The next step, is repairing Robotron 1715 K7222 monitor. Initially, I tried to plug in the monitor to the computer and see what happens ( probably a bad I idea ). After turning it on, I saw no signals that the monitor is heating, so the first thing to investigate was is there any voltage from the power supply applied at all. After opening monitor case I found that fuse was ok, but really corroded. I have removed the fuse and the fuse holder had no continuity due to corrosion. In my junk box I found another fuse holder and replaced it.
After a second try to power it up with a new fuse holder, I heard some crack and magic smoke. Usually it is not a good sign 🙂 After further inspection I found that flyback transformer has huge cracks on it, not good.
Now a couple of things I have to do:
- test monitor with external PSU with flyback transformer removed just to see if anything else works.
- if it works, then there are two options:
Some image from forum regarding replacement flyback transformer connection:
First, I removed UHA78 flyback transformer.
Then, peeled off the outermost coil. According to instructions I have found, guys are cutting it. But as mine was cracked, it peeled off easily without cutting, with a help of screwdriver and pliers.
Then started winding the flyback transformer coil. I made a jig for that: bench vise, drill, corc, a couple of magnets, and old phone with a magnetic counter app. You can get the picture from the picture 🙂
The first coil – I winded everything, then decided to seal it with epoxy but have made bad epoxy mix – it did not cure. Thrown away.
The second coil – one wire broke in the middle of winding process, I fixed it, but then one wire sliped away. Decided to dump it.
The third coil – I have 3d printed core for it with a cap. That one worked out well.
After testing without the outer coil, I noticed smoke in resistor R12. I guess the voltage is too high on it. tested resistor V3, it seems to be ok. Another guess is signal IS that goes to resistor V3 is too high.