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Thread: Commodore PET 4016 CRT Problems??

  1. #1
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    Default Commodore PET 4016 CRT Problems??

    I went back to approach fixing my PET again. This is where I'm at currently.

    I installed 3 new ram chips and their sockets.
    PETTEST2.BIN is installed.

    As you claim it should...the signals are all 5V until about ~6s after it boots of which VSYNC is about 4.4, HSYNC is around 3.5V, and 0.3V on Video-IN. Seems normal enough...so I think I can conclude the board is atleast partially functional. Maybe V-in should be higher...but it seems as the CRTC is being initialized so I don't think its much of an issue. I could always disconnect the resistor to the grid to see if a raster forms.

    Anyway...based on that I measured around the CRT board. Got proper voltages on the cathode(glows), HYOKE and VYOKE coils, along with proper voltages on E18, E30, E400, and the -115V rail once HSYNC kicked on. The only voltage so far that has come up wrong is Q761(by R762 on zimmers-321448 schematic), where the base never comes back down to 12.9V, rather stays at 18V. Other transistors on the board do similar but kick down once HSYNC comes in and the HOT can start generating voltages.

    So the tube currently does this: The tube starts glowing...a dot appears on the screen, eventually gets quite bright over a second or two. The beam drops down and a raster that makes the entire width and about half the height flashes then it dies. I've found the tube to make a sorta crackling sound...I had an old trinitron that did this so I think its the HV discharging? Also some slow motion video on my phone....I heard the while from the CRT since it was slowed down by 8x, and it was stable for a teeny bit, then start to wharble or something...sounded...unhealthy? Anyway the raster appears(1 run) and the picture dies. No more dot even. I'm suspecting loss of HV....though the grid voltage is on which would also cause that.

    I'm lost on where to go now from here. I'm still betting on the CRT being bad....I showed the video of it running to someone at VCF and they said caps on the tube. I checked them with my multimeter and they mostly seemed good. I'm confused on where to go from here though.
    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. #2
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    I disconnected the video signal while leaving them all intact. Got a good raster and no crackly sound. Now is the video causing HV failure(ie...no wierd sound), or is the CRTC outputting garbage video. Given that HSYNC and VSYNC are being properly generated...I see no reason why there shouldn't atleast be SOME picture, right?
    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. #3
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    The video and the H/V shouldn't interact with each other - unless the E30 Voltage is being compromised in some way?

    Dave

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    Once HSYNC kicks on, E30 seems to be rock solid. Also checked many of the caps on the board with my multi meter in capacitance mode...they seem to check out for the most part.
    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. #5
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    I did read that in your previous post regarding E30 though.

    The problem with using a capacitance meter is that the capacitor could break down when subjected to a voltage across it (especially in the higher-voltage stages, but not exclusively there).

    Dave

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    The video signal is only reading 0.3V, shouldn't it read something higher...PETTEST2 should be increasing the voltage on screen. Could the part between the CRTC and the video out be bad? The VIDEO signal drops from 5V to 0.3V when HSYNC starts.

    Would recording a video of the machine with and without VIDEO be helpful?
    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.

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    I checked more of the circuit on schematic 8(dot gen & even display), clock seems okay, data appears to be flowing into UA2 from UA3, though I'm not sure anything is coming out. Input 12 of UC2 seems a rather low 0.4V, figure it should be atleast over 1V if there is a video signal.
    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.

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    Okay, I injected a separately generated test signal from an Arduino into the video line and got an expected pattern. I can safely assume the CRT is fully functional. Also given that the CRTC is creating a proper HSYNC and VSYNC after a very repeatable ~6-7s, I think the CPU, RAM, CRTC and ROM have to be properly functional. From here...it should be somewhere in the character rom/dot gen part of the circuit, right?
    Last edited by twistedpneumatic; April 6th, 2020 at 07:01 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.

  9. #9
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    Are your 4 video RAMs (2114) on sockets? If you remove them, you should get a checkerboard pattern on the video data line. If you disconnect the J7 Connector for protect the CRT, check the data line for about 2.5VDC average on your meter.

    But I do not see how any signal on the video line since it is a TTL signal can disrupt the raster scan.

  10. #10
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    Quote Originally Posted by twistedpneumatic View Post
    I checked more of the circuit on schematic 8(dot gen & even display), clock seems okay, data appears to be flowing into UA2 from UA3, though I'm not sure anything is coming out. Input 12 of UC2 seems a rather low 0.4V, figure it should be atleast over 1V if there is a video signal.
    No, if the screen is primarily blank, I think one will see a very low voltage on the video line (mostly zeros shifting out).

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