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Thread: PET 2001-32N BS, with horizontal lines.

  1. #1

    Default PET 2001-32N BS, with horizontal lines.

    Hello everybody!

    First of all please forgive me for my not so well English, it is not my first language. So...

    Before two days i got my precious Commodore PET 2001-32N BS. Till now i never had the chance not even see one in real and finally i got one! I take it faulty with horizontal lines on the screen. Photo is from others member thread cause today i take apart the monitor from the case, but i have the same issue.

    dsds.jpg

    What i find so far is that i missing -5V on ram IC's. I check the voltage regulator and i have output 0v. The input is -10V. One of my questions that i dont want to forget to ask, is that on schematic i see that pet 2001 use 7905 regulator. In my case my motherboard have 7906 regulator. Is that normal?

    5444.jpg

    What i did after i saw the missing -5V, is to test the HM4716A-4 Ram's with my arduino tester one by one. Luckily all Ic's are in working order. So, i just clean very well the IC legs and put them away till i fix first the issue with the regulator.

    Photo 5-12-19, 12 48 17-min.jpg Photo 5-12-19, 13 25 19-min.jpg Photo 5-12-19, 13 25 23-min.jpg Photo 5-12-19, 13 26 15-min.jpg Photo 5-12-19, 14 47 01-min.jpg

    What other things i found in the board are some "mods" that may be tell something to you.

    First there is a cut on trace that going at UG6 74LS107 Pin12.

    Photo 5-12-19, 10 15 52-min.jpg Photo 5-12-19, 10 33 56-min.jpg

    Also, there is a bypass cable that connect the previous UG6 74LS107 Pin12 with the UH6 74LS107 Pin2.

    Photo 5-12-19, 10 33 13-min.jpg Photo 5-12-19, 10 16 12-min.jpg

    Finally, the is one more cut on the trace that normally connect the UH1 74S74 pin12 with UF1 74LS08 pin3.

    Photo 5-12-19, 10 16 22-min.jpg Photo 5-12-19, 10 33 22-min.jpg



    All this i can tell for now. The reason i take apart the monitor from the case in because i read that is very risky to do test's with the original monitor. I am waiting the schematic from here http://www.vcfed.org/forum/showthrea...230#post597230 to connect an external monitor to the motherboard.

    Should i replace the faulty voltage regulator? But with what, 7905 or 7906? And check after the replace the -5v if they come back on board.

    I have a voltmeter and i just buy a logic probe, eprom programmer, mini oscilloscope (in the way), in case i need them for the repair.

    Thank you all very much for your time reading all this and i really cross my fingers for your help!

  2. #2
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,339

    Default

    Welcome to the party!

    Don’t apologise for your English... Your English will be better than our Greek!

    Yes, you need to get the -5V rail back to working order, otherwise you will have no working RAM.

    Before I give you any recommendation of my own, let me digest your text and photographs tomorrow to see what I can see in the modifications. It is getting rather late in the UK at the moment, and I am sure it is much later in Greece!

    I also hate people who do mods and then don’t leave documentation within the machine telling the next owner what they have done. But that’s another story...

    Dave

  3. #3
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,744

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    If you disconnect the J7 video connector, your monitor will be safe. With no horizontal drive signal from the mainboard connected to the video board there will be no high voltage. There appears to be no video data being generated. Hopefully that is a problem in the mainboard which can be easily fixed.

    Yes, when you replace the 7905 regulator, check that +5V, -5V, and +12V are all working. Then we can see if the 6502 CPU is running. The jumper from G6-pin12 to H6-pin2 is connecting signal /RA1 as the trace was cut to G6-pin12. Perhaps the trace was shorted to something and had to be abandoned? You would want to scope G6-pin12 and see that /RA1 is OK. It will be a square wave of 500 KHz.

  4. #4

    Default

    Thank you both!

    I did a mistake. The neg voltage regulator is indeed 7905c and not 7906. Sorry about that. He have both numbers and this confuse me. To be honest, when i read 7906 in the beginning i didn't read the other number.

