resman
Veteran Member
This is another part of the Paradise storage room collection and it lives, too! However, getting this up and running was about the most challenging project I've dealt with. It was very dirty (like the TRS-80 Model III) and required lots of cleaning.
When finally powered on, I heard the beep but no video. Nada, zip, zilch. Screen didn't even get slightly bright - no flash when powered off. Pulled everything apart (I hate working on CRTs) and was fortunate that someone had uploaded both the operators manual and the service manual. Nothing obvious was wrong with the monitor control board and all the voltages looked fine. The operators manual described how to connect an external composite monitor to the digital board (a nice little 6502 single board computer) and I was able to verify functionality through that port.
Pulled off the monitor controller board and poked around it. There was a service bulletin to replace one of the caps, and I could see where that was already done (very sloppily). In looking for shorts, I stumbled across the horizontal drive transformer and noticed one side was reading open. Huh, not going to do much good without any current flowing through it. I pulled it off and looked for a replacement transformer but couldn't find a match. With nothing to lose, I cut the surrounding tape and peaked inside. Scraped away the wax that was covering the posts and noticed that one of the posts had the coil wire wrapped around it, but wasn't soldered! The other three were. So I soldered them together and got continuity through that side of the transformer. Tucked everything back together, re-attached the transformer and to my amazement, it worked!
What are the chances this monitor controller board ever worked? It obviously had been attacked by a technician to replace the capacitor - flux and solder splatter everywhere. Or maybe it worked for awhile with just a crimp fit on the transformer post and when it failed they assumed it was the capacitor, then gave up after that didn't fix the issue. Well, whatever, I made all the video adjustments mentioned in the manuals to get a nice display. Except you can see the image is shifted to the left a little. Anybody know how to get it centered (short of reprogramming the CRTC)? There aren't any other pots to trim on the monitor controller board.
Dave...
When finally powered on, I heard the beep but no video. Nada, zip, zilch. Screen didn't even get slightly bright - no flash when powered off. Pulled everything apart (I hate working on CRTs) and was fortunate that someone had uploaded both the operators manual and the service manual. Nothing obvious was wrong with the monitor control board and all the voltages looked fine. The operators manual described how to connect an external composite monitor to the digital board (a nice little 6502 single board computer) and I was able to verify functionality through that port.
Pulled off the monitor controller board and poked around it. There was a service bulletin to replace one of the caps, and I could see where that was already done (very sloppily). In looking for shorts, I stumbled across the horizontal drive transformer and noticed one side was reading open. Huh, not going to do much good without any current flowing through it. I pulled it off and looked for a replacement transformer but couldn't find a match. With nothing to lose, I cut the surrounding tape and peaked inside. Scraped away the wax that was covering the posts and noticed that one of the posts had the coil wire wrapped around it, but wasn't soldered! The other three were. So I soldered them together and got continuity through that side of the transformer. Tucked everything back together, re-attached the transformer and to my amazement, it worked!
What are the chances this monitor controller board ever worked? It obviously had been attacked by a technician to replace the capacitor - flux and solder splatter everywhere. Or maybe it worked for awhile with just a crimp fit on the transformer post and when it failed they assumed it was the capacitor, then gave up after that didn't fix the issue. Well, whatever, I made all the video adjustments mentioned in the manuals to get a nice display. Except you can see the image is shifted to the left a little. Anybody know how to get it centered (short of reprogramming the CRTC)? There aren't any other pots to trim on the monitor controller board.
Dave...
