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.....