Another RCA CT-100 discovery B8001599

[Penthode]

Originally Posted by dtvmcdonald

I do very strongly recommend that you don't use the approx 6 mH specced
for the Q channel ... it seriously ruins the chroma reaponse. Get one that's in the 2.7 or 3.3 mH range.

After thinking about this a bit, I believe the large inductance was to specifically limit the Q bandwidth to roll off at 500kHz. The I channel is 1.5 MHz wide.

Remember the I and Q are transmitted vestigial sideband in quadrature which means extending the Q bandwidth will lead to quadrature crosstalk. Further in this early design both Y and I channels have delay inserted to align with the limited bandwidth Q channel: Y a larger delay than I. Reducing the peaking coil inductance will extend the Q bandwidth which will reduce the delay.

All sets I believe following the 21CT55 used equa-band narrow channel (500kHz) chroma demodulation apart from my CTC5 Deluxe which has a equa-wide-band chroma demodulator. This means the CTC5 Deluxe intentionally introduced quadrature crosstalk in what appears to be an attempt to increase marginally chroma resolution. The crosstalk is noticeable at abrupt color transitions. (There was a 1955-56 RCA review I recall reading about this many decades ago. I need to look for it).

So I am going to investigate this further. Certainly I can do some tests with the peaking in circuit resonance with the technique outlined in my Measurements Lab. Grid Dip Meter applications notes pertaining to video amplifier design which outlines how to determine peaking coil inductance in circuit.

[old_tv_nut]

Originally Posted by Penthode

After thinking about this a bit, I believe the large inductance was to specifically limit the Q bandwidth to roll off at 500kHz. The I channel is 1.5 MHz wide.

Remember the I and Q are transmitted vestigial sideband in quadrature which means extending the Q bandwidth will lead to quadrature crosstalk. Further in this early design both Y and I channels have delay inserted to align with the limited bandwidth Q channel: Y a larger delay than I. Reducing the peaking coil inductance will extend the Q bandwidth which will reduce the delay.

All sets I believe following the 21CT55 used equa-band narrow channel (500kHz) chroma demodulation apart from my CTC5 Deluxe which has a equa-wide-band chroma demodulator. This means the CTC5 Deluxe intentionally introduced quadrature crosstalk in what appears to be an attempt to increase marginally chroma resolution. The crosstalk is noticeable at abrupt color transitions. (There was a 1955-56 RCA review I recall reading about this many decades ago. I need to look for it).

So I am going to investigate this further. Certainly I can do some tests with the peaking in circuit resonance with the technique outlined in my Measurements Lab. Grid Dip Meter applications notes pertaining to video amplifier design which outlines how to determine peaking coil inductance in circuit.

I think you've got it right. Since the Q coil is large, it's resonant frequency probably is also more critical - too low and you could bite into the 500 kHz bandwidth and get ringing. Going the other way, a little quadrature distortion could be hard to see on a small screen.

[Penthode]

Going through the chassis replacing capacitors and the odd resistor. The previous owner replaced a number of capacitors it appears years,ago with orange drop Spragues but left long leads on the replacements. Tidyi g up as I go along.

I saw a little wire jumper under the chassis I initially thought was jumping an old fuse. Found it bridged the power switch. I pull out the switch and found it quite charred. It was a Centralab switch and potentiometer so I proceeded to look though my box of old parts. Found many exact physical replacements. However I found that all the other switches fitting the control were only 3 amps and the burnt out switch was rated 5 amps. Think about it: this set dissipates 475 watts which means it must drawabout 4 amps! To avoid any physical modification to the control, with the 3 amp swich unit now married to the old CT100 control, there is space underneath to fit a 15 amp @ 115v Potter Brumfield relay.

I amazes me the power this beast consumes!

[Penthode]

Making further progress. Hoping to within the next couple of weeks power up the chassis minus the CRT.

Replaced most of of the paper capacitors. This chassis takes a lot and awaiting an order from Digikey to complete.

Reforming the can electrolytics: all coming up with low leakage after a few hours feeding a 10mA current limited supply. It looks like this set has sat dormant for the past 40 to 50 years as all the earlier replacement capacitors are quite old themselves now. Despite that, the electrolytics came back from almost a dead short to holding full rated voltage with less than 100uA leakage in just three hours. Not bad. We will see how they fare.

The design uses early cartridge electrolytics for DC coupling in the chroma stages and despite coming back with low leakage, I am substituting new ones. The 4uF 450v units are on order from Digikey.

My power switch solution entailed installing a 120vac relay. The relay contacts are rated at 25A at 120vac. It was interesting to note the original Centralab 5A switch on the volume control physical form factor matches the standard 3A switches. 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?"

Anyhow, the photo below show the fitment. The replacement 3A switch on the volume control now only has to energize the relay.

I shall be substituting 3A silicon diodes for the old Federal selenium rectifiers. And still considering various solutions for the super hot voltage dropping resistors.

The KRK12 tuner unit has been fully dismantled, cleaned and lubricated and is ready to re attach to the main chassis.