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Thread: PET 2001-32N Nonlinear Picture

  1. #1
    Join Date
    May 2007
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    Free Country, USA
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    Default PET 2001-32N Nonlinear Picture

    Hey y'all, I set up my PET 2001-32N for the first time in awhile, and with newfound confidence, I decided to tackle the issues plaguing the unit.

    The most glaring of which were the following:
    1. The picture feed on the display would drop out, and come back with a smack (figured it was a solder joint)
    2. The top line or two of text would be squished horizontally (sounds like a capacitor)
    3. The top few lines of text wouldn't be at the very top of the screen, and would also be squished vertically

    I put it on the bench, opened it up, and replaced electrolytic capacitors C3, C9, C12, C13, C15, C17, C22, & C23, which has solved problems #1 & #2. I would have replaced the remaining 3 electrolytics if i had them in my stockpiles. Problem #3 was not solved by the capacitor replacement...

    IMG_20170407_035247867_HDR.jpg
    So I cranked up the brightness knob past where you would normally want to set it, and the horizontal lines across the screen showed me more about problem #3. The whole vertical deflection is nonlinear.
    monitor schematics.jpg
    I've hooked up my scope to a few spots to see what kind of wave forms I get, and they mostly look okay, with proper voltage ranges. But based on the crude drawings in the schematics, I'm seeing that the wave forms near the vertical yoke (pictured), and near the base of Q10 aren't quite the right shape.
    IMG_20170410_032614178_HDR.jpg

    Long story short, I'm not sure what the next step in my repair process should be. Any ideas what I should test & try next?
    Last edited by CommodoreKid; April 16th, 2017 at 09:09 PM. Reason: schematics didnt upload right
    Reassembling a Hero 1, fixing another Hero 1, putting together an IBM 5170, fixing a spare VIC-20, among other things...

  2. #2
    Join Date
    May 2007
    Location
    Free Country, USA
    Posts
    117

    Default

    Update Time!

    IMG_20170417_031346985.jpg
    I threw together a composite video adapter, and hooked it into the PET to confirm that the signal going to the monitor was alright. The linearity issue was not present on the other display
    IMG_20170417_034637556.jpg
    I've ordered the remaining 3 electrolytic capacitors, including the odd 10μF nonpolarized electrolytic capacitor. I've also ordered replacement transistors for the vertical drive section.

    I'll keep y'all posted on if those new parts fix the nonlinearity.
    Reassembling a Hero 1, fixing another Hero 1, putting together an IBM 5170, fixing a spare VIC-20, among other things...

  3. #3

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    Is the 12 Volt supply OK ? Maybe Q8 is bad ?

    Otherwise just check each of the components in the vertical circuit, non electrolytic capacitors, resistors, transistors and such.

  4. #4
    Join Date
    May 2007
    Location
    Free Country, USA
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    The 12V supply is looking fine actually. Clean stead 12V, with very reasonable ripple. I've tested a few of the resistors and diodes, however I don't have a good way to test the transistors at present. I can check the other capacitors when I've got everything apart for the transistor upgrade.

    I agree, Q8 sounds like trouble, so I'm swapping that out when the parts arrive.
    Reassembling a Hero 1, fixing another Hero 1, putting together an IBM 5170, fixing a spare VIC-20, among other things...

  5. #5
    Join Date
    May 2007
    Location
    Free Country, USA
    Posts
    117

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    Good news, everyone!

    I replaced all five transistors in the vertical deflection circuit (Q6-Q10), along with the two remaining semi-common electrolytic capacitors on the board. Sure enough, I have crisp clean linear video displaying on the screen!
    IMG_20170501_072239578.jpgIMG_20170501_072445760.jpg
    Look how beautiful that is. Honestly, I don't see any reason to replace the nonpolarized electrolytic at present, considering the video looks as good as it does.
    Reassembling a Hero 1, fixing another Hero 1, putting together an IBM 5170, fixing a spare VIC-20, among other things...

  6. #6

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    Very nice!

    I wouldn't do any more work unless something goes wrong.
    Be polite and I may let you live.

  7. #7

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    Looks great and if it ain't broke, don't fix it

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