Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Just an update after 4 1/2 years of regular use. Only problem in the last 4 1/2 years was the KRK2 fine tuning shaft. I made a new one out of brass riveted to the original fine tuning disc.

Still using all of the original chassis mount electrolytics. 78 years and still fine. The restoration entailed only replacing the wax caps that needed replacing eg subject to a a large DC across them. A three resistors had drifted affecting operation. Tested transconductance on all tubes and replace a few with original period NOS RCA tubes.

The main point is the originality of the electrolytics. They survived because the set was not plugged in after the set sat unpowered for over 50 years. Slowly reformed the capacitor dielectrics and they are all still fine. One axial lead electrolytic was bad and was replaced.

Here are a couple of snapshots off air made this afternoon. Can you name the film?

[Penthode]

Thought I should resurrect this old thread considering the significance of the originality of my 1947 721TS. I resuscitated this set nearly 5 1/2 years ago and this was to be an experiment to see if a fairly well preserved set could be reliably operated replacing parts only what were absolutely necessary.

In the resuscitation of this set, it retains all but two of it's carbon resistors. About 95% of the paper dielectric capacitors were changed. The ones that were retained were shunted by resistances which swamped the leakage resistance of the capacitors. It retains some of its original tubes. I carefully selected RCA period tube for replacement so it is a fully RCA tubed set. When I received the set 10 years ago, it had sat in a heated attic in a house near Schenectady for just over 50 years. Upon pulling it apart I found a small mouse nest under the chassis and fortunately it caused almost no damage. The set was not powered from the early 1960's until after it's resuscitation in 2020.

However, of major consideration is this set retains all of it's original electrolytic capacitors. This thread describes the extra effort I took to preserve the capacitors. I am not averse to changing the electrolytic ic they test bad. But slowly reforming the dielectric on each electrolytic in this set brought them all back with very low leakage.

So the bet was on as to how long they would last? I power the set on average once a week and let it run for a couple of hours. I decided to check the capacitors this evening and can report that they all appear fine.

Here after 5 1/2 years, after the careful reforming of all the original electrolytic cans, are some off air shots. (I did replace the original 40uF axial lead electrolytic but all the chassis mounted cans are now 78 years old.

[old_tv_nut]

Gorgeous pics!

[Penthode]

Thanks.

[hardpanman]

"The Lady Vanishes".

[dtvmcdonald]

I notice this thread return today. I have a comment on electrolytics.

Earlier this year I opened up and checked the electrolytics on a ~1975 device I have that has run with no repairs for over 250000 hours. They check out fine, low leakage, low ESR, and within spec capacitance. This set ran continuously except for long vacations for over 30 years. It is a Hafler 280 watt audio amp. I also checked the amp's performance, and it meets specs.

Just to be sure, the words are "two hundred fifty thousand hours". The caps are mounted well away from the transformer and heat sinks, and it has been well ventilated.

[stuben]

I’ve had the same experience with equipment that used Sprague Atonms The blue aluminum electrolytics with axial leads. I used to own a dental laboratory which I opened in 1980. One furnace that I fire porcelain in heats a small chamber to 2000 degrees F. Since it was installed it’s never been turned off except for a week or two during vacations. It’s well ventilated with heat sinks to protect critical components. I’ve never bothered to check the capacitors as it still works the way it should nearly 50 years old now.

[Penthode]

Next month it is 6 years since I reformed the electrolytics. I still regularly use the set and all the chassis mount electrolytics remain fine.

Recall the set remained in an attic for over 50 years unpowered. I attribute this the electrolytic capacitor survival to slowly reforming the dielectric. When the capacitors have been reformed, the current should be no more that 200uA at full rated voltage.

Note when I began reforming the capacitors, each began as a near dead short. The initial 10 milliamperes was reached at about 15 volts! Over a period of about six hours, the current dropped slowly until after six hours, the current was 200 uA at the capacitor full rated voltage. Periodic testing showed that the current remained at around 200 uA which is normal for these types of electrolytic capacitors.

Here are some photos 721TS this morning on it's 79 year old electrolytics. The video source this time is a VHS tape of of the motion picture "Mr Smith Goes to Washington" played on my 1977 RCA VBT-200 VHS machine: the first VHS machine introduced to the US market.

[old_tv_nut]

[Penthode]

It was six years ago in June 2020 I restored this set.

To recap, I intended to make this a thoughtful restoration. That is I only replaced what was absolutely necessary. I would only replace what I felt necessary to affect the operation of the set.

I attempted first to reform all the electrolytics. Amazingly all the chassis mount can electrolytics reformed well with minimal expected leakage. An axial tubular 40uF.electrolytic used as the audio output screen decoupling capacitor was bad and replaced by a modern unit.

I next used my transconductance tube tester to check and replace weak and defective tubes.

Analysing the circuit, I next only replaced the paper capacitors that were not shunted by a low resistance. In other words, if the capacitor was in a high impedance circuit or there was a high dc voltage normally across it, the capacitor was replaced.

Lastly, resistors were only replaced if they caused impairment to the sets operation. After the capacitors were dealt with and when the set was powered, I would look for any obivious operation faults which would point to a bad resistor. Then even if there was no apparent fault, I would measure and compare the voltages recorded in the RCA service notes.

In the past six years, I expect it has operated for about 2000 hours. The only issue I have had was a worn 1/4" brass fine tuning shaft. the excessive play made it difficult to fine tune. Recall this is a split sound non-intercarrier set and tuning for best sound was difficult. The solution was replacing the brass shaft on the KRK 2 tuner variable capacitor. That was two years ago and it remains excellent since the fix.

Here are a couple of snapshots from tonight's Saturday Night Live.

[ronl]