Another RCA CT-100 discovery B8001599

[Penthode]

I have been considering replacement of the Selenium rectifiers. They are connected in a full wave volage doubler arrangement. I plan to replace them with a pair of 1N5208 diodes which are rated at 3A with a peak inverse voltage of 800volts.

First I thought I should test the old Federal units to determine their current state and performance. I calculated the typical full B supply current from the schematic diagram as indicated in the photos below. The full load current appears to be 600mA and I used my adjustable low voltage bench supply to see what the forward volatage drop of the selenium rectifier is at 600mA.

I was surprised to find the drop only to be 10 volts. If I was to substitute silicon diodes and ignoring the junction drop of about 0.6 volts, the series resistance to effectively account for the diode substitution works out to be 15 ohms. Power dissipation would be 10 x 0.6 = 6 watts so I should get away with a 10 watt resistor.

I am curious as to the failure mode of selenium rectifiers. I understand the forward resistance rises with age. But 15 ohms at 600mA does not seem bad and both rectifioers have almost identical forward resistance. I suspect that the rectifiers may reach a point where the dissipated heat leads to a thermal runaway situation. Will need to do more research on this.

Not: In my forward voltage drop test, I arbitrarily used 10 ohms in series as a current limit as I brought up the voltage supply. The voltage aimed for across the series load resistance was 6 volts which woulkd man 600mA is flowing.

[old_coot88]

Dunno how true it is, but I've heard tell that reverse leakage in aging seleniums can contribute to heating. Might be urban legend though.

[Penthode]

Logically, the power dissipated by reverse leakage would be minimal unless the selenium rectifier broke down. My understanding is that the forward resistance increases with age. And as the forward resistance increases, the power dissipation would increase. Thermal runaway would occur if the resistance increased with temperature.

[Penthode]

I have completed the paper capacitor replacement as well as the disassembly and reassembly of the IF stages to clean out the mouse nest. Last two remaining jobs:
1) Replace/fix the open peaking coils.
2) Arrive at a solution for the dropping resistors (ballast).

Peaking coils: All of the white encapsulated coils are open. The white concrete like covering of the coils was a really bad idea. I managed to salvage two by carefully crunching off the white exterior and fishing for the coil ends. There were signs of green "vertigris" at the wire ends. I managed to find the ends and reattached. However I still need a couple of 1000uH and the 6900uH coils.

I reflected on the design and have dismissed the substitution of different coil values. The I and Q channels have predetermined bandwidths of 1.5MHz and 0.5 MHz respectively. The coils in the I and Q channels facilitate the different bandwidths hence the values must be maintained. Further, the wider I channel has a delay inserted to align the narrower Q channel so it is important to ensure the bandpasses remain as designed to minimize display chroma registration and alignment.

I am awaiting the delivery of a spool of AWG44 wire and contemplate making some coils myself. Once installed, I shall sweep I and Q channels in the course of overall alignment and testing of the chassis prior to attaching the 15GP22.

2) Ballast Resistor: I have decided to mount the 315 ohm 50W and 800 ohm 25W resistors on a heat sink assembly which will plug into the ballast socket. I decided again using the chassis itself as a heat sink and/or mounting the ballast resistors over or near the selenium rectifiers as I want to keep the heat source isolated as much as possible from the CRT bell. I am concerned the temperature source will be detrimental to the CRT.

Also note I decided against using the selenium rectifiers. I have left the rectifiers in situ disconnected and immediately below them have installed a tag strip to which is attached two 3A 800piv rectifiers and a 10 ohm 25W dropping resistor.

I shall be assembling the ballast replacement this week. The electrolytic cans all check okay and the capacitors have been reformed. I should be on target for a first power up this next weekend.

[Penthode]

I have been making progress with the peaking coils. I have managed to repair three and found a way to accurately measure and recreate the missing coils.

I have been looking for a way to accurately measure peaking coil inductance. The best solution which is repeatable is resonating it against a known capacitance. I am using the setup below.

I am series resonating so as I adjust the oscillator, I will find a dip when resonance is found. I am using a 5% 820pF mica capacitor. (I could not find a 1% tolerance cap). The capacitor is slightly higher than 820pF so my readings are consistently a few percent below. But it is close enough.

The repaired 1.9mH measured resonance an inductance of 1.85mH. The 820uH inductor measure 800uH. and the 1.0mH coil measure 980uH. So all is good.

I am now in a position to recreate the 6.7mH and another 1.0mH to complete the set.

When the set is back together, I shall sweep the chroma and luma channels with the new peaking coils to ensure this CTC2 chassis meets spec.