    Now about the Oscilloscope, i order a cheap one. The DSO FNIRSI-150 15001K, the fully assembled one.

    With this specifications:

    Maximum Real-time Sampling Rate: 1MSa/s
    Analog Bandwidth: 0-200KHz
    Sensitivity Range: 5mV/div-20mV/idv
    Maximum Input Voltage: 50Vpk (1X probe)
    Input Impedance: 1M ohms/20pF
    Accuracy: 12 Bits
    Record Length: 1024 points
    Coupling Modes: DC / AC / GND
    Time Base Range: 500s/Div - 10us / Div
    Trigger Modes: Auto, Normal, and Single
    Trigger Position: Center of Buffer
    Supply Voltage: DC 9V(8-10V)
    Current Consumption:~120mA @ 9V
    Main Board Size: 94 * 65mm / 3.7 * 2.56in
    Analog Board Size: 65 * 47mm / 2.56 * 1.85in
    Screen Size: 52 * 40mm / 2 * 1.57in
    Package Size: 14.5 * 10 * 3.7cm / 5.7 * 3.94 * 1.45in
    Package Weight: 179g / 6.33oz


    I hope i can do my job with this one.

    Thank you!

  5. #5

    Default

    Quote Originally Posted by daver2 View Post
    ... Your English will be better than our Greek!
    Όχι όλοι μας, φίλε μου Και καλώς ήρθατε ... Muttley Black. Γεια σου.
    PM me if you're looking for 3½" or 5¼" floppy disks. EMail “ ” For everything else, Take Another Step

  6. #6
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,339

    Default

    What Dave says!

    The regulator should be a 7905 and NOT a 7906. Someone may have tried to 'fix' this before and caused more problems for you in the process. EDIT: I see you have identified the correct part now. The 7906 may be the date code (1979 month 06).

    As Dave says, make sure you have healthy power rails before doing anything. If you already have the RAM removed, don't plug it back in until you have sorted out the -5V. You may find there is a problem on the board and you don't want to damage the RAMS if you can avoid it.

    The oscilloscope you have chosen is similar to what I have. It is OK for video signals and slower - but nothing much faster. We have fixed ilpaninaro's 8032 PET before with a similar oscilloscope and a bit of ingenuity !

    Dave
    Last edited by daver2; December 6th, 2019 at 07:32 AM.

  7. #7
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    You are the exception that proves the rule Stone !

    Dave

  8. #8

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    Dave, I was fortunate enough to have lived on a Greek Island for several years.
    PM me if you're looking for 3½" or 5¼" floppy disks. EMail “ ” For everything else, Take Another Step

  9. #9

    Default

    Power rail always have capacitors on them. These can be shorted. Since you have a voltmeter, I'm sure it can read ohms. With the power turned off, measure the resistance from the output of the regulator to ground. If it is shorted, the resistance will be really low, usually less than 10 ohms.
    Two things can cause 0 volts. An open regulator is one. A short to ground is the other.
    Dwight

  10. #10

    Default

    Quote Originally Posted by Dwight Elvey View Post
    Power rail always have capacitors on them. These can be shorted. Since you have a voltmeter, I'm sure it can read ohms. With the power turned off, measure the resistance from the output of the regulator to ground. If it is shorted, the resistance will be really low, usually less than 10 ohms.
    Two things can cause 0 volts. An open regulator is one. A short to ground is the other.
    Dwight
    I agree with Dwight, it's far more easy for a capacitor to short than for a 7905 to fail, but all can happen anyway
    I'd check for shorts on the -5V rail before changing the regulator. Since your board has tantalum capacitors, one shorted capacitor is almost certain.
    I've had shorted tantalums in quite a few PET repairs.
    The "mods" may actually be corrections on a bad run of 2001N PCBs, I *seem* to remember having seen something similar on one PET I've got to repair, but I can't really recall the details now.
    I would first fix the -5V problem, then see what happens next.
    Frank

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