Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

It has now been three years since I resuscitated this set. Recall all of the original electrolytics except for a replaced 40uFd axial lead unit are original. Most of the paper dielcetric capacitors were changed. I left a few units which were bypassed by resistors less than 100Kohm. And a few resistors which I found drifted when it affected performance. I then decided to run the set regularly and report from time to time on it's performance over the years. It is well past 1500 hours.

The set has developed two problems:
1. The original KRK2 tuner fine tuning has a bad back lash which make it difficult to fine tune. I believe it has to do with the slop in the shafte which I have to address. The second is a horizontal sync issue which causes the horiztal oscilaator phase to shift briefly and jump back. It is very momemtary and occurs rarely. Have no idea what it may be and keeping an eye on it.

But this set has been incredibly reliable and delivers a very good picture for a set which is 76 years old. Here are a few snapshots from over the air CHCH TV channel 11 this morning.

[Penthode]

Latest update on the 721TCS.

From the original 2020 restoration:

All the original can electrolytics are still fine with no problem That makes them 76 years old!

I left many of the original wax capacitors which pose no threat due to leakage. The ones I left are shunted by resistances 1M ohm or less and pose no DC threat.

Only two resistors changed asthey affected performance (horizonal plate resistor and video amp plate resistor.

An axial lead 40uF capacitor changed due to leakage.

Recent service:

KRK2 tuner fine tuning problem - slop due to wear in the brass tuning wand worn making it hard to tune for best sound (recall this is a split sound set). I made a new brass wand and attached it to the original tuning shaft disc. The repair fixed perfectly the fine tuning issue.

Horizontal jumping. the picture would jitter horizontally due to what appearred to be a time constant change in the circuit. Changed 6SN7 oscillator tube which fixed the problem.

Set remains working fine after regular 3 1/2 years use on all original electrolytic capacitors.

Here is a screen shot from this morning with the signal from an over the air signal via an ATSC set top box.

[old_tv_nut]

Originally Posted by Penthode

...
Here is a screen shot from this morning with the signal from an over the air signal via an ATSC set top box.

Beautiful!

[kvflyer]

Originally Posted by Penthode

Latest update on the 721TCS.

From the original 2020 restoration:

All the original can electrolytics are still fine with no problem That makes them 76 years old!

Interesting.

I recently restored (But did not align) my 630TS and found that half of the original electrolytic capacitors tested good (out of circuit) for leakage; the other half were no good. I restuffed the bad ones and reconnected those testing good. I could see that those which tested good were a different manufacturer (or design or whatever, just different). Sure made things easier.

I guess they may be from the same era/manufacturer?

[Penthode]

Originally Posted by kvflyer

Interesting.

I recently restored (But did not align) my 630TS and found that half of the original electrolytic capacitors tested good (out of circuit) for leakage; the other half were no good. I restuffed the bad ones and reconnected those testing good. I could see that those which tested good were a different manufacturer (or design or whatever, just different). Sure made things easier.

I guess they may be from the same era/manufacturer?

The capacitors in this 721TS initially looked bad. But the set had been stored away and left dormant for 55 years when I bought it. I slowly reformed the capacitors over the course of four days. The key is to limit the current to just under 10mA and slowly bring up the voltage until it meets or slightly exceeds the maximum rated voltage. I would then let it sit at the maximum voltage for a few hours and notice the current continue to drop until it was no more than 200uA at maximum voltage. All of the capacitors came back that way.

However, I have a Canadian market RCA CTC5 color TV. The set was manufactured in Camden NJ and in Montreal it was modified for 25Hz power. The modification included replacing the power transformer on the main chassis with a 25 Hz filament only transformer and then adding a separate small additional chassis with a massive B supply transformer, an additional filter chole plus two Canadian mad Sprague 80uF 500v electrolytic capacitors. The main chassis had Camden installed Mallory capacitors.

When I restored the set in 2014, the Mallory capacitors reformed dielectrics were very low leakage 100uA or less. The Sprague's were close to 1mA leakage at 500v. I left them in and 8 years later one shorted and destroyed a pair of 5U4 rectifiers. I felt at the time the Sprague's were marginal. But the Malloys remain fine.

[Penthode]

It has just reached four years since the restoration. The restoration preserved all the original chassis mount electrolytics and carbon resistors. Only the paper capacitors not shunted by resistors were replaced.

Sixteen paper capacitors and an axial lead 40uFd and capacitor were replaced. Two out of tolerance were replaced with identical type carbon resistors.

The only mechanical issue was a loose worn brass fine tuning shaft which I remade and replaced. A full IF RF alignment was done.

I switch on the set regularly and it remains in excellent operating condition. The snapshots are from an off air signal this evening.

The aim of this thread was to see if the original 77 year old electrolytics would stand up to modern use. As I pointed out earlier in this thread, I took care reforming slowly the original electrolytics.

[Alex KL-1]

Impressive. And equally impressive is the lifetime of these electros!