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Apple IIc Monitor Tube displaying strange behavior

VERAULT

VERAULT

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I recent bought two Apple IIc monitors (A2M4090). Both in different states of neglect. I was able to get one of the monitor boards working perfectly. If I connect one of the CRT tubes It displays fine; I can make adjustments as needed. When I connect the other CRTs (which is the bare CRT and deflection yoke only) The picture is mashed up and really bizarre. See here: 20190718_134933.jpg The image displayed is the "Check Startup Device" from an apple IIGS. If I try and tweak the adjustment pots I cant resolve the picture. Its squeezed and rolled over and the eges have these 3 collapsed portions. Even the line of the apple swinging back and forth is wavy on the left side of the screen.

I circles the areas I mentioned iicmon.jpg

Sorry for the grainy images, lighting wasn't good where I was working.

Could it be an issue with the magnets? Maybe some fell off? Again my other CRT works perfectly fine on this board.
 
Also does anyone have a service manual, parts list or full schematic for the apple iic monitor a2m4090? Apparently there are different revisions; I can only find the sams computer facts manuals for the iic monitor however the layout of the board and the component numbers are different.
For instance on the Sams computer facts the flyback transformer is T502 but on my boards its T702; this may seem minor most other things (diodes for instance) are way off as well.

edit: after some looking I definitely need the parts list. Seems these ceramic (??) Diodes are all disintegrating. One is missing near the flyback (it exists on the working board) and one is clearly broken on the PSU board. I suspect alot are bad as they are falling aparts (strange diodes only are doing this. 20190718_152021.jpg 20190718_152051.jpg
 
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That is bizarre. The only thing I can think of that would cause that, since the board is tested OK, is a partial short in the yoke. The yokes are the same model, right?

Adjustment magnets would not cause it to be THAT far off.
 
SomeGuy said:
That is bizarre. The only thing I can think of that would cause that, since the board is tested OK, is a partial short in the yoke. The yokes are the same model, right?

Adjustment magnets would not cause it to be THAT far off.
Click to expand...

They look identical, thats as much as I know. Id rather not yank it off it I can avoid it. Break in the winding possibly?
 
Going back to the topic of schematics and parts lists I need; I have confirmed that the diodes (must be from exposure to moist environments) are all corroding and falling apart.. some are literally just legs with no component. The ones remaining are "corroding" out the posts:20190718_152900.jpg

On the working board I noticed a copper heatsink with 4 diodes (bridge rectifier?):20190718_152803.jpg On the non working board two are missing one is cracked and only one remains: 20190718_153044.jpg

These ceramic diodes have no marking other than a blue or black band.. so unless I can find a parts list Im pretty screwed.. Its a shame. as is I can get roster so I know replacing these bad diodes would get me that much closer.
 
VERAULT said:
Break in the winding possibly?
Click to expand...
Well, the fact that you have some deflection suggests there is current going through it. A short between winding levels (I'd guess the vertical deflection) might cause insufficient deflection and a random part of the windings would be inactive, so it would not draw straight across. All the while pulling too much current from the board, so wacky things would happen with the horizontal too.

Not much you you can do about it, I think, but the tube would still be good.
 
So you are leaning to the yoke being bad? Thats going to be a nightmare to find. Can I use the yoke from an a2m2010 (apple monitor II)? I have two spare yokes of those (retrieved from cracked CRTs)
 
The a2m2010 is a 12" CRT, and the A2M4090 is a 9", so no. A replacement yoke would at least have to be from the same size CRT. The early Macs used a similar 9" CRT, I wonder if they used the same yoke and connectors?

I'd think there would be a part number on the yokes somewhere that you could compare. The two yokes on the IIc monitors *should* be the same part. (If for some reason they are not, then the board might not be compatible)

One thing to check, you could check the resistance across the yoke winding and compare it to the other monitor. A short would result in lower resistance, BUT it might only short when there is high voltage applied. It might not be the worst thing to pull the yoke off and look for visible damage. (photograph where all the magnets and stuff are first)

I would be worried though, that if there any additional shorting occurred that it might damage your good board.
 
If the yokes are the same part, one must have some shorted turns in the vertical coils. The H scan coils look OK probably. Sometimes you can see a turn of wire on the yoke near the surface with other are crossing it at nearly 90 deg,with gaps, if you see a configuration like this you can try to move the wire a little , sometimes the shorts can be near the surface and you can get lucky. The CRT itself is probably fine.
 
You can compare the resistance of the vertical coils.

You can also swap coils without removing them. Either swap tubes, or, route wires between the two so that with both monitors powered up, they are driving the coils of the opposite monitor. It sounds ridiculous but it's a valid diagnostic test.
 
I'd replace the diodes before I started blaming the yolk. The diodes are damper diodes to prevent the deflection voltage across the coil going negative and ringing, which is what I suspect is happening.
The voltages across those coils is not terribly high but the speed of the diode will be important on the horizontal yolk coil because it runs at over 15KHz. Looking at the diodes in question they look identical to the fast recovery diodes uses in the apple power supplies.

Something like a HER305 should do the trick.
 
Having reread the OP, I think my tube swap suggestion has already been carried out. In that case, it must be the yoke.
 
Thanks for all the insight Guys. I will do a resistance check on the yokes as well as resistance checks. David_M I will try those Diodes as well thanks.
 
Hugo Holden said:
If the yokes are the same part, one must have some shorted turns in the vertical coils. The H scan coils look OK probably. Sometimes you can see a turn of wire on the yoke near the surface with other are crossing it at nearly 90 deg,with gaps, if you see a configuration like this you can try to move the wire a little , sometimes the shorts can be near the surface and you can get lucky. The CRT itself is probably fine.
Click to expand...

If the problem is in the vertical coils, they're also easy enough to be rewound even by hand, they are the external ones. I'd try rotating the yoke 90 degrees to see if the symptoms rotates also by 90 degrees on the screen.
Vertical coils are sometimes connected in parallel, maybe some connection is missing, or as Hugo said, it's a short somewhere.

Frank
 
iz8dwf said:
If the problem is in the vertical coils, they're also easy enough to be rewound even by hand, they are the external ones. I'd try rotating the yoke 90 degrees to see if the symptoms rotates also by 90 degrees on the screen.
Vertical coils are sometimes connected in parallel, maybe some connection is missing, or as Hugo said, it's a short somewhere.

Frank
Click to expand...

That is a good thought, I have the schematic but I'm not at home. Most of the time in smaller monitors it is the H coils that get wired in parallel to keep the resistance low and they don't require as much total inductance either, but if it was a design where the V coils got wired in parallel, then it is possible that one could be open circuit and it is not a short.
 
Rotating the yoke won't help... Swapping coils (horizonal to vertical) is probably not a good idea.
 
KC9UDX said:
Rotating the yoke won't help... Swapping coils (horizonal to vertical) is probably not a good idea.
Click to expand...

Yes I agree, generally the H coils have a much lower inductance and resistance than the V coils, so it's not a good idea to swap them.
If the OP has an inductance meter it's another good way to compare the V coils on the two yokes, aside from checking their DC resistance.
 
I think the idea was just to turn the yoke to see if the same pattern persists, but rotated. Although I am sure it would.

No H/V plug swapping.
 
Hugo if you have the schematic when you are home, please let me know. Here are some photos of the CRT. What is that strange outer coil? An inductor? 20190719_230433.jpg20190719_230439.jpg20190719_230414.jpg
 
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