Electronics > Projects, Designs, and Technical Stuff
Adding composite input to an old B&W TV
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flimshaw:
Hello hello,

I saw a video on 8-bit-guy about adding RGB inputs to a color CRT, and was wondering how difficult it would be to convert an old black and white RCA tv I have (AXR 120Y - https://www.radiomuseum.org/r/rcataiwan_solid_state_axr_120y.html) into a composite monochrome monitor for vintage computers. I've done a fair bit of tinkering with CRTs and know the safety precautions and general principles, but I can't find a schematic for this one and I'm not sure if the signal I want is there, and if so how to find it. Here's the block diagram I'm working from:



My assumption is that somewhere on this board antenna signals are decoded to a normal composite signal ("video detector") and then sent off to get amplified and sprayed on the CRT. and at that point between "video detector" and "amplifier", I could cut the trace and inject an external composite signal. Is that a safe assumption, or would the signal on the board not necessarily be a standard-level NTSC composite signal? I don't want to bother going too far if I'm vastly simplifying the problem.

Thanks in advance!

Muttley Snickers:
Couldn't you just use a modulator to convert the composite video signal from the computer to an RF channel in the same manner a VCR would ?.   
SiliconWizard:
You'd probably have to work at the "video detector" level. I'm not completely sure its input (which would be the output of the IF amplifier) is a proper composite signal. If it is, you know where to insert your composite input. You will likely need some signal conditioning.

To figure this out, first thing IMO would be to locate the "video detector" block input on the board, and take a look at the signal on a scope.
flimshaw:
Thanks, I poked around with my scope and found a video signal eventually. So it seems like the detector has nothing to do with the IC, and is instead implemented with some discrete components inside that little can. I snipped what appeared to be the location where the signal left the can and went into a transistor amplifier circuit and then on to the TV, and I was able to then inject a signal into the TV and it seems to mostly work.

The image is not *too bad*, way sharper than the RF adapter, but there is still a fair amount of snow on it. I'd expected to get a pretty snow-free image once I bypassed all the tuner circuitry. Does this mean there's some additional source of noise coming in that I would have to track down and cut? Or perhaps my expectations were high. Could it still be getting a noisy sync or something from the RF bits?

bob91343:
Perhaps the level you are injecting is too low.  And you need to make sure the detector output isn't added in with the signal.
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