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PET 2001-8 Ripple Effect Display


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  PET 2001-8 Ripple Effect Display by Bill Degnan - 10/30/2019 17:15
Snapshot in mid motion. Need to diagnose cause of a rippling screen that reminds me of a flag waving in a slow wind. Click image for larger view.

See video:

Going with assumption the brightness, contrast, vertical hold are ok and the screen does not shrink. It looks like a flag slowly rippling in the wind because a cap(s) is failing but not failed entirely.

Probably unrelated is the fact that I had to bang on the chassis when the system was first powered on in order to get the display to show anything. There may also be a cracked solder joint somewhere.

Conclusion - The entire display needs to be worked over, test caps and components, etc.

  Testing components by Bill Degnan - 10/30/2019 19:55
Ethan Dicks Says:

"...Considering you have both that wide shimmy and the horizontal roll, I'd be looking at more than one failure.


Look around the vertical hold circuit - check for potentiometers - they could be dirty or have a bad solder joint.
As for the wide shimmy, check the PSU for ripple - the input is ~18VAC IIRC, from the transformer, but locally, the CRT board has a simple linear regulator to +12VDC. The caps right there by the connector are ones I'd look at. Also check the ripple at the large eletrolytic in the base to ensure that's not complicating things with noise/ripple. ..."

NEXT - results of testing...

I think I have a spare good 23000 uF 15v cap, we can start with the big cap near the transformer.

  Smoothing Cap Replaced by Bill Degnan - 11/08/2019 21:50
commodore pet 2001-8 smoothing cap replaced
The leaky 23000 uF 15V smoothing cap near the transformer was replaced by a 22000 uF 15v that was tested to be in spec (low ESR and over 24000uF). Click image for larger view.

Replacing the smoothing cap did not fix the screen ripple problem but the original cap was leaky so it was worth doing anyway. Note that I tested the fix with two other similar uF caps to make sure the results were duplicated.

NEXT: Review the specs, trace the problem down.

  Display board repairs by Bill Degnan - 02/05/2020 22:48
commodore pet 2001-8 display board 4700uf cap
This 4700uF 25v cap was found to be disconnected from the analog display board. That might be an issue, eh? ... Click image for larger view.

commodore pet 2001-8 display board
Cool photo of the video control board... Click image for larger view.

  Repair completed by Bill Degnan - 02/06/2020 11:50
Commodore PET 2001-8 screen warble repair
I did not have an exact match to replace the original 4700uF 25v cap, so I used a 4700uF 35V instead, which is better than the the original. Only issue, I had to install a patch wire to connect the positive end to the display control board. Click image for larger view.

Commodore PET 2001-8 screen warble repair
Here is a veiw of the inside of the display electronics after the repair. Click image for larger view.

Commodore PET 2001-8 screen warble repair
And the screen no longer warbles. Click image for larger view.



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