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Thread: PET 8032 - CRT board issue

  1. #1
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    Default PET 8032 - CRT board issue

    Hi guys,

    after about an year with almost no time for my beloved PETs, I'm back again.

    I'm trying to fix an issue that basically is the same in two different 8032 CRT board: the R753 resistor is burned out (and, in one of the two boards, the R752 too). I tried to put another resistor, but it starts to overheat immediately (even if it's 1 watt instead of 1/4 watt). The CRT worked, but I had to power off it soon.

    I've seen there are two capacitors (C752 and C753) that could be involved, but I checked them and they're ok. The two diodes close to R753 and R752 are ok too.

    Please somebody here who has some skill with CRTs could kinldy take a look to the schematics and tell me what should I check?

    The CRT diagram is: http://www.zimmers.net/anonftp/pub/c...032/321448.gif


    thank you

    --Giovi

  2. #2
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    Hi Giovi,
    Nice to hear from you again. This issue of burnt 56 Ohm resistors has been seen before. One of the analog experts here has solved it. We'll have to wait for him, but I think it had to do with bad capacitors. What method did to use to check the caps?
    -Dave

  3. #3
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    Yes, there are two ‘massive’ threads already on this topic.

    Can I suggest looking them up and seeing if anything looks familiar with your PET first? You should be able to find the thread by looking up my posts and working from there.

    Capacitors are the general problem...

    Please, please do not replace these resistors with higher powered ones. They are (effectively) fuses, so replacing 1/4 W with 1W will just mean that the fault current will persist for longer and, potentially, do more damage !

    Dave

  4. #4
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    Bit of time to have a look at your problem between Church and lunch...

    Yes, welcome back to the Commodore thread...

    R752 feeds the +400V supply. The most likely cause of a problem here is C752 breaking down under high voltage. As Dave_m has asked, how are you testing these capacitors? If with a multimeter, it is possible that it appears as an open circuit on a multimeter - but when subjected to a high voltage it breaks down and presents a low resistance to the power supply (hence drawing a large current).

    R753 candidates are (as you have stated) C752 and C753. Note that R753 is also in series with R752 (so any fault at the 400V end could burn-out R752 and/or R753). Especially if you have replaced one of these with a higher wattage part.

    I would also check for PCB 'tracking' or 'growths' that may also present a short circuit (or low resistance).

    If the above doesn't prove fruitful, I would remove R753 and initially check all the waveforms and voltages through the line driver part of the circuit from the Horizontal Drive input connector from the PET main board through to the primary of T721 for adherence to the oscilloscope traces and voltages as described in the manual.

    Especially, check the output voltage from the regulator IC901 to make sure it hasn't gone short circuit and feeding more than +18V d.c. to the monitor circuits.

    Dave
    Last edited by daver2; January 26th, 2020 at 04:03 AM.

  5. #5
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    Hi Dave & Dave!

    I checked the caps just removing and testing by a caps meter. The value is close to the nominal, but honestly I don't know it they works fine under high voltage.
    I think a cheap and harmless test is to replace both caps with fresh ones.

    I used higher wattage resistor just because I used what I had in my toolbox. I will put right wattage resistors too.

    As Dave(r2) suggested, I will check IC901 too.

    It will take a little, because here in the badlands (lol) it's hard to find electronic replacements.

    I will tell you the results asap

    thanks for now
    Giovi

  6. #6
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    No problem.

    Just a thought...

    With the power off (and everything discharged) you could just check around with your multimeter on resistance and see if there are any short circuits (or low resistances) to 0V/GND from the junction of D752/C752 and/or the junction of D253/C753.

    Dave

  7. #7
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    Quote Originally Posted by daver2 View Post
    No problem.

    Just a thought...

    With the power off (and everything discharged) you could just check around with your multimeter on resistance and see if there are any short circuits (or low resistances) to 0V/GND from the junction of D752/C752 and/or the junction of D253/C753.

    Dave
    Good idea, I will do! Thank you!

  8. #8

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    Daver 2 is on the right path also check the positive end of D752 goes to E400 which supplies voltage to
    the focus and screen of the CRT check controls for a short or low resistance to ground or components
    feeding the CRT...Colin

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