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Thread: Commodore 64 Power supply characterization

  1. #1
    Join Date
    Nov 2017
    Location
    Australia, NSW
    Posts
    20

    Default Commodore 64 Power supply characterization

    Hi all,
    I found at the back of my vintage computer collection a Commodore 64 console. It has the 251137 Rev.B motherboard.. I also found the power supply that came originally with it. I decided to check the power supply first, so it wouldn't blow up the console if it was faulty. This is the type of Power Supply it came with..

    C64PowerSupplyLow.jpg

    I decided to put the 5V on a load and gradually check the voltage drop as I increment the load in 100mA increments up to it's rated spec i.e. 1.5 Amp. This required me to hook the Din plug to the power load. I only have 5 pin Din plugs and I didn't want to risk a short using paper clips or crocodile clips, so I fabricated two connectors out of two spade plugs used for automobile connections.. See the following photo for how to do that.

    ConnectorOverview.jpg

    Then I attached it to the load and began the testing.. Here it is at it's max rated output 1.5 Amp.

    Testing_1_5Amp2-Low.jpg

    While testing I also used my thermal imaging camera to check the temperature.. After it had been running at it's max rated capacity for about 5 minutes I took this image.

    FLIR0621.jpg

    It's showing that the two rectification diodes are about 109 Deg Celcius. Pretty hot me thinks!

    The output characterization wasn't too bad i.e. it only dropped to about 5v.

    C64 Power Supply Graph.png

    The 9 V AC are fine, but I don't dare using this power supply as it may blow up given it's age. I've ordered a 5v 2A switch mode power supply I plan to insert into this one, which hopefully will be a longer term solution. I didn't realize how hot these diodes can get, so be aware if using this pack!

    Simmi

  2. #2
    Join Date
    Nov 2017
    Location
    Australia, NSW
    Posts
    20

    Default

    Well,
    I hooked the system board up to my power supply along with the AC volt from the power pack. I then tapped into the AV and Audio out and hooked into an LCD TV I have..

    AVandSound.jpg
    Soldered to the underside of the board where the AV connector is located..

    When I powered up the board, the LCD monitor flashed that it had a PAL signal, but otherwise it was black. I checked all the voltages and they were good. I then took out my trusty thermal imaging camera and after a few minutes of being powered on, I recorded this image..
    FLIR0628.jpg
    As you can see, the hottest chip on the board is the SID chip.. So I took that out of it's socket and then voila!

    Bootup-Screen.jpg

    Looks like it's working again, at least without the SID chip! I now just need another one of those and couple of key stems i.e. two of the keys are broken off..
    Keyboard.jpg
    Once I have the key stems I can test the keyboard. I wonder if I can 3D print them, has anyone had good experience with that?

    Simmi

  3. #3

    Default

    You can also replace the 5 Volt regulator for a more modern switchmode variety like the Recom series.
    Add a 5.6 Volt TVS over the output for extra peace of mind. And maybe replace the filter cap if it is marginal.
    Further advantage is that you can remove the cooling bracket completely and the supply will run much cooler.

  4. #4

    Default

    I just did this with a 5-pin C64 - I replaced the insanely hot and having problems 7805 at VR2 with a CUI V7805-1000.

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