Apple Multiple Scan 17 Display Purple Tint

[grieverheart]

I recently picked up a Quadra with a Apple Multiple Scan 17 Display. The Quadra was not working, it clearly had corrosion from leaking caps so I replaced them and luckily it chimed and worked properly.

The next problem to tackle was the screen. When it's turned on it's purple, but after waiting some time, without doing anything, it suddenly regains all colors. Another symptom is that it sometimes seems to momentarily blink. I confirmed the monitor was the problem by testing the monitor on a different Macintosh, which it again showed this purple coloring, and also by testing the Quadra with a different monitor, which seemed to work fine. The cable itself looks fine, I played around with it and the connector but they don't have any effect on the tint.

I checked the boards for any leaking caps and can't find anything. Everything looks pristine, with the exception of some dust, and the caps are not bulging. The solder joints also all look shiny and can't find any signs of cracks. What are some things I can to to troubleshoot? Do have some recommendations for soldering to some test points to see on which board the problem is? If I am to replace caps, which board should I start with? You can see pictures of the boards here.

[terrorinstinct]

A dying cap isn't always a bulging/vented cap. Electrolytic capacitors from the 1990s are past the end of their useful life and it's not uncommon for the electrolyte to dry out and cause a reduction of capacitance and/or increase in ESR, especially hot capacitors in a CRT. The fact that the issue fixes itself as the monitor "warms up" strongly suggests something between the RGB output IC and the green cathode is marginal.

The first thing I would do is find a service manual for this monitor. Unfortunately CRTdatabase only has an entry for the Multiple Scan 20, and the service manual is an Apple technician manual (read: useless) instead of a proper manual for TV repairmen. The second thing I would do is rustle up some cold spray and spray down the area around the RGB drive amps and see if that makes it worse. That should help you confirm the source of the problem. On the other hand you may want to just make a cap list for yourself and recap the entire board. If it fixes your problem, great, if it didn't then you at least eliminated a future point of failure.

[grieverheart]

A dying cap isn't always a bulging/vented cap. Electrolytic capacitors from the 1990s are past the end of their useful life and it's not uncommon for the electrolyte to dry out and cause a reduction of capacitance and/or increase in ESR, especially hot capacitors in a CRT. The fact that the issue fixes itself as the monitor "warms up" strongly suggests something between the RGB output IC and the green cathode is marginal.

Wouldn't the color in this case come back gradually instead of suddenly?

The first thing I would do is find a service manual for this monitor. Unfortunately CRTdatabase only has an entry for the Multiple Scan 20, and the service manual is an Apple technician manual (read: useless) instead of a proper manual for TV repairmen.

Unfortunately, as you also found out, I cannot find a proper service manual with board schematics. Perhaps some other Sony monitor exists with the exact same boards. Who knows.

Since the last post, I reflowed several solder joints, especially connectors, the neck connector, and some components along the green line on the neck board. Also replaced a couple of capacitors on the green line. Unfortunately the problem remains and it doesn't regain the color anymore, although it did this before, so I'm not sure it's due to the things I did.

Is it possible that the green gun is dead/dying or is that not very likely?

Anyway, perhaps I'll try to replace all the capacitors on the neck board for which I currently have replacements and see if I get lucky. Otherwise I'll have to order the rest and continue trying. By the way, would I need to replace the capacitors on the deflection board?

Finally, would it perhaps help to buy one of those cheap oscilloscope kits to try and see where the green signal disappears?

[tongshadow]

This has faulty guns written all over it. Get a tube analyzer/rejuvenator and check for shorts.

[grieverheart]

This has faulty guns written all over it. Get a tube analyzer/rejuvenator and check for shorts.

I made some progress. So the nice thing with these neck boards is that in principle the red green and blue parts of the board should actually be clones of each other. So put my multimeter in diode mode and checked the pins R.In, G.In, and B.In, and found that R and B were reading the same, but G was different. This leads me to believe that there is a faulty component somewhere along the green part. Following the path with my multimeter, I end up at IC001. If I'm understanding the PCB correctly, all RGB inputs to the IC read the same value in diode mode, which means that IC001 might be faulty!