Also, since I have adjusted the internal pots on my VIC-20 motherboard to make it look good on a 1702 monitor, when I use the VIC with an RF modulator the colours are bad. Actually, on the B&W TV they look reversed! The background is black and the letters are white upon boot. Again, this is a useable setup for doing diagnostic stuff in the workshop, but it could be improved upon.
So, I was mulling over the idea of adding a composite input to the TV.
Firstly, I must say I know about the dangers of working inside a TV (flyback transformer and other dangerous voltages)....so this is probably not something I will try to do. But I was wondering about how possible this would be theoretically.
What are the differences between a monochrome monitor and a B&W TV? Both expect a luma signal only. The colour signals are ignored.
The difference is that the B&W TV has a tuner/RF demodulator in it, and the monitor doesn't. The monitor also is a little sharper in terms of picture quality, and might have a higher persistence phosphor coating inside the CRT. But a black & white TV is much sharper than a colour TV for text purposes.
So....inside a B&W TV the tuner module accepts a signal from the antennae input, then splits this signal into luma and audio. The internal tuner then sends the luma signal to the CRT and the audio signal to an amp and the speaker.
If it would be possible to find out where the RF modulator outputs the luma signal to the TV's circuit board, wouldn't it be possible to 'hijack' this line and route it to an externally mounted RCA jack, to which one could connect the luma signal of a computer like the VIC-20? One could also do this with the audio line.
Another reason why I think it would be great to do this, is that I have a 40/80 column board for the VIC-20, but only one monochrome monitor. If this monitor breaks I will have difficultly finding a new one....however I see RF input only B&W TVs all the time at thrift stores for $4 each. I think they might make a nice cheap monitor for such a display. I think the monitor in the original PET 2001 was a Sony B&W CRT (maybe not a TV tube, but something similar).
I did a Usenet search, and found a post by Dave Haynie (the D. Haynie?) on this subject:
I also found this post on Usenet by a fellow name M. Zenier which makes me think that it's not worth it to try this hack unless you really know your way around the inside of televisions:Yes, no, maybe. You can't connect anything reasonable directly to the
picture tube -- don't do it. When I was 17, I got my first home
computer (an Exidy Sorcerer -- anyone recall this one), and hacked a
composite video port into my B&W TV at the time. Not a bit deal: I
drove down to the local radio supply store, got the "Sam's Photofacts"
on that model (Hitachi something-or-other), found the composite
signal. I routed the output side to an SPST switch, the common of that
switch to the input side of the signal, and the other pole of the SPST
to my circuit. It was basically just a high speed, single transistor
amplifier, and I messed around with gain on it until I got a decent
level (and luckily, didn't blow out the TV).
On a related thought: has anyone ever tried hooking a B&W security monitor to a VIC's luma output? These are generally high quality displays....maybe they would be sharp enough for 80 columns.A problem is that TV's are cheaply built and may not have any isolation from the power line. Plug your TV in backwards and your computer blows up. (This happened to some poor soul up here in Seattle about 10 years ago, and the local club got together and scrounged up all the chips necessary to rebuild his altair)
Some older sets run with a rectifier right on the power line which feeds
everything. These are bad news. Newer sets use the Horiz. drive as a
switching power supply so most of the circuitry is isolated.