Princeton HX-12 Monitor Color issue

[rvdbijl]

Hi Everyone,

I have a Princeton HX-12 CGA monitor that's exhibited a weird problem ever since I got it a few years back. It would not display one of the CGA colors until it's warmed up for about 30 mins after which it suddenly works fine. So when it starts up, the text on the screen is magenta. After a half hour it suddenly switches to white.

I opened up the monitor to look for anything obvious (wires loose, popped or bulged caps, or anything like that, and I didn't see anything obvious). Does anyone have any idea where I should start looking to fix this behavior?

I have only 2 CGA monitors, and this is one of them. I'd like to keep it going for as long as I can. Picture is good on it beyond that intermittent problem...

Thanks!

 

[modem7]

Does anyone have any idea where I should start looking to fix this behavior?

Post warm-up time, you could try the use of a non-conducting freeze spray.

 

[rvdbijl]

I haven't tried to use anything like that before. Is the trick with that just to spray it on a component and see if it reverts back to not displaying one color? I was hoping for a more targeted approach.

 

[modem7]

I haven't tried to use anything like that before. Is the trick with that just to spray it on a component and see if it reverts back to not displaying one color?

Yes. Initially, sections are sprayed at a time to identify the section that contains the cause. Then, within the section, you target each component one by one. You get the idea. For temperature sensitive symptoms, it can reduce identification/repair time considerably, particularly if the circuit diagram is not available.

It does not always work. For example, when you spray an IC, you are cooling the pins and the package (plastic/ceramic), not directly the actual chip within the IC. For some ICs, a lot of time needs to be spent on the IC, and in some cases, even that is not enough.

For temperature sensitive symptoms, post a good visual inspection (which includes solder joints), reseating connectors, etc., I am going to try freeze spray first; I might just identify the cause within minutes.

I was hoping for a more targeted approach.

Per [here], the three channels of colour (red, green, blue) information coming into the CGA monitor are separately amplified and adjusted to suit the CRT. In your case, something is happening to the green channel (and it could be anywhere along the chain).

A technician would probably use an oscilloscope to compare the three channels at various points along the chain.

Each monitor is different. In the IBM EGA monitor, most of the subject electronics is in a metal cage (for shielding purposes), as pictured at [here].

 

[rvdbijl]

Ok. Sounds like I'll be looking for some freeze spray and give it a shot!

Thanks for your suggestions.. Hopefully I'll be able to identify the malfunctioning section or component. Downside of this monitor is that it's all packed in pretty tight with the high voltage stuff in the way of the main circuit board... It'll be fun!

Thanks!

 

[Hugo Holden]

. So when it starts up, the text on the screen is magenta. After a half hour it suddenly switches to white.

Hello,

This suggests the fault is anywhere in the channel that amplifies the signal driving the green CRT gun. If you have a white display/character and the green gun goes off, you get red & blue only giving Magenta.
So the fault could be anywhere in the green amplifier chain. Anything that ultimately takes the green CRT gun's grid negative, or cathode positive with respect to the grid, will cut off the beam.

There are places where the green signal relies on mechanical connections, these are obviously the input connector itself, often some bias setup potentiometers for the CRT gun's grid and or cathode (the track wipers on those potentiometers) and the CRT socket and CRT pin connections for the green gun/channel. So check all those. If you are going to unplug the CRT's base socket be very careful not to rotate the assembly off axis when you unplug it, there is sometimes a fragile glass exhaust tip on the CRT base and you don't want to snap that off. If you are not experienced with removing CRT base sockets, might be better to avoid that for now. Often they are sealed with a blob of silicone rubber that needs to be removed too.

If there is an electronic intermittent fault that is temperature dependent, you may well find it with the freeze spray technique is a good suggestion.

If not, you would be able to find the fault by tracing the green signal through the circuitry from the input lead/connector and buffer IC's all the way to the green CRT's gun amplifier, with the scope, and find out where the signal is disappearing or the fault occurring.