Hi everybody! tired of wavy and small image at a little TV I have, I'm trying now to watch replica 1 image at my amstrad CTM-644 monitor. It has not AV input, only RF, so I made an "interface" with an UM1233 modulator (from an old zx spectrum).
I connected to replica video output and I almost see something, playing with tunning and vertical hold... but I didn't get
an useful image. what could be the problem? maybe something about PAL-NTSC compatibility (monitor has PAL input). I wondered about signal from replica too, it could be too strong or too weak for this modulator
if anybody knows about video modulators, please help me 
Luis
I don't think the problem is too weak or too strong. I think the problem is the signal itself. The signal is a generated signal from a microcontroller. It is not perfect by any means. It works with most TV's (even a lot of PAL TV's) but not all.
Doing some research on the UM1233 shows that you need to use it with a manual tuning TV. Is that what you are using? Isn't that the same modulator used on ZX-81's too? You might try and use a sup-r-mod from an Apple II. This was an aftermarket add-on used on some Apple II's. What a great project. I'll dig one up and try it out!
Vince
yes, this monitor has manual tunning. It has an add-on modulator Amstrad did to this monitor to allow using it as TV. It converts RF signal to RGB (monitor input amstrad computer uses)
I was thinking about forget this add on modulator and 1233 and try to convert AV signal to RGB, but searching at net I found maybe it's very diffcult (or expensive) to do..
about 1233, it's the same used at zx81, with no sound input. If I can get it works, it would be perfect! (amstrad monitor has an amazing image quality)
I will research and tell you if I have progress
about apple II, it sounds like perfect, but unfortunately that computer is almost impossible to find in spain..
I think this is a great project too
thanks very much for your reply
Luis
maybe this could be useful
http://www.allelectronics.com/cgi-bin/item/MOD-6/810/RF_AV_MODULATOR_.html