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Thread: dead 8032 with strange behaviour

  1. #21
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    If the reset circuitry doesn't work - then there is no chance of the PET working.

    Solder a normally open pushbutton either across C51 - or between pins 7 and 1 of UD16 (555).

    Under normal circumstances (after the PET has been powered on for 30 seconds or so) measure the voltages/logic levels at:

    UD16 pins 6 and 7 - both should be close to 5V.
    UD16 pin 3 should be LOW.
    UE15 pin 1 should be LOW.
    UE15 pin 2 should be HIGH.
    UD15 pin 5 should be HIGH.
    UD15 pin 6 should be HIGH.
    UB14 pin 40 should be HIGH.

    Depress (and hold) the push-button.

    UD16 pins 6 and 7 - both should be close to 0V.
    UD16 pin 3 should be HIGH.
    UE15 pin 1 should be HIGH.
    UE15 pin 2 should be LOW.
    UD15 pin 5 should be LOW.
    UD15 pin 6 should be LOW.
    UB14 pin 40 should be LOW.

    Release the push-button.

    Capacitor C15 should charge up over a second or so and the signals levels should revert back to what they were initially.

    Please post your results.

    Dave

  2. #22
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    Here the results...

    BTW I haven't a new 74LS17 (UD15) so i replaced it with a 74LS07, should be a 100% replacement


    UD16 pins 6 and 7 - both should be close to 5V.-- 4,84v
    UD16 pin 3 should be LOW. -- HIGH
    UE15 pin 1 should be LOW. -- HIGH
    UE15 pin 2 should be HIGH. -- LOW
    UD15 pin 5 should be HIGH. -- LOW
    UD15 pin 6 should be HIGH. -- LOW
    UB14 pin 40 should be HIGH. -- LOW


    Depress (and hold) the push-button.


    UD16 pins 6 and 7 - both should be close to 0V -- 0V
    UD16 pin 3 should be HIGH. ok
    UE15 pin 1 should be HIGH. ok
    UE15 pin 2 should be LOW. ok
    UD15 pin 5 should be LOW. ok
    UD15 pin 6 should be LOW. ok
    UB14 pin 40 should be LOW. ok

    Oddity of the day: it becomes to make a "fizzle" from the speaker, sometimes, randomly. In first I was worried because it's that kind of noise made by a burning resistor or IC. Strange, because the 6522 isn't in place (I removed 6520s and 6522; they're ok, I tested them, but just in case). I replaced UD5, but the noise continues. Some electrical noise over some line?

  3. #23
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    EDIT: I changed R24 that seemed bad (actually, after removed I tested it, and it seems ok...) and things get slightly better:

    UD16 pin 3 should be LOW. -- ok
    UE15 pin 1 should be LOW. -- ok
    UE15 pin 2 should be HIGH. -- ok
    UD15 pin 5 should be HIGH. -- ok

    UD15 pin 6
    should be HIGH. -- LOW
    UB14 pin 40 should be HIGH. -- LOW

    Now it seems something related with UD15. Tomorrow I will check again the above scheme; I also have some doubts about R25

    No more noise from the speaker.

    Time to sleep!

  4. #24
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    Based upon your last post I would check the following:

    R15 (1k pull-up to +5V). The 7407/7417 are open collector devices. So if this resistor is open circuit - or the PCB track damaged - then the /RESET signal will never go high.

    UD15 (7417) could be faulty. Yes, I know you have replaced it - but it still could be faulty (unlikely - but possible).

    There is a short circuit somewhere on the /RESET line dragging it to GND/0V/LOW. With the PET switched OFF, measure the resistance from UD15 pin 6 to GND/0V. A low value would indicate the presence of a short circuit - so you will have to look at the PCB carefully with a bright light and a magnifying glass to see where the problem is.

    The fact that you are stating that you get a different result after replacing R24 (even though R24 seems to be OK when it is out of the board) indicates to me potential PCB tracking/soldering issues.

    Dave

  5. #25
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    I can't get the rid of the reset circuit, I don't know why.

    The 555 gives strange results at multimeter and probe.

    Pin 6,7 and 3 pulses; pin 3 sometimes pulses, then stays high for a while, then pulses again, apparently randomly.

    I also did some measurement removing the UE15 and UD15, just to focus on the reset circuit... and I also removed UE15 and UD16 sockets and verified there was no short circuits or something wrong under them.

    I tried to chang the three caps: C51 with an 1uF 100v electrolytic, C50 with a 0,1 uF (labelled 104) ceramic and C52 with a 0,01uF (labelled 103) ceramic.
    I have about 2,50 Volts at UD16 pin 6 and 7, but removing the IC I read 4,89 at the socket. --->> Oddity: on the good and working 8032 I read 0v at UD16->pin 6 & 7 after boot ... will check again, but I'm quite sure of that.

    I checked with various (at least 5) different 555.

    I can't see any problem on the PCB and I don't understand what's going on with that. The circuit is quite simple and I expected to fix it quickly....

  6. #26

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    Pull out all chips with a /RESET connection.

  7. #27

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    Quote Originally Posted by giobbi View Post
    I can't get the rid of the reset circuit, I don't know why.

    The 555 gives strange results at multimeter and probe.

    Pin 6,7 and 3 pulses; pin 3 sometimes pulses, then stays high for a while, then pulses again, apparently randomly.

    I also did some measurement removing the UE15 and UD15, just to focus on the reset circuit... and I also removed UE15 and UD16 sockets and verified there was no short circuits or something wrong under them.

    I tried to chang the three caps: C51 with an 1uF 100v electrolytic, C50 with a 0,1 uF (labelled 104) ceramic and C52 with a 0,01uF (labelled 103) ceramic.
    I have about 2,50 Volts at UD16 pin 6 and 7, but removing the IC I read 4,89 at the socket. --->> Oddity: on the good and working 8032 I read 0v at UD16->pin 6 & 7 after boot ... will check again, but I'm quite sure of that.

    I checked with various (at least 5) different 555.

    I can't see any problem on the PCB and I don't understand what's going on with that. The circuit is quite simple and I expected to fix it quickly....
    smells like damaged PCB tracks and/or hole metallization damaged making no contact to GND on pin 1 of the 555 and/or surrounding capacitors.
    Solder iron is the worst enemy of any vintage repair.

    Frank

  8. #28
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    Quote Originally Posted by KC9UDX View Post
    Pull out all chips with a /RESET connection.
    extracting UE15 and UD15 shouldn't cut the /RESET line between 555 and the other chips?

  9. #29
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    Quote Originally Posted by iz8dwf View Post
    smells like damaged PCB tracks and/or hole metallization damaged making no contact to GND on pin 1 of the 555 and/or surrounding capacitors.
    Solder iron is the worst enemy of any vintage repair.

    Frank
    I understand...

    Even if I'm quite sure I didn't damage the PCB (I have a good unsoldering station and I take a lot of care doing that; at least I didn't damage any tracks), I will check every solder with a multimeter to be sure; of course there's a problem and it must to be fixed.

  10. #30
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    You say above that you replaced C51 with an electrolytic. The parts list calls for a low leakage electrolytic - or a tantalum.

    If the capacitor is too leaky - it may not charge up enough to operate the 555.

    Either that, or a faulty PCB...

    Dave

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