Another RCA CT-100 discovery B8001599

[old_tv_nut]

Beautiful!

What percentage of the work would you say has been done towards first light?

[Penthode]

Originally Posted by old_tv_nut

Beautiful!

What percentage of the work would you say has been done towards first light?

What percentage? I feel I am almost to the point where the real restoration may begin. Re instituting the removed chassis section will take another full day to do it properly. And then replacing all the paper capacitors and tidying up previous repairs. The power supply needs work: what to do with the ballast resistor And all the filter electrolytics. RCA used excellent quality electrolytics in the late 40's and early 50's so I will be curious how the CT-100 filters will be. And there is refinishing the cabinet while the weather is good.

I would like to think I have nearly reached the 50% point before the first power.

I have learned a lot about the curious nature of the CTC2 chassis design. It does not feel like a big run chassis from a number of little anomalies I have found such as component placement. Certainly with the high number of power resistors, at 475w it is not a power efficient design.

[ARC Tech-109]

In the day when an uninsulated home was heated with electric resistance another 475 watts was not a big concern

[Penthode]

Getting the subchassis's prepared for putting them back together. And scrupulously going over connections and components.

I make ote of the Peter Deksnis alert for the 6BK4 HV Regulator replacement for the 6BD4. As he discovered, pin 8 on the tube socket is used as a tie point for the original 6BD4 circuit. Substituting the later 6BK4, it has a pin 8 interconnection to the cathode.

https://www.earlytelevision.org/Deks..._6bd4-6bk4.htm

I added a tag strip wiring point to carry the connection away from pin 8 of the tube socket as,depicted below. After this last item, I recounted the HV cage back to the main chassis.

The CRT socket HV leads for the electrostatic focus and convergence electrodes had deteriorated and,were,about to fall off. I missed out on the one recently for sale on eBay. Taking the socket apart, revealed the HV electrodes encapsulated in a molded plastic piece. I had to break the plastic surround to remake the connection and in the process of making a small plastic surround replacement.

The vertical integrator leads were corroded from my mouse friends to the point one lead easily disintegrate and dropped off. For those familiar with these things, it is a Central "Couplate" number PC-101. I decided to remanufactured one as it has only a few passive components: a 002uF, two .005uF and 22k and 8.2k resistors. I ordered from Digikey some surface mount chip components I will put on a little board less than the size of the original couplate.

[Penthode]

I assembled my replacement for the PC-101 couplate this evening. Parts cost about $1. Cheaper than the Centralab unit.

Surface mount resistors and capacitors on a scrap board. The 0.01uF cap is a small ceramic disc mounted on the rear.

[Penthode]

Making more progress on the CT100. The reunited wiring is complete! Spent over a dozen hours reuniting the HV cage and the front section including the IF amplifiers to the main chassis. Laboriously checking photographs, my notes and the schematics (as there are variations) finding where the approximately 35 wire and components originally went. The photos help ascertain the original lead dress.

Some of the original wiring looked messy. I tried to clean up some of the worst congestion but ensured, especially around the IF sections I had to fully remove, that it was as close as possible to the original layout.

Next comes the replacement of the remaining paper capacitors. I tested a few and the leakage is incredible. It only goes to prove that plugging in any set of this vintage or older without addressing the paper caps is crazy.

As for the electrolytics, I aim to see if they will reform with low leakage. It will be easy to weed them out and if there is any doubt, out they go.

Here are some photos where this CT100 is now. Note the lower right which was previously the mouse abode.

[old_tv_nut]

Great!

Glad to see you recognize the importance of routing to proper function and are avoiding changes for the sake of visual neatness in the high frequency areas. You probably also know that routing and grounding points can affect the coupling of sweep currents (horizontal ringing "jail bars") into the video, but thought I'd mention it here for others who may be newer in the hobby.

[Penthode]

Been going through the chassis this evening. Every one of the white blob peaking coils is open circuit: 180uH, 1.0mH, 1.6mH, 1.9mH and 3.6mH. The biggest ones are in the I and Q chroma channels.

I have an NOS Meissner 180uH replacement. But it is difficult sourcing the others. Anyone know a good source of supply?

I found on Amazon a spool of 44 AWG magnet wire and may end up winding them myself. Easy to measure the inductance but achieving the correct Q may be a bit of a challenge.

[dtvmcdonald]

As to peaking coils: don't try to find NOS ones. Just get the small to medium
size (size of old fashioned 1/2 or 1 watt carbon resistors) cylinderical axial
ones from Bourns or API Delavan. Get shielded if they have them. Make sure
the self resonance frequency is high enough.

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.

They don't look right but work very well. You may need to search both Mouser and Digikey to find all values.

The picture on my CT-100 is the best recommendation for these.

The coil's Q is not a problem, all of those coils are plenty good enough because they
have ferrite forms. The shielded ones also have ferrite outer shields.

[dcl0]

I was able to get an assortment of peaking coils quite reasonably from Moyer electronics in PA to solve the same problem I had with my Hallicrafters set.

[Penthode]

Originally Posted by dcl0

I was able to get an assortment of peaking coils quite reasonably from Moyer electronics in PA to solve the same problem I had with my Hallicrafters set.

That is good to know. I will check it out. Thanks.

[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.