I need some help with a Colecovision a friend has asked me to look at, it's a PAL unit and doesn't have a composite mod.
I don't have a TV with analogue RF input so I am scoping pin 36 (luminance output) of the video chip in an attempt to see if it's working. It behaves in 3 different ways when I turn on the power switch...
1. Sometimes it seems to power up ok, with sensible looking signals on the luminance (pin 36) and colour difference signals generated by the video chip
2. Mostly the video chip does not give good output, the luminance signal is then stuck high. The CPU appears to be running ok and can be reset using the console's reset button but this does nothing for the video chip. If I briefly pull low pin 34 (Reset) on the video chip it then seems to re-initialise properly and gives a luminance output on pin 34.
3. Occasionally the video chip goes into an unstable mode, it gives a triangle-ish wave (approx 66 khZ) on the luminance output pin
The only pattern I've noticed around which of these 3 'modes' the video chip goes into is that I can get 5 in a row good behaviour 1's when I start things up from cold and 5 in a row bad behaviour 3's when the console has been running for a while.
The video chip is receiving +4.75 V at Vcc pin 33 but with up to 0.6V p-p of noise
The 4116 ram chips are getting -5.19V, +4.85V and 11.7V.
I did wonder if the +4.75V is a little low. I tried powering this rail from my bench supply at 5.20V (measured at the supply) but the console drew 450mA, which seems a lot, and resulted in only 4.2V on video chip Vcc pin 33. It does sometimes start ok even with this low voltage.
I removed the power switch, cleaned and scraped the contacts but it did not improve things. I even tried bridging the switch contacts by soldering a short across the switched pins but that did not affect things either, that ruled out the switch being at fault for me.
I see activity on the Z80 address and data lines but I never see outputs change on IC 74LS138 (marked as U6 on the NTSC schematics, which are here:
https://wiki.console5.com/wiki/Colecovision).
I'm not sure what to try next and welcome suggestions. So far I suspect a new TMS9929A video chip may help but I'm reluctant to buy a new because the current one does seem to function ok when I reset it, I'm wondering if the unpredictable behaviour results from something not allowing the video chip to initialise properly when the console is powered up.