AOC CM 312 monitor problem.

[Anderslober]

Hello forum. Just a quick question. I need to know the value of R750 and ZD750 on the crt board of this cm312 monitor. I cant find a good SM.
I hope to get some help. Thanks. See picture for position. Picture is from forum.

[sean0118]

Hey, I've attached the schematic I have, but the value for ZD750 is unreadable. Maybe 6.8V ?

I still have my Commodore CM312, so can open it and have a look this weekend 

edit: 12V rail so the zener is probably 12V?

[Anderslober]

Wow…thanks for the great info! Highly appreciated if you can check too. 👍🍻 R750 is 75 ohm. Zd750 possibly a 12v zener.
The resistor and zener were both fried.

[PA0PBZ]

I think, looking at other commodore monitor schematics, that the zener is 93D39-68 which would make it 6.8V and that makes no sense on a 12V line    So 12V would be a good choise, probably the zener shorted and smoked the resistor.

[Anderslober]

Yes…I changed to a 12v zener and new 75 ohm resistor. I have power and crt elements are glowing, BuT, no life in display….ideas welcome….I have checked anode cap..all is good….still no display…..

[BlownUpCapacitor]

If there is no beam at all, check the pre acceleration voltage. If that still checks out good, that must mean CRT is working. Or is it?

To check if the CRT will still even display any kind of beam, disconnect the wires to the electron gun grids. Manually drive the grids and see if a beam appears.

[Anderslober]

Pre-acceleration voltage…..where exactly would I check this on this cm312 monitor? Does the anode cap transformer ever go bad?
Strange this…..

[sean0118]

Yes…I changed to a 12v zener and new 75 ohm resistor. I have power and crt elements are glowing, BuT, no life in display….ideas welcome….I have checked anode cap..all is good….still no display…..

I measured mine and get 12.7V across the zener so your replacement should be correct. Resistor on mine is also 75ohm.

I'm not sure if my CRT is fixed (I need a CGA card to test) but I do get a raster with no input connected. Let me know if you want me to measure any other voltages for comparison. 

[BlownUpCapacitor]

Pre-acceleration voltage…..where exactly would I check this on this cm312 monitor? Does the anode cap transformer ever go bad?
Strange this…..

I wouldn't know for this exact CRT monitor without the service manual, but you can try finding the pin to measure from by looking at the electron gun construction.