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Converting CBM 8032 PET to switching power supplies

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    Converting CBM 8032 PET to switching power supplies

    Picked up a 8032 with bad transformer a while back, my son posted about it here.

    Primary winding is open circuit, didn't see a thermal fuse anywhere. Bottom line is fixing the transformer seems like a non-starter, exact replacements are unobtainium and likely failure prone during the shipping process (old and heavy), and individual transformers of sufficient amp ratings are expensive and I'm not sure I can actually fit them all given the all the different voltages needed here at sufficient amperage.

    So, as much as it pains me to hack this up, I'm thinking of going with an +18V AC-DC SMPS to feed the monitor, and then some DC-DC buck converters (probably xl4016 based) downstream to get +12V and +5V, and finally a LM2662 based negative converter to get the -5V for the DRAM. Total cost via my favorite Chinese vendors is around $20. Judging by the max current delivery capacity of the existing linear regs, I'm guessing a 18V 10A supply, two 8A bucks for +12V and +5V, and finally 200ma for -5V should easily do the job, even factoring in Chinese amp ratings

    Will need to pull the linear regulators, some of the existing buffer/storage caps (to prevent inrush surge currents at power up), and the rectifier bridges so I can reuse the transformer connector block to make things look somewhat tidy.

    Thoughts? I feel dirty already, but a working computer in this case seems better than one just collecting dust.

    Did you look into this option?
    Be polite and I may let you live.


      If you're going to bypass the regulators anyway why not just use an AT supply with a boost converter for the monitor?

      On the other hand, why not just leave the PET board as is and set your boost/buck adapters to deliver the original voltages (+9, -9 and +16) to the original connector and ~21V to the monitor? DC voltages will just pass through the bridges, albeit with a little voltage drop.



        Originally posted by MikeS View Post

        On the other hand, why not just leave the PET board as is and set your boost/buck adapters to deliver the original voltages (+9, -9 and +16) to the original connector and ~21V to the monitor? DC voltages will just pass through the bridges, albeit with a little voltage drop.

        Mike's solution also has the benefit of producing the +9V required for the cassette motors. See Schematic sheet 3.


          Originally posted by dave_m View Post
          Mike's solution also has the benefit of producing the +9V required for the cassette motors. See Schematic sheet 3.
          Good point on the +9VDC and the tape drive, was curious why that was sent along on the power schematic,

          I thought about leaving all the linear regs in initially, but sort of felt wrong heating them all up with nice regulated DC input. That being said, would be a lot easier and keeps things more "original". Maybe I'll give that a try first, pretty simple to wire up. I'm just always paranoid about killing the triple voltage DRAM of this era. Seen lots of lower memory issues on Spectrum ZX's when the +12 and -5V regs go bad and only +5V is left powering them.

          KC9UDX, regarding using individual transformers again, getting the three of them (21VAC, 9-0-9 18VAC center tap, and 16VAC) at adequate amperage (there's over 5A's of potential +5VDC regulator on my board between the main board and the 64K RAM expansion card), taking into account the Hammond tech notes and the massive smooting caps (especially on +9VAC), just didn't seem cost effective looking in either the Hammond or Triad Magnetics options from places like Mouser or Newark.

          The original transformer is/was a beast.


            It's alive!

            Ghetto rigged it all up.


            +21V DC feeding the monitor through the rectifier bridge and rest of the buck converters.

            +16V DC feeding the +12V regulator through the rectifier bridge.

            +9V DC feeding the +5V regulator directly, bypassed the rectifier bridge since there's an exposed pin on the power connector for it. No point in heating some diodes.

            -9V DC feeding the -5V regulator through the rectifier bridge.

            Came right up, drawing 1.7A on my lab power supply.


            All the board voltages look good, ran fine for about 30 minutes while my son got the disk drives working. Now to order a 24V supply that will fit in the case and 3D print some mounting brackets for this mess and new input switch (with fuse) on the back.


              Good work!


                Very nice. I'd like to see the results when it's all permanently installed and perhaps a list of components.
                WANTED: CBM-II hardware or software, PET software


                  Finally got around to finishing this project, details here



                    Originally posted by hideehoo View Post
                    Finally got around to finishing this project, details here


                    Very nice! Thanks for the info! Would you be able to share the 3d bracket files?
                    WANTED: CBM-II hardware or software, PET software


                      Thingiverse link is in the blog post, but here it is again.


                      These were quick a dirty designs my son did. They work fine, but the Mean Well PSU mount could use a little more meat around the mounting holes and extra standoff's on the two corners to better support the PSU. I'll try to dig up the AutoDesk files and upload them also if anyone wants to modify them in the future.