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Green raster circle, now nothing

F

fm2606

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Joined
Apr 17, 2021
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13
Location
Florida's Space Coast
I have a TRS80 M3. When I 1st got the computer about 1 month ago and turned it on nothing came on the screen but the drive sequence seemed to be correct. I replaced the RIFA caps on both power supply boards, cleaned up the computer a bit and put it all back together. Initial power up was promising as a green rasterized circle appeared (see link below) and the drive sequence again seemed correct. By correct sequence I mean the bottom drive comes on, then the top drive comes on momentarily and then the bottom drive again and that's it. If I press DELETE and RESET the sequence starts over.

Now however, nothing comes on. With the video cable attached to the video board the bottom drive comes on longer than the normal sequence, then the top drive comes on and this time runs for about the same amount of time as the bottom, it then stops and then the bottom drive comes on again longer than what seems it should. DELETE and RESET do nothing in this set up.

If I unplug the video cable, the normal drive sequence starts again and once completed if I press DELETE and RESET the sequence starts over.

I have 12V at pin 7 on the video cable. The two 4-pin connectors on the CRT power board have +/- 5V and +/- 12V.

When I tore down the computer for a cleaning I never did open up the back plane that holds the motherboard and drive controllers.

Any suggestions on how to proceed to trouble shoot this would be greatly appreciated.



https://imgur.com/2r1kUZ2

Thank you
Curt
 
Interesting, I have not seen that circular distortion before. Seems like an issue with the CRT or the video board for sure. I would start by swapping out the top with another Model III or 4. I'm curious, what are those gray panels inside of the drive openings?
 
The circular effect could be from someone/thing moving the deflection coil on the neck of the CRT. If you move it toward the face of the CRT, the raster should fill the screen. Just be careful moving it if you have the computer on, the HV is several kV.

Tom
 
As for voltages, the high KV level is on the second anode lead, going to the side of the tube. This usually doesn't have enough current to kill you but will make you jump, possibly doing secondary damage, as your arm rakes across the broken tube stem that your uncontrolled arm snapped.
The yoke has lower voltages but these can be a hundred plus volts with enough current to kill. At least these voltages are not going to jump a half inch to snap you. Often times they are on clearly visible terminal strips. Even they are no an issue if you keep the rest of your body isolated from a current return path.
Anyway, it is likely that the clamp has failed, holding the yoke on the neck. This can happen when the old plastic breaks apart. You may have to find a way to replace that part.
Color screens will be effected for color purity by the position of the yoke as well as other purity adjustment. Usually the red uses the strongest electron beam. One adjust for the best purity of the red ( use a magnifying glass ). Color alignment is also done with green/red needing the best at the sacrifice of some blue misalignment.
I should note that is the image has significant lose of focus as well, it could be a voltage problem. Color requires voltage regulation.
Good luck.
Dwight
 
Dwight Elvey said:
A
Anyway, it is likely that the clamp has failed, holding the yoke on the neck. This can happen when the old plastic breaks apart. You may have to find a way to replace that part.

Good luck.
Dwight
Click to expand...

Finally got around to looking at this again. Pictures on imgur of the possible culprit. It is definitely cracked and the crack facing the front is wider than that in the back. The back is almost a hairline crack. https://imgur.com/a/C8dfExp

Also the bulb does not light up at all when powered on.

How hard are these to come by?

Would this explain why both drives run almost continuously when the video cable is plugged into the board?

Part of me wants to just get rid of it but it is so close to working. But then I just bought a bunch of old Commodore computers so torn between what to do. I want a working computer and not a vintage paper weight.

Anyhoo...thanks to everyone for their replies.
 
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