Another RCA CT-100 discovery B8001599

[old_tv_nut]

'I can picture in my mind RCA Victor engineers calling the Centralab engineers in around 1953 asking them to beef up the specs from 3A to 5A and the incredulous Centralab people replying "What the heck are you powering?" '

It's the bread/bagel switch!

[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.

[old_tv_nut]

Going nicely!

Edit: If you short the 820 pF you could also measure the self-resonant frequency, although you will have to correct the result due to the scope probe capacitance.

[Penthode]

Originally Posted by old_tv_nut

Going nicely!

... If you short the 820 pF you could also measure the self-resonant frequency, although you will have to correct the result due to the scope probe capacitance.

Measuring the self resonant frequency is an interesting thought, The original peaking coils are machine wound in a pattern to minimize self capacitance. My winding is by hand bunch wound which inevitably will affect self capacitance.

It would be interesting to compare a coil I make by hand with a NOS Merit machine wound coil which is first made to match resonance with the 820pF and then tested for self resonance. The peaking coil is designed to resonate with the circuit self capacitance hence my winding may skew in-circuit performance.

This will all become apparent on the final I and Q channel frequency sweeps.

[Penthode]

I knocked together a CT-100 ballast resistor replacement on the weekend. It plugs into the ballast socket.

I bought 350 ohm 50w and 800 ohm 25w chassis mount resistors and attached to a heatsink I bought on Amazon for another project. The octal male plug was from a relay and was mounted on an angle so that when plugged into the CTC2 chassis, the fins sit upward.

I used fiber glass spaghetti on heat resistant wire to the socket. The aluminum bracket attaching the heatsink to the octal plug was fashioned from a scrap aluminum plate or bracket my my scrap bin.

It will plug into the rear facing CTC2 ballast socket above the power transformer and will deflect and isolate the heat source away from the 15GP22.

The chassis is complete apart from two peaking coils. This evening with the chassis on the bench and everything connected except for the CRT, I plan to insert the tubes and provide the power up of the chassis in in over 40 years.

[Penthode]

Last night I powered up the chassis. The power supplies came up and with the new ballast resistors installed, the voltages appear correct.

The only immediate issue was no audio. I had expected the rush of noise from the speaker and traced it to a short from the plate of the 1st audio IF to ground. Unsoldered and removed the plate connection and the short went away. Reconnected and I heard the rush of noise on power up and then it disappeared. Reasoned that the RF and IF were being biased off so a slight rotation of the AGC control brought up the rush.

Connected the Zenith set top box with its modulator on Channel 3 and I can hear program audio. Looks like all my video audio IF rewiring was okay.

Have made preliminary measurements through the chassis and all seems well. The high voltage is at 22kV and the regulator is working as I can swing it up and down with adjustment. Horizontal sweep is on frequency. The photo below the chassis is running and you will see the Heathkit VTVM reading 22kV with my Heathkit probe. (I bought the Heathkit IM18 VTVM as,a kit in 1971: hard to believe I have been using it for 51 years!)

My ballast arrangement is fine. With the heat sink, the temperature levels at 55 degrees Celsius which is much lower than I had anticipated. And that is with the correct voltage drops to the two B plus branches.

I still am missing a 1mH and 1.9mH peaking coil. The 44AWG wire arrived and so I think I will have a go at that tonight.

The chassis has run about half and hour with no issues.

[dtvmcdonald]

The easiest and best way to measure the self-resonant frequency of a coil
is with a grid-dip meter and freqency counter or SDR receiver to get frequency accuracy.

That's if they are unshielded. If shielded, feed from a signal generator
through a 270K resistor and measure with a very low capacitance scope probe:
hook a 0.5 or 1 pF cap inseries with a short piece of coax. IF picky you can of course calculate the unloaded frequency.

I used both methods on my commercial ones for my CT-100 and they agreed.
I suspect you will be appalled with the frequency response of the Q channel.

[Penthode]

Originally Posted by dtvmcdonald

The easiest and best way to measure the self-resonant frequency of a coil
is with a grid-dip meter and freqency counter or SDR receiver to get frequency accuracy.

That's if they are unshielded. If shielded, feed from a signal generator
through a 270K resistor and measure with a very low capacitance scope probe:
hook a 0.5 or 1 pF cap inseries with a short piece of coax. IF picky you can of course calculate the unloaded frequency.

I used both methods on my commercial ones for my CT-100 and they agreed.
I suspect you will be appalled with the frequency response of the Q channel.

Yes I have my Measurements Lab Grid Dip meter and plan to use to test it in that manner. However I have had consistent and pretty accurate results resonating the coil with my signal generator and frequency counter.

What is appalling about the Q Channel response? It is suppose to cut off at 500kHz. I was hoping to perhaps tailor it once I do the sweep.

I have been testing the circuits. I have audio and I can see some composite video. The Horizontal oscillator is properly locked but I have concerns about the vertical lock. I first need to do a complete IF alignment as I expect the IF response to be pretty horrible having had to reconstruct the strip.

I next need to fashion a jig to inject the frequency sweep to the mixer input.

[AlanInSitges]

I for one am very anxious to see the first power-up post-peaking-coils.

[Yamamaya42]

Originally Posted by AlanInSitges

I for one am very anxious to see the first power-up post-peaking-coils.

In other words... it has piqued your interest!

Could not resist! :P

[Penthode]

Originally Posted by Yamamaya42

In other words... it has piqued your interest!

Could not resist! :P

Brilliant!

Haven't gotten arond to addressing the peaking coild yet. Hopefully this evening.

Neverthe less have been letting the chassis run. I reformed the electrolytics before powering and they seem fine. All the voltages seem correct so far up to the sync separator. I can see composite video on the scope and horizontal sync good. V ertical a bit touchy. Remember I recreated the couplate PC-101 integrator network. Seems the problem goes back to the vertical sync separator where the voltages are a bit low and the sync output appears a bit low.

The HV is holding at 21 kV. The vertical convergengence transformer seems fine so far.

A longer run for over an hour produced the new resistive ballast unit temperature reaching 75 degrees celcius. Too hot to leave your finger on but a lot less than the 125 degrees I measured when it was clamped to the chassis.