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Color Classic no raster; black image and booting

belzrebuth

Well-known member
Hi all,
I recapped a color classic analog board and after replacing a faulty TL431 it came back to life with a fair bit of pincushion distortion and a very bright screen.
I got no brightness control from the front buttons or the OS so I turned G2 voltage down (pot on the analog board) in order to fix the brightness issue.
I had to keep my G2 in a certain level in order for the image not to flicker which I don't think is normal but the computer was on about 2-3 hours after that working without the cover and the fan on.
Trying to adjust pincushion the pot was like having no usable range so I decided to touch a bit the solder joints on the pot itself and various other transistors in that section.
In retrospect I think that probably the chip which does the pincushion correction is responsible for that issue..
After turning the computer back on I got no image while I hear it booting, accessing the disk etc.
5V and 12V are okay.
My joints were clean; I even cleaned the flux after I was done so no soldering issues.
I'm not sure what happened but I lost all video.

If I turn G2 up I get horizontal (retrace) lines; a lot like what you would get if you disconnected the video input entirely.
Another thing which may act as clue is that each of these lines consist of three RGB lines so each thicker retrace line has a red green and blue line inside it all in parallel.

I checked all connectors and they're fine.
I don't have any schematics for this board so I don't know what voltages I can measure to verify the analog board is working.
I tested all the transistors I touched out of circuit and they are fine.
I would appreciate any input on how to proceed with this..
 
Last edited:

belzrebuth

Well-known member
Here is an image of what I get in the display if I crank the brightness pot on the analog board..
I don't think I've ever seen a CRT acting like this so I don't know how to interpret this image.
These are not standard single retrace lines.
The voltages on the analog side of the analog board PSU are +9V -9V and +24.
I don't know what they should be but they seem stable and the +- ones are almost the same.
The jungle IC gets about 7.7V.
The B+ voltage is about 75V.
The parallel zeners which often get burned are reading 18V on their cathodes.
The zener below them also reads 8.2V or so so it's okay as well.
IP11 78L05 reads 5V.
Things I noticed:
If I remove the video cable coming from the logic board I lose the image entirely.
I have control over the image using the pots on the back of the CRT so this is not a fixed thing.
If I remove one color from the video chip rgb output (desolder the corresponding inductor on the logic board) I still get the 3 RGB stripes so those RGB and the ones coming from the LB are not directly related.
If I cut the composite signal coming from the LB I get no image at all.

cc_image.jpg
 
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