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C128 No Video Kicking My Butt

B

bluebarntech

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Oct 20, 2024
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I've been at this on and off for a couple months now so It is time to ask for help. Have a C128 with a new power supply. Lights up, no video, no signal at all. I have gone through the troubleshooting chart and can't make heads or tails. I've socketed the major chips and ran them through my Chip Tester Pro.

I'm down to no sync or color signal on the 8564. No response at all on my scope on those pins. Chip tester says it is Passed NTSC. Easy answer would be to just substitute but no extra chips laying around for me. Do I just need to buy another 8564 even though this one "tests" fine? Or could something else be inhibiting it from outputting anything?
 
What's displayed on the standard 40 cols? (I.e. do you have a white line on the left side of the screen?)
Also, did you verify that the 9v ac input voltage is present? It has a fuse in the power supply, and this gets turned into the +5v that drives the Z80 cpu.
The Z80 cpu is the first to come up and configure the machine to work, so if the z80 isn't working correctly, nothing else will come up.
 
Actually, I had a DCR where the 9V AC was bad (fuse in the transformer had blown) and the only symptom was the TOD clock not updating.

As far as 80 columns, if you have a male DE-9 plug and a RCA phono plug, you can make a quick and dirty monochrome cable and plug it into any composite video input. Connect pin 7 (monochrome) to tip and either pin 1 or 2 to sleeve (ground).
 
dabone said:
What's displayed on the standard 40 cols? (I.e. do you have a white line on the left side of the screen?)
Also, did you verify that the 9v ac input voltage is present? It has a fuse in the power supply, and this gets turned into the +5v that drives the Z80 cpu.
The Z80 cpu is the first to come up and configure the machine to work, so if the z80 isn't working correctly, nothing else will come up.
Click to expand...
Nothing at all on the 40 cols. No sync, no picture. Yes, 9v AC looks good. The Z80 is getting power and I can see activity on my scope.
 
ClassicHasClass said:
Actually, I had a DCR where the 9V AC was bad (fuse in the transformer had blown) and the only symptom was the TOD clock not updating.

As far as 80 columns, if you have a male DE-9 plug and a RCA phono plug, you can make a quick and dirty monochrome cable and plug it into any composite video input. Connect pin 7 (monochrome) to tip and either pin 1 or 2 to sleeve (ground).
Click to expand...
Ahh, great idea. Thank you!
 
ClassicHasClass said:
Actually, I had a DCR where the 9V AC was bad (fuse in the transformer had blown) and the only symptom was the TOD clock not updating.

As far as 80 columns, if you have a male DE-9 plug and a RCA phono plug, you can make a quick and dirty monochrome cable and plug it into any composite video input. Connect pin 7 (monochrome) to tip and either pin 1 or 2 to sleeve (ground).
Click to expand...
DCR might be different, I've had several 128s with no 9vac, with power light, showing black screen only.
 
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