Adding Composite Video Input to my RCA CTC21 Chassis

[loud41]

I just got a 1967 RCA HH-860F aircraft carrier console! Only problem is that the automatic tuners on the CTC21AC chassis are on the fritz. They constantly cycle the channels. I don't think the set has ever been serviced as all the original tubes are still in place, including the crt. Very clean inside apart from dust. Came from a non smoking home too! I'd like to just ditch the tuning circuits and pipe in a composite video signal as I'd just like to use the set as a "monitor" for my old gaming consoles. I want to keep everything in place so it's all reversible.

I restore antique radios as a hobby, but this is my first vintage TV. I figured it would be similar to adding an aux input to a radio chassis, but after looking at the Sams schematic for the CTC21, boy my head hurts lol.

I see the video output tube, but it looks like it wants a strange input voltage signal. The 2nd video amplifier seems to want a more standard 2v signal, but the 1st video amplifier wants 3v? Should I utilize those two amplifier circuits? Either way I'm going to need to boost the signal from the gaming consoles as they output 0.7v peak to peak.

Or should I just deal with the tuner circuits and get them working? Pretty sure the UHF and VHF motor gears are stripped as it just positively wails whenever it activates, and the UHF tuning knob is frozen in place. The VHF tuner cycled a few times, but now that wails as well without spinning the dial. The motors constantly are cycling, picture signal or not. It's very annoying lol. I don't want that automatic feature anyway as I'll only be using either channel 3 or 4 if I end up using my VCR as a modulator.

Lastly, save the lectures on high voltage. I have great respect for these devices already. I'm an idiot, yes, but not completely stupid. I take great pride in my work and am always very safe with everything I do. If there's something that needs to be discharged (like the crt tube for example) or whatever, let me know. Just save the diatribes on how these devices kill everyone that touches them except the cable guy.

[old_tv_nut]

Generally, video has to be introduced in place of the video detector, before the first video amp. (The composite video needs to be injected before the sync and chroma take off points and luminance delay line.) You need to duplicate normal detected peak-to-peak amplitude (around 4 volts or so in many sets) and the DC level.

[Electronic M]

Probably easier to fix the tuners. If you are only ever going to use CH3 just unhook the motors from power and manually adjust the VHF tuner to CH3...may want to clean the CH3 contacts, and you'll probably need to set VHF fine tuning.

It probably isn't much harder to fix the power tuning system, but that's not necessary.

[loud41]

I see. I've attached a small screenshot of the schematic showing the location of the video detector for any who might be interested.

Yeah that's kinda what I was thinking as I've been staring at the back of this unit. Unplug the motors, clean the pots and contacts, and not really mess with everything else unless I have to. I don't want to disturb more than I really should.

[damen]

There is an audio muting transistor mounted on a terminal strip at the top of the tuner mounting assembly just above the VHF tuner. It depends on an negative and positive voltage to work correctly, it's also connected to the tuner programming switches. If you disconnect the motors it may be necessary to disable this muting circuit. There's a small coil (it looks like a carbon resistor) connected to the terminal strip and the volume control terminal, just disconnect one end of the coil.

[old_tv_nut]

Originally Posted by loud41

I see. I've attached a small screenshot of the schematic showing the location of the video detector for any who might be interested.

Yeah that's kinda what I was thinking as I've been staring at the back of this unit. Unplug the motors, clean the pots and contacts, and not really mess with everything else unless I have to. I don't want to disturb more than I really should.

That detector puts out a 3V p-p video signal, with sync negative as shown in the schematic. It would be easy to build a video input amplifier with a gain of 3. However, the DC level is also negative, with whites at about -0.3 volts and sync tips at -3.3 volts. To duplicate this, you would need to add a negative supply, which the chassis most likely doesn't have, to reference DC restoration of the sync tips to minus 3.3 volts. Not impossible, but a bit of a bother.

[loud41]

I ended up disabling the tuner motors, cleaned the pots and that made a world of difference. I have everything working, and can fine tune ch03 which is nice.

I've figured out that my horizontal sync seems pretty screwed up. I can manage to just barely tweak the horizontal adjustment knob to get the image to sit still, but the color will shift/drift, or the sync will jump as the image changes. Sometimes it loses sync altogether, and I have to start all over. I'm guessing I should, at the very least, test the 6cg7 tube (horiz. osc/afc). I might just replace it... I've also read that the horizontal phase detection diodes could also be a culprit, but I'm just parroting what I've read.

[Electronic M]

If the entire image is hard to get horizontal lock on check that tube and replace the AFC diodes. If the monochrome has solid lock and good locking range on the control but the color has poor lock may have to adjust the chroma reference osc coil... You'll need a plastic hex alignment tool (count turns so you can get back).
Sometimes poor color lock can be exacerbated by the horizontal lock being poor or the image being locked at an extreme edge such that the burst is not being gated to sync the chroma osc.

[ARC Tech-109]

Checked your AGC by chance?

[loud41]

Originally Posted by ARC Tech-109

Checked your AGC by chance?

It actually ended up being the AGC. I gently adjusted it, and that ended up making a huge difference. Great suggestion! Thank you!