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Thread: Commodore PET 4016 CRT Problems??

  1. #21
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    Yes I am using pettest2. Also the time to initialize the CRT is like very exact. It is the exact same period of time, every time. I'm inclined to believe this isn't an error, though I could be wrong.

    I really don't think anything is wrong with the HSYNC or VSYNC output. Of course I can try to measure it with my Arduino anyway, as that's the only tool I have right now. Also, even if HSYNC and VSYNC weren't okay, I would still expect to see a picture on the display, even if its a garbled mess.
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  2. #22
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    Arduino as a Freq. Counter, HSYNC is 20kHz, VSYNC is 60Hz
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  3. #23
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    Quote Originally Posted by daver2 View Post
    You say in post #7 that you are seeing data between UA2 and UA3 and that you see a clock on the shift register. Do you have the load signal as well? It is no good having the clock to shift the bits if it didn't load them in the first place.
    3.45V steady on load. Doesn't change when the CRTC initalizes.

    Datasheet for 74166 shows that its high unless it loads...this should be normal? I'm starting to wonder if the shift register is dead?...
    Last edited by twistedpneumatic; April 7th, 2020 at 11:38 AM.
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  4. #24
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    Quote Originally Posted by twistedpneumatic View Post
    3.45V steady on load. Doesn't change when the CRTC initalizes.

    Datasheet for 74166 shows that its high unless it loads...this should be normal? I'm starting to wonder if the shift register is dead?...
    3.45V is most likely a correct negative going pulse every 1 uS with the duty cycle of about 75%. A good /LOAD SR signal.

    Even if the shift register was dead, wouldn't it be stuck at a 1 or 0? This should not cause a bad raster scan.

    This is a very interesting problem!

  5. #25
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    I did check and even after it flashes and "dies", there is still an HYOKE and VYOKE present, so it's scanning alright, even if there is nothing displaying.

    Wait ... Why wouldn't it being stuck at 0 cause a blank screen?
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  6. #26
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    Quote Originally Posted by twistedpneumatic View Post
    I did check and even after it flashes and "dies", there is still an HYOKE and VYOKE present, so it's scanning alright, even if there is nothing displaying.

    Wait ... Why wouldn't it being stuck at 0 cause a blank screen?
    Well yes or a bright screen as i'm not sure of polarity of video signal, but not craziness. Is your brightness turned too high?

  7. #27
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    The brightness was at its max, yes, but it didn't seem too high. Just about as bright as other tubes turned all the way up. It was just to see if I could see a raster from it being up but nope. Video signal is 0V black, 5V white(green). I turned it down to a reasonable level currently.

    Also pin 11, UC2 is 0.2V, and never changes once it initializes. (output of the XOR of the shift register and UB2)

    EDIT: 12 and 13 are also 0.1-0.2V. I'm thinking the problem resides before this point.

    EDIT 2: Just to confirm again, checked the lines into the shift register, many of them have different voltages in the 0.3V-2V range, and they change a lot as the machine boots. I could see if I could set the Arduino up as a crude logic probe. It runs at 16Mhz, so I think I should get atleast an effect bandwidth of a few hundred Khz.

    EDIT 3: Just checked pin 13 of the shift reg. not to be shorted to ground...just to make sure.
    Last edited by twistedpneumatic; April 7th, 2020 at 12:59 PM.
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  8. #28
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    Quote Originally Posted by twistedpneumatic View Post
    Also pin 11, UC2 is 0.2V, and never changes once it initializes. (output of the XOR of the shift register and UB2)

    EDIT: 12 and 13 are also 0.1-0.2V. I'm thinking the problem resides before this point.
    No, 0,0,0 on the XOR is OK. It is there to invert the video data if bit 7 of video RAM is set. On a power on screen LSD7 is not set.


    After boot, Video RAM is cleared so the data in lines to the shift register will be mostly low even after the Commodore welcome screen. It may be hard to tell there is data using the voltmeter. A logic probe can tell. Make your Arduino into a pulse detector/pulse catcher where you can clear it and see if it catches a new pulse. Your Arduino is proving useful!

  9. #29
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    Arduino was polling a signal until the CRTC started, at which point it stopped finding any pulses. I know VSYNC was at 15 something KHz before the CRTC started when it dropped to 60Hz.

    The inputs to the shift register are very active according to the 'duino

    The output (13) to the shift register is dead as a doornail.

    I'm probably going to do a lot component buy tonight since I also needs caps for my Mac Portable. Outside of the shift register, what else would be a good to buy, just because shipping right now is a hassle and I'd like to just order once and get some extra ICs.
    Last edited by twistedpneumatic; April 7th, 2020 at 03:01 PM.
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  10. #30

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    Quote Originally Posted by twistedpneumatic View Post
    Arduino was polling a signal until the CRTC started, at which point it stopped finding any pulses. I know VSYNC was at 15 something KHz before the CRTC started when it dropped to 60Hz.

    The inputs to the shift register are very active according to the 'duino

    The output (13) to the shift register is dead as a doornail.

    I'm probably going to do a lot component buy tonight since I also needs caps for my Mac Portable. Outside of the shift register, what else would be a good to buy, just because shipping right now is a hassle and I'd like to just order once and get some extra ICs.
    Your experiment with the arduino says the video monitor is working.
    The shift register is not shifting any thing.

    Places to check next.
    Is the shift register getting clock?
    Is the data changing on the load input?
    Is there any changes on the data inputs?
    You've said you don't have a scope but do have a logic analyzer. Use it.
    Dwight

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