused for 60 years, the reforming takes hours. It is an electrolysis process you cannot hurry. In this case the capacitors as would be expected would initially test an almost dead short because in the sixty years the dielectric would have disappeared. That is why even applying a Variac is murdering the capacitors. The key is a gradual process that will take about 24 hours.

I limit the current to no more than 5mA at the start. By the time the 450v was reached, the leakage was about 200uA at full rated voltage. Then test capacity and if it is within 20% of the rated value, it is good to go. The important thing is patience.

So I dug up the crusty old space Fada 630TS clone chassis I had and removed two electrolytics. This set sat in a shed for years and the cabinet was shards. The chassis had a lot of rust and the capacitor were pretty grubby when I removed them. There appears to be a date code which suggest the capacitors were made within a week of each other in August 1947.

I put them onto the Sprague TO-6 capacitor analyser power supply to reform them. After 24 hours each of them reached filled rated voltage with a leakage current for each section below 100uA. And tested capacity and all were within 10%. I have never won a lottery and I do not think this is sheer luck. The success is patience and not using the Variac!!

I am going to punish one of these capacitors by running them at and for good measure 5 to 10% above rated maximum voltage to see if I can get it to fail. The other I will keep it at maximum rated 450 volts and then dissect it. Certainly we should not expect to see a picture similar the Banderson's which was apparently insufficiently reformed before applying normal operating voltages.

Here are the candidates. And remember, this is not one off's or luck. I am finding a consistency here.

The ones that the leakage didn't drop fast on got replaced with new capacitors... meaning some potentially reformable caps got changed.... despite that the best of the original caps still eventually failed. If the ones that don't have measurable leakage on my C3 from the beginning are going bad then the ones that were leaking sure as hell are not going to last.

[JohnCT]

Originally Posted by Electronic M

If the ones that don't have measurable leakage on my C3 from the beginning are going bad then the ones that were leaking sure as hell are not going to last.

Back when I became a full time bench tech (70s), I was changing can caps almost daily, and these coming from TVs with a quarter of the years on them that we have now and despite having a fully formed oxide layer from daily use...

Every parts store back then had a full line of Cornell-Dubilier, Arcolytic, Mallory, or Sprague drop in cans.

NOS, used stock, reformed.. they're all time bombs - even back then when they were only a few years old. It's not a matter of if it's a matter of when.

John

[Penthode]

The 721TS had only another 7 hours this past couple of days reaching about 80 hours. No sign of the capacitors failing.

Have been running one of the 630TS capacitors I had reformed at its full rated 450 volt rating. Will leave this for a week and monitor the leakage current.

One consideration: the late 40's RCA sets had full wave power supplies with capacitor of relatively low capacitance e.g. 40uF to 80uF at 450v. There are a number of factors in the design that put less stress on the electrolytics in these early sets: ripple current in the full wave rectifier, low deflection angle tubes which required less defection power etc. In the fifties, the designs pushed the limits of components to keep the consumer purchase cost down. The electrolytics were stressed with higher ripple currents, increased capacity in the same form factor. I know the RCA CTC11 color set I have has had to have all its electrolytics changed. The voltage doubler capacitors have been changed twice!

So there are various factors at play here. There is no one size fits all solution and I am only citing the specific capacitors used by RCA in their sets in the late 40's in this instance.

As this is a hobby endeavor and I am speaking from a hobbiest point of view with regards to conservation as opposed to wholesale rebuilding, intelligent assessment of the components is advised. The 1949 RCA 8T243 television which I am still running on its original capacitors I have owned for 50 years and the set still performs fine.

[Tim Tress]

I have been servicing radios, TV sets, and hi-fi/theatre/jukebox amps for 50+ years now; the problems with electrolytic capacitors were well known, even back then. Today, that stuff is 50-90 years old, and those old caps are like a time bomb. The survival of a "dry" electrolytic capacitor is largely due to the integrity of the rubber seals on the case. Once the electrolyte dries up and corrosion sets in, it's dead.

When servicing for a customer or commercial account, reliability is of the utmost importance, to avoid the dreaded "callback". Hobby work is different, of course, but when a reformed capacitor fails anyway and has to be replaced, it's time wasted.

Except for commercial theatre-sound and jukebox amplifiers, most consumer equipment back then did not have an AC fuse, and a shorted capacitor can take out the power transformer. New capacitors are cheap insurance, and I add a fuse in the AC line for more protection.

[Penthode]

And update on the 721TS running its original capacitors. It has now reached 350 hours. I am writing today because I pulled the chassis to make a minor mode e.g. reduce the value slightly of output transformer primary parallel capacitor at the plate of the 6K6. This is provide the audio with a bit of a treble boost. I find they liked muddy audio in the '40's.

I looked at all the original electrolytics and they are remaining cool. I disconnected two to measure leakage and they are well below 1mA.

