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I am repairing an Apple IIGS RGB monitor, A2M6014, and need your help.

J

jk92

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Jun 27, 2023
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Hello. I got this monitor, A2M6014, yesterday and I have a problem with it. The seller said it is working fine but it actually is not.

What I can tell is that the screen scrolls left to right constantly and rapidly.

There are three adjustable knobs in the back, V-HOLD, V-HEIGHT, H-HOLD. First two seems normal, it doesn't really help with the situation but they seem to be doing their job.

But then I turn the H-HOLD knob, it gives me very weird symptoms.

When I turn it to clockwise, the screen gets darker and darker as if I am turning a brightness knob and eventually I cannot see anything.

If I turn it counter-clockwise, it makes the screen brighter and when it reaches to the end, the monitor turns off. With that position, I cannot turn on the monitor back. No power no nothing.

I have to move the knob to around the center to power on the monitor.

I measured the resistance for the pot, VR502, and it shows around 4.5k ohm. The monitor schematic shows 5k so it is okay I guess?

That means something else is the problem but I do not know where to start.

Please share your knowledge with me. Thank you.
 
Does the IIgs drive another monitor correctly? (I'm assuming you know the IIgs is good.) If so, then it sounds like the monitor has a failed component in the horizontal circuitry somewhere. Have you opened it up to look for obvious capacitor failures?
 
I do not have any other monitor for IIgs but its composite output works fine with my TV. It reads floppy disks, goes to OS screen. I really wish I can hook it up with my VGA monitors but connectors are totally different.

Inside the monitor is surprisingly clean. No dust at all. As far as I can tell, there is no leaking capacitors. Well, at least not visible from their top side.

There are several pots that need a screw driver to turn and they all do nothing much.

I am curious why H-HOLD controls the brightness. Does “normal” IIgs monitor do that too? Probably not. I will post some pictures videos later.
 
Part of the reason for the H-Hold's behavior is that the horizontal flyback transformer also generates the high voltages the CRT requires (B+, screen, focus, etc.)

The monitor shutting down when it gets to a certain point means the high-voltage shutdown circuit is engaging (as it should - prevents the CRT from becoming an X-ray generator).

The issue seems to be either the V / H sync pins are not making a connection or there is a failure in in the horizontal oscillator / sync stage.
 
Yes, I've downloaded the schematics. The red box in the picture shows the pot for H-HOLD.

resistor.png

If I really try to fine tune the pot, I am able to get a readable screen but still moving as you can see in the video.


I've tried 3.3k ohm pot but no luck with it as well. I guess the problem is something else as you guys mentioned.

Honestly, I cannot find what and where the oscillator is and it is not easy for me to understand the schematic.

What should I look for now?
 
I'll need to look over the schematics and the data sheet for the LA7822 chip further.
One quick thing to try, see if any adjustment of the horizontal centering control helps to achieve lock on the horizontal hold (but I'm thinking it might not help).
Do you have a capacitance meter? Unsolder C502 (keep a record of where the - lead is) and check its capacitance (1µF). Do the same for C519 (22µF) (don't forget to discharge it before touching both leads). While C502 is out check the resistance values of R505 (2.2k), R506 (47k) and R507 (12k).
 
The Apple II Family Service Procedures vol 2 has a section that covers this monitor (p 385-453). Note that Acrobat Reader has real problems with this file, I had to use Okular on Linux to view it. It says that if the horizontal hold is adjusted too far out of range it will shut down, as you have observed. The corrective action is to turn off the monitor and place the horizontal hold adjustment back to center position. Not much else useful in the troubleshooting section other than to say replace the main board and/or power supply board. Since you do get an image displayed with reasonable intensity and area the power supply board is probably good. Thus you'll have to diagnose to the failing components on the main board.
 
Your missing horizontal sync. Video comes in IC501 pin 14 and there is an internal sync separator. The standing vertical floating bar means the horizontal is the correct frequency, just not sync'd.
Maybe C501 is open ?

Larry G
 
hcenter.jpg

The Horizontal Center does not really do anything. It moves the screen a little bit but nothing else.

cap.jpg

I found C501 and C502 deep inside the board. I have no access from the back because plastic housing is blocking them. I have to take the whole board out in order to unsolder them.

However, I just checked their value while they are still in position as seen on the picture. C502 gives me 1µF as expected. Now, C501 on the other hand, gives me a value of 0! No nothing.

I don't know it is because I am measuring it while it is sill on the board but definitely suspicious. I might have to remove it, check the value again, and replace it with 1µF 50v cap I have.

Take that board out of the metal housing will be a challenging job for me. Wish me luck!
 
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