G07-CAO Repair

[SirPeale]

These will be the notes as I repair this monitor.

Received chassis via mail.   Monitor had been rebuilt via a Bob Roberts kit. 

Upon powering I noted that it was not firing up properly.  No heater, but high voltage is present.  B+ ~100VDC.  Other side of R901 was 150VDC. 

Found the VR to be bad.  Replaced that, it started making really odd noises before blowing F901.

HOT blown now.  Fantastic.  Replace that, and using my "lightbulb fuse" I fire it back up.  Bulb comes on dim, but ever increasing brightness. STILL no heater.  Did get the B+ around what should be about normal if a fuse were there, and it was adjustable.  Then it starts making odd sounds again, so power off.

Go over the heater circuit really well, double check the cap orientation, reflow tons of solder joints, and fire it back up.  Now the B+ is @ 50VDC on both ends of R901.

Walking away for now, will tackle again later.  Any insights?

[SirPeale]

WEIRD.  I let it sit for about 15 minutes, and thought I had an insight.  So I fired it up, and the first thing I noted was that the heater was on.  It fired right up.  That's just goofy.

Next: replace the fuse and hook up video.

[SirPeale]

Now I know the reason for the 50VDC B+.  The lightbulb fuse had a wire come off it, so it was acting like it was completely open.  Though I thought it should be completely dead when that happens.

[lilshawn]

20 minutes... not bad.   

usually takes me 20 minutes to get wrangled up to go and fix it. 

[SirPeale]

The "noises" I mentioned before?  Today I started it cold, and no picture due to no heater.  I thought that was really odd, so I was probing, metering the B+ when the sounds started again.  This time I didn't shut it down.  They sped up, then suddenly stopped...and it fired right up, heater came on.  So whatever is going on with it is thermally related.  Don't think it's a solder joint, I've gone over them several times.

The B+ is now a rock-solid 120VDC.

Remember the fuse that blew?  I didn't have any 1.25A pigtail fuses, so I removed the fuse, drilled holes for and installed a fuse bracket.  In the event the fuse ever blows again you don't have to unsolder it to change it, just pop it out.  I've had the brackets for years and I always wondered what I'd do with them. 

Got video to come up, but colors were appearing and disappearing.  Also, I got it to lock onto a picture!  At least for a few moments - it wouldn't hold.  Some kind of connection issue, it appears since it would lock on when I flexed it slightly.  I removed the interface board to replace the caps, and was greeted with a metric ton of broken solder joints.

The caps on the interface board were replaced, and I resoldered every solder joint on it.  Now the colors are all stable, but still can't get the picture to lock.  Sync connector on pin six gets it to stabilize somewhat, but pin five has no change. Combining the syncs does nothing, and it did before I touched up the solder joints.  I visually inspected to see what I could have done, but I see nothing out of the ordinary.

That's it for now.

[SirPeale]

Okay...I fixed it...I think.

Yesterday I went over most of the chassis and resoldered the solder joints.  It seems to have fixed the problem with the slow start, but I couldn't do anything about the sync.

Today I went over every remaining solder joint with no luck. I did find a severed trace, and reconnected it - but it didn't help.  On examination it appears that the cut was intentional, so I put it back.

Then when connecting the sync I bridged the H and V pins (again) and got it to come on for a second, but couldn't repeat it for whatever reason (why, I do not know, as my fix makes it happen permanently).   So I took a small length of wire and bridged the H and V sync pins, and it locked right in, no fuss, no muss.

I let it run for an hour before shutting it down.  I want to see this get as cold as a start as possible since I want to see if it does that thing again where it takes five minutes to start.  Today it didn't, and that was after the solder rework.  I hope that's fixed it.