I haven't spent much time on the two electrolytics I pulled from a junked 630TS chassis. I had been running these capacitors rated a 450v on a 450 volt supply. I left them 48 hours and the leakage remained about 0.2mA. I had put them aside the last few months and will continue my test on them. Ultimately I planned to tear them apart to look inside but they have held up well.

I think the key is the patient reforming. Never run even a variac to a set 70 years old until reforming the capacitors. While reforming, monitor and limit the current to no more that 10mA at the start and monitor regularly as the dielectric reforms. At the start of the reforming process on the 721TS, the voltage remained at about 10 volts at 10mA for about 10 minutes until it started to climb. In about 6 hours the current at 400 volts was about 1.5 mA and leaving it overnight brought it down to less than 0.5 mA. Testing today the couple I tested the leakage remains the same.

The key is patience. My background is electronics engineering and I have been repairing vintage electronics since the late '60s. In the last 50 years, I have done my share of electrolytic replacements. I am only pointing out that I do not immediately condemn the capacitor until I test it out. I recently restored a 1954 HP High Voltage 0-600 volt Regulated DC power supply and found four of what I would have expected to be quality capacitors bad and had to restuff them. So I insist this doesn't always work. But if good quality Sprague or Mallory electrolytics are used, it is worth giving them a chance.

[Penthode]

I have an 721TS update. It's not about the filter capacitors this time. The set has about 350 hours since my minimalist restoration and the filter capacitors are still fine).

This is about a nagging sync problem. I am surprised how well this little set works despite it being a pared down design. The sync separator is very basic: a sync amplifier fed from the video amplifier (12AU7) plate to a 1/2 6SN7 Sync Amplifier followed by a 6AL5 sync clamp to1/2 6SN7 sync separator. The set is pre-AGC with the contrast adjustment controlling gain in the RF/IF section of the receiver.

The problem stems from a relatively narrow adjustment range affecting horizontal sync. This set features the earliest version of RCA's classic a later widely licenced Synchroguide Horizontal AFC without the stabilization coil. I checked pin 6 (cathode follower sync feed) as I varied the contrast control and found that the sync separation diminished either side or a very narrow range of contrast adjustment. In other words, the sync separation was not very good.

This is not suprising considering the limting factor of cost in the design. The sync clamp or limiter works well as it keeps the sync tip at at fixed DC level. I tested and swapped the 6SN7 with no improvement. And the capacitors all tested good resistor values good.

I was tempted to swap the low amplification factor (20) 6SN7 with a (70) 6SL7. But this would require further redesign as the amplifier gain would be too great.

Picture 1 is the Sync Section of the 721TS with the modification applied.
Picture 2 Is the 621TS Sync Separator included for comparison.
Picture 3 is the separated horizontal sync.
Picture 4 is a picture off air of TVO this evening via an RCA branded set top box. The set is running behind where I am writing this.

I ultimately considered altering the cut off of the sync separator portion of the 6SN7: by applying a pull up resistor of 510kohm as depicted, the clipping action would remain wider with video gain changes. The result is that the waveform depicted was maintained at a somewhat wider range of contrast adjustment. This will be displayed in the video as less video pulling of the horizontal sweep as the contrast control is adjusted.

The sync separator cathode after adding the resistor, showed less ingress of video (in the range depicted with the white arrow) over a wider range of contrast control adjustment.

I would be curious how other with the 721TS finds this problem with contrast adjustment. I have included the circuit of the 621TS which is very similar to the overall design to the 721TS. The 621TS uses the earlier 6H6 as opposed to the 6AL5 for the sync clamp. And the 621TS is still pre-Synchroguide and would be curious how the contrast adjustment affects it.

I have added the resistor and am happy with the overall result.

Below:
Picture 1 is the 721TS sync separator diagram with the modification made.
Picture 2 is the 621TS sync separator included for comparison.
Picture 3 is the 721TS sync separator output horizontal sync waveform with the range depicted where you will see video ingress when the contrast control is adjusted.
Picture 4 is the set sitting behind me as I write. It is receiving this evening TVO (TV Ontario) via an RCA Branded set top box.

[Penthode]

II know I am winding everyone up by posting this. But the set has I expect past 400 hours. In my pandemic office at home where I work, I have been runni g the set about six hours a day. CHCH-TV runs old sixties,/ seventies shows all day.

I had the set apart at about 350 hours to check the electrolytics. The two I tested (rectifier output) leakage was about 400uA at full rated voltage which is normal.

The only electrolytic which was bad was a tubular axial mount 25uF at the audio output supply. This,was replaced as it failed my original test. All others have remained original. And about half of the original paper capacitors I left if they were in the circuit shunted by a resistance of 100k ohm or lower. All the other paper capacitors have been changed.

A couple of resistors,were changed as they were out of tolerance and affected performance.

Here,are so.e snapshots from this afternoon oat via an RCA ATSC set top box.