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Thread: Commodore PET 4016 CRT Problems??

  1. #11
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    1) It appears there are only 2. Its a 40 column machine...wouldn't the 80 have 4?

    2) No they aren't in sockets.
    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. #12
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    That seems right because every once in a while when I turn off the PET, i get a few dots of green on a line and then it shuts off. I'm assuming this is the grid shutting off before the heater is fully cooled down so a few electrons spit out before it dies. Also an injected video signal works just fine...so it must just be outputting a low signal.

    Sort of side note: I know that those V-RAM chips go bad often, but is it possible that one of the additional 74 series chip went bad...maybe the shift register??
    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. #13
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    Quote Originally Posted by twistedpneumatic View Post

    I know that those V-RAM chips go bad often, but is it possible that one of the additional 74 series chip went bad...maybe the shift register??
    Sure, and it is also possible that a bad RAM or ROM is keeping the the PET from running properly and it is outputting a blank screen.

    Can you summarize what is happening when the video data line is connected to the video board along with the proper timing signals? Is the screen blank or ??? How about when the video line is open? I am not understanding the problem. How did you 'inject a video pattern? was it in sync with the timing signals? Was it a TTL level signal or ?
    -Dave
    Last edited by dave_m; April 6th, 2020 at 10:52 PM.

  4. #14
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    Okay, there are 3 scenarios:

    1) The connector is plugged into the board, the CRT is fed HSYNC,VSYNC and Video from the PET. Raster flash then nothing. 0.3V at the video pin and nothing on the display. 30V at the grid.

    2) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and NO VIDEO(left open, not grounded). Boots up to a solid green screen.

    3) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and VIDEO from the ARDUINO. Literally I disconnected the jumper from the PET main board and stuck it in pin 13 of my Arduino. The Arduino is outputting a random square-wave at a few dozen kHz. Striped lines appear on the CRT as expected. Video responds to changing timing on the Arduino(ie...higher frequency, narrower stripes, and vice versa)
    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.

  5. #15
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    If you set the CRT at maximum bright, you should see some green nebula and diagonal lines. If not, probably the CRT isn't working.

    I had the same problem with both my 8032: a broken resistor in the high-voltage line, because of a bad electrolytic.
    If it's your case too, you should download the the 40xx manual from Zimmers and take a look to the high voltage section (page 76 of the pdf manual from Zimmers).
    Check the resistors and if you find any broken, check the capacitors close to them.

  6. #16
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    I did notice that the brightness changed the picture size, which AFAIK is a dead give away for something wrong in the power circuitry, likely caps. Which cap/resistor did you replace?
    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.

  7. #17
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    Also, if the video signal is completely dark...ie grid on full, shouldn't the brightness be worthless since the electrons are already being blocked?
    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. #18
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    Quote Originally Posted by twistedpneumatic View Post
    Okay, there are 3 scenarios:

    1) The connector is plugged into the board, the CRT is fed HSYNC,VSYNC and Video from the PET. Raster flash then nothing. 0.3V at the video pin and nothing on the display. 30V at the grid.

    2) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and NO VIDEO(left open, not grounded). Boots up to a solid green screen.

    3) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and VIDEO from the ARDUINO. Literally I disconnected the jumper from the PET main board and stuck it in pin 13 of my Arduino. The Arduino is outputting a random square-wave at a few dozen kHz. Striped lines appear on the CRT as expected. Video responds to changing timing on the Arduino(ie...higher frequency, narrower stripes, and vice versa)
    We have to make sure the HSYNC and VSYNC from the PET are correct or you may damage your CRT/ video board. Can you make the adriano into a counter to see if H is 20 KHZ and V is 60 Hz?

  9. #19
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    This is an interesting fault.

    Post #14 item (2) clearly indicates to me that it is the video signal from the PET to the monitor that is the problem. Disconnecting the PET video signal causes the monitor to pull the video signal to +5V (white level) via a resistor. I would temporarily stop looking at the monitor for now.

    There was something strange you said back in post #1 about the PET taking ~6s to "start-up". This does seem to be a bit long to me... Is the RESET line holding things up - or is the ROM not working properly and 'eventually' the CPU manages to initialise the CRTC to get H/V drive I wonder? If this is the case, what has the PET tried to display on the screen? If it has managed to clear the video RAM - we will see a "black cat in a coal cellar at midnight"... Need to check through the ROM source code - but it may be that the EDIT ROM initialises the CRTC and clears the screen but (if page 1 RAM is faulty - the stack) the subroutine return from the EDIT ROM back to the KERNEL causes the CPU to crash'?

    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.

    Just thinking of scenarios...

    Either work forwards from the character generator through the logic chain to see where the video signal disappears or work backwards from the video signal to see where it appears.

    Dave

  10. #20
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    Quote Originally Posted by daver2 View Post

    There was something strange you said back in post #1 about the PET taking ~6s to "start-up". This does seem to be a bit long to me...
    Daver2,
    I think our OP is using the old pettest2 which tacks on the 6545 CRTC initialization at the end of the first pass of the Eudi's program. It could very well take 6 Seconds to get to the initialization. Bad programming (mine!), I did not have have a 6502 assembler handy when I did it. I just tacked some machine code to the end of the binary code and then jumped to the beginning.

    I told our OP in a private message to switch over to your (pettest2kV4) in the EDIT socket, but he reported he does not have access to a prom programmer.

    -Dave
    Last edited by dave_m; April 7th, 2020 at 11:09 AM.

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