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Thread: cbm 8032 rusty and dead

  1. #21
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    Quote Originally Posted by Xchip View Post
    But I found another problem, the cpu signal on the PIN17 signal AB8 is abnormal, it could be a short circuit or a defective chip but I have not yet understood where the problem is, I have already replaced on that line a 74ls244 and a 7425 without success
    The source of address line 8 signal (AB8 ) is the CPU (UB14), but the 6502 chip rarely fails, and you say you replaced the buffer at UD14 (74LS244). If you have another 6502 CPU then try replacing UB14, but the problem may be an intermittent short on the AD8 line caused by solder splash, bad solder joint, bad IC socket, etc. Look with magnifying glass and if nothing found, reflow the solder joints on socket UB14 or replace socket. You are doing good!

  2. #22

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    after so much work I found two broken logic chips: 74ls02 and a 74ls00 that were partially working. the current situation is that on the video an unstable image appears with some dots but nothing readable, every reset always performs the same thing so the computer must be working because the CRTC is programmed by the CPU and starts the video signal generation, with good probability I made a mistake or some capacitor is at a loss as I have already verified several times, if you have useful suggestions to check the signals with the oscilloscope I am grateful.
    now I'm close to the solution of this unfortunate computer.

  3. #23

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    The CRTC running indicates that the CPU got far enough to initialize the CRTC. It may or may not be working after that. The processor is likely good but you may still have a problem with RAM or ROM.
    Some pictures of what you have would help to know where to start looking. A better indication of the processor working correctly would be for you to type in a proper command and see if the command does anything to the display? It doesn't have to display correctly, just respond in some way. That would help to isolate a display problem from a code problem.
    Dwight

  4. #24

  5. #25

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    I forgot to say that the PET sounds, when it is turned on or after the reset the buzzer is heard, then the computer correctly boots, I found a missing resistor, r50 47 ohms, after putting it I have something on the screen as in the picture above moment I finished the ideas, I bought the 6502 system test of the tynemouth software but nothing changes on the screen, you hear a different sound then perform the various test cycles

  6. #26
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    Quote Originally Posted by Xchip View Post
    I forgot to say that the PET sounds, when it is turned on or after the reset the buzzer is heard, then the computer correctly boots, I found a missing resistor, r50 47 ohms, after putting it I have something on the screen as in the picture above moment I finished the ideas, I bought the 6502 system test of the tynemouth software but nothing changes on the screen, you hear a different sound then perform the various test cycles
    If you installed the Tynemouth 6502 diagnostic board in the CPU socket, and do not get a proper video output, I suspect something is wrong with the video data generation on schematic sheet 8. There could be a problem with the video RAM, 6545 CRTC, video latch, character generator, shift register, parallel to serial shift register, or several gates. Or perhaps you still have a problem with a short on the Address bus or Data bus.

    Let's start with the buses. Remove the diagnostic board or 6502 from the 6502 socket. Remove the J7 video cable from the mainboard. Power on the PET, and with a scope, check that you have about 1.5 VDC on all the address lines. If that passes, we'll have to float the data lines to check them for 1.5 VDC. If you have no EPROMs installed in the ROM sockets, you can ground a signal called NO ROM to float the ROM outputs. You can find this signal on ROM UD6-pin 21. Check that all the data lines are about 1.5 VDC.

    If that passes, we can go on to the video data circuits as you are getting incorrect video data.

  7. #27

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    now I'm at a standstill, I found a defective chip but no change, no screen image suddenly stopped playing when turned on, with the diagnostic rom it starts up but the video is completely wrong I checked sram, crtc, cpu , rom all and ram but nothing wrong, you have to start over to understand I checked as suggested by the previous post but it all seems in order maybe I did something wrong in the controls

  8. #28
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    So, how come R50 was missing?

    This is one of the major clock lines into the video circuit, so with R50 not present it was never going to work...

    Has this machine been 'worked on' previously?

    The other thing is (assuming this is a univ2 machine) this clock line can be configured by a link (J1 or J2) upstream of R50 (for 40/80 columns). Is this link (and the others) in the correct position for the type of monitor you are using? Just thinking, if someone has removed R50, is it possible they have also reconfigured the links?

    Dave
    Last edited by daver2; August 7th, 2019 at 10:16 PM.

  9. #29

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    Quote Originally Posted by Xchip View Post
    to me, Hsync doesn't seem to be correct, maybe also Vsync. How did you adapt the peculiar scan rates of a CRTC PET to that monitor? I use an old MDA monitor with a little board adapter to display CRTC PET's video output. Scan rates are almost identical to the ones used in MDA PC cards, one ttl sync signal has to be inverted and the video output needs to go to both video and intensity pins of an MDA monitor.

    Frank

  10. #30

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    Quote Originally Posted by daver2 View Post
    So, how come R50 was missing?

    This is one of the major clock lines into the video circuit, so with R50 not present it was never going to work...

    Has this machine been 'worked on' previously?

    The other thing is (assuming this is a univ2 machine) this clock line can be configured by a link (J1 or J2) upstream of R50 (for 40/80 columns). Is this link (and the others) in the correct position for the type of monitor you are using? Just thinking, if someone has removed R50, is it possible they have also reconfigured the links?

    Dave
    you are right to think so, in fact the card is rusty and many components were damaged as r50, in fact so corroded that it was gone now, I saw a condenser light up like a light bulb.
    The configuration is 80 columns has remained so unchanged, the problem is certainly a short circuit or an interruption this requires a great knowledge on its operation, knowledge that I do not have.

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