Hi all. I have an issue with an Color Composite IIe monitor, model A2M6021 that I was hoping someone on the forum could help me with.
I have had the monitor for about three years now. Has worked flawlessly with my IIe since then. Recently, it started to develop a high voltage arcing issue. I would hear an occasional snapping sound, see a flash on the screen, and smell an acrid odor coming from the back of the monitor. After opening up the monitor, giving the surfaces a good dusting using anti-static brushes and compressed air, I was able to find the source of the arcing. It is located at a particular pin of the tube socket on the CRT neck board. I will attempt to post a few photgraphs of the area.
I am not familiar enough with troubleshooting monitors to know where to begin with this. I was considering desoldering and then resoldering that pin, to see if the solder joint might be the issue. But I thought I would try getting some advice before proceeding. I was able to download the Apple Services Procedures manual from the Internet Archive, which has some information but has no wiring schematics or reference voltages I can use to do testing. I am flying blind here, and hope that someone on the forum can point me in any useful direction.
Thanks in advance!
What is this V-shaped thing on the board exactly where the arc is forming?
The V-shaped thing is a cutout through the circuit board itself.
Oh, I see. To me initially it looked like a piece of wire on the board. :)
Yes, resoldering should fix the arcing.
Yes, the image was a bit deceptive. But thank you for the advice! Will resolder tonight and fire her up again. Cross fingers ...
The arc is coming from the G2 (SCREEN) electrode. It may be caused by a loose connection to the neck board, but could also be a sign of excessively high voltage at the SCREEN or FOCUS grids discharging through a spark gap. A high voltage probe would allow these voltages to be measured.
Thanks robespierre. I do have a B+K HV-44A high voltage probe that I use to discharge the CRT at the anode. It has a voltage meter that reads up to 40k. How would I test the volgage at the SCREEN and FOCUS grids, and what values should I be expecting?
If you put the - clip of the HV probe on the clip that touches the gray outer CRT coating (called the "dag"), and the + side on the screen or focus pins on the CRT socket (the G2/screen is the one with the cutout, supplied by the red wire from the flyback, and the focus is the white wire from the flyback that goes directly into the "bump" of the socket), the G2 should be roughly 200–600 VDC, and the focus roughly 4–8 kVDC. The exact voltages are listed in data books based on the CRT tube number, which I don't know.
The design of the socket may make it hard to reach the focus anode (at its high voltage it looks like special measures were taken to cover and shield it). Also make sure you wear rubber gloves when measuring operating CRTs, and don't bring your hands any closer than necessary.
That cutout is quite intentional. It creates an air-gap spark arrestor to protect the screen grid circuits and the CRT itself from an over voltage.