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Thread: Commodore Pet 8032SK with monitor not working

  1. #231

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    Dave is really great and has much more patience than I have. He knows the machine very well.
    I do have a question that you might be able to answer. In the schematic towards the lower right of the diodes in question, there is an unusual circuit. I see an electrolytic all by itself. I would have expected there to be a resistor in parallel with it. Can you examine your video board and confirm it is as in the schematic on the web, no resistor in parallel?
    As for why the resistors failed, I'm blaming it on smokers. The resistors in question would have acquired dust and tobacco smoke over time, because of the high voltage. That and a little bit of humidity might have made for a destructive goo.
    Thanks
    Dwight

  2. #232
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,194

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    >>> Finally 8032Sk it's ok!!

    Great news, another one rescued!

    >>> Thank you very much for the precious help you gave me!!

    No problem.

    Of course, when you had my PETTESTER installed you could have tested the keyboard then. Make use of the diagnostics...

    Dave

  3. #233
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,706

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    Great team effort on the video repair!
    Kudos to Daver, Dwight and Hugo for their expertise and especially to The Paninaro for his dedication.

  4. #234
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,213

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    Whew! Great work, guys!

  5. #235

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    Good evening,
    i cleaned keyboard inside and now works correctly
    Thank you again to all!


    These pictures are for Dwight:
    20191106_211405.jpg,20191106_211426.jpg,20191106_211431.jpg

  6. #236

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    Even though the schematic shows it in the right lower corner, the circuit in question is mostly under the yoke. I was hoping to see a resistor that didn't have a match in the schematic. I've looked at the voltages and don't know how it could drop in voltage without a DC path to ground.
    Dwight

  7. #237

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    Quote Originally Posted by Dwight Elvey View Post
    Even though the schematic shows it in the right lower corner, the circuit in question is mostly under the yoke. I was hoping to see a resistor that didn't have a match in the schematic. I've looked at the voltages and don't know how it could drop in voltage without a DC path to ground.
    Dwight
    Dwight,

    As far as I can see with this circuit, maybe, the capacitor is charged by the transistors emitter current and would reach the peak value there, plus the average DC component there and perhaps a little less if the charging is not complete due to the emitter resistor. But at flyback the polarity reverses, at the transistor's base, and the transistor is cut-off, but at this point the diode there rapidly discharges the capacitor below the average value, therefore the capacitor probably does not require a parallel discharge resistor.

    It would be interesting to look at the transistor's collector voltage on the scope, if it has a parabolic form, it might be a focus correction circuit for the CRT beam, but going against this notion is that it appears to connect to(and modulate) the Gun's anode voltage not the focus electrode voltage (unless its just the way the CRT is drawn). Another possible option could be is an H blanking amplifier, but it would be a very odd connection because normally blanking is introduced at the CRT's grid or cathode and the polarity of the voltage generated looks incorrect. It would probably all be clear with the scope on it.

  8. #238

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    It just bothers me. It doesn't look like anything some monitor designer would create.
    Dwight

  9. #239

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    Quote Originally Posted by Dwight Elvey View Post
    It just bothers me. It doesn't look like anything some monitor designer would create.
    Dwight
    Dwight,

    Off the topic of this thread (but shouldn't matter as the OP's computer is fixed now) it is amazing what kind of interesting and oddball and sometimes downright brilliant circuits turn up in vintage TV's and monitors.

    The best one I ever discovered was a beyond brilliant horizontal deflection circuit used for a 1948 American Admiral vintage electrostatically deflected TV set. The designers appeared to achieve the impossible; they generated two 450V peak to peak anti-phase linear sawtooth voltages for H deflection, from a mere 250V supply rail, and did it with just one triode and a pair of small transformers! I had to write it up. The circuit is on page 4 and the description in this article of how it works:

    http://worldphaco.com/uploads/Admiral_TV..pdf

    It was not documented in any of my American TV textbooks (Grob, Fink etc). After years of looking I discovered it was invented by Faudell & White.

  10. #240

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    Quote Originally Posted by Hugo Holden View Post
    Dwight,

    Off the topic of this thread (but shouldn't matter as the OP's computer is fixed now) it is amazing what kind of interesting and oddball and sometimes downright brilliant circuits turn up in vintage TV's and monitors.

    The best one I ever discovered was a beyond brilliant horizontal deflection circuit used for a 1948 American Admiral vintage electrostatically deflected TV set. The designers appeared to achieve the impossible; they generated two 450V peak to peak anti-phase linear sawtooth voltages for H deflection, from a mere 250V supply rail, and did it with just one triode and a pair of small transformers! I had to write it up. The circuit is on page 4 and the description in this article of how it works:

    http://worldphaco.com/uploads/Admiral_TV..pdf

    It was not documented in any of my American TV textbooks (Grob, Fink etc). After years of looking I discovered it was invented by Faudell & White.
    Thanks for sharing Hugo! That's highly interesting!
    Frank

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