taos23,
I've written up a PDF document that is on my Dropbox account with the following link: (Anyone with link can access for some duration.....)
https://www.dropbox.com/sh/8zcx0rhgzu58rc1/AAA24YpJ4f5Dw9Hjzbzs5FeWa?dl=0
Download everything at that link. 4 items.
Print the PDF after viewing, and have your yellow marker ready to mark the places I have in RED on the PDF. Those are the KEY PLACES.
I've tested the basic program on my Model 4 so I know 100% it works.
It shouldn't take long to locate the problem socket, trace, IC, or bad solder joint.
I'm restoring a model 3 as well and it seems to have a power supply issue.
If it's a Tandy Power Supply (I'd have to go look at the exact model number) be sure to turn the Power Supply over and VERIFY
that the PINS that are soldered to the Supply have a good solder flow and no circles around the pins or weak looking solder joints.
That was a problem with the Tandy Power Supplies.
I'm always reluctant to pull the motherboard because of those thin flex cables--I ruined one!
Yes, those are a PAIN, until you know the secret. Get yourself an old wooden pencil that is about 5" long.
Lay the wooden pencil in the bend of the flex cable and hold the flex cable tight to the wooden pencil
as you remove the flex cable. Insert the cable the same way, by wrapping the cable around the wooden
pencil and insert the cable into the connector. It's easy to do now!.
TROUBLESHOOTING M4:
If you just use an OHM meter to OHM the Trace (and IC socket) for the Char Gen IC to the Shift Register IC
I'm 99% sure you will locate your problem. It could be a cold solder joint, bad IC Socket for Char Gen IC,
cracked or broken trace, or bad Shift Register IC or bad IC socket for Shift Register.
If you have an O'scope or Logic Probe you should be able to trace the signal by running the Basic Program.
It shouldn't take more than 20 minutes to isolate the problem.
Thanks for the update.
Larry