Just got a TRS-80 Model I (4K Level I) bad video!

[DistantStar001]

The title pretty much says it all, even so, it's a bit of a beater.  This is my second Model I (the other is a 16k Level II).  This one has a few broken plastics on the bottom, and all but one of the standoffs from the inside of the top have either fallen off or shattered into pieces (however, and fortunately, I appear to have all those bits).  Three of the keys are missing (1, 2, Break).  I do have an enter key, but it appears to be the wrong one as the little square mount is on the wrong side of the cap.  Unfortunately, I have no idea where to get replacements at this point.

 

Still, the biggest issue is the video.   At first, I thought it was a black screen.  I plugged it into an LCD and the TV acknowledged a signal, it was just black.  I tried adjusting the two resisters on the power side (R1 and R2) with no effect.  Then I tried connecting it to my Amdek 300, and this is what I got:

 

 

The lines seem to dance vertically on the screen.  I think it might be the vertical and horizontal sync, but my knowledge of this machine isn't extensive enough for me to be sure.  So am I missing something?  And can anyone steer me in the right direction?

Edited July 13, 2022 by DistantStar001
Correction

[DistantStar001]

Fixed!

 

I had to replace the IC's responsible for video sync and integration:

Z6: Horizontal sync: 74C04

Z57: Vertical sync: 74C04

Z5: Signal integration: 74C00

 

Thanks to TRS-80.com for the instructional video:

https://www.trs-80.com/wordpress/repairs/trs-80-repairs-model-i/

[MrDave]

Congrats ! ?

[Bruce Tomlin]

Ugh. I tried to fire up my old TRS-80 the other day to figure out some details of a video mod I had made. The only power supply I could find already had the fuse blown, had to hot wire around it. Then I turn it on and I'm getting a bunch of 1-inch horizontal lines with bad sync. I guess I've got a project ahead of me.

 

After taking 15 seconds looking at the schematic, I think I probably should start with the same chips. I didn't even realize that 74HC was around in 1978.

 

EDIT: oh that's 74C, but I'll try 74HC first anyhow

Edited July 21, 2022 by Bruce Tomlin