Restoration Commences on 1947 RCA Victor 721TCS

[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.

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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.

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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.

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I work from home these days. I switch on the set and tune it daily to an OTA station via my little RCA STB to a local channel that runs 60's -70's shows. It is on about four hours most weekdays. I run it for a few hours on the weekend as well. Still running on the original "properly reformed" electrolytics. Must be in the region of 700 hours now.

This set is a good little performer. A problem occurred a few weeks back when the video went away. Traced it to the last video IF plate load resistor opened.

Recall this was a set I only changed the paper capacitors that I deemed stressed. That is with a high DC voltage across. I left a bunch of original capacitors used for bypass that have only a few volts across. I also left the vertical integrator caps. One electrolytic did not reform: it was the radial lead electrolytic bypass to the audio output screen grid. It had to be replaced all other electrolytics reformed to low leakage at full rated voltage, so I left them to see how long they would last.

Two resistors have been replaced: the aforementioned last IF plate load and the plate supply dropper to the Horizontal Oscillator.

Needed to touch up the video IF alignment after the IF load resistor replacement. Did it by eye by checking to see the carrier is at 50% by taking note of the brightness and the detail and rocking the fine tuning back and forth. A half turn of the last IF coil slug did the trick. Helps that it does not have AGC.