Image Map Image Map
Results 1 to 5 of 5

Thread: Converting CBM 8032 PET to switching power supplies

  1. #1

    Unhappy Converting CBM 8032 PET to switching power supplies

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

    http://www.vcfed.org/forum/showthrea...highlight=8032

    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.

  2. #2
    Join Date
    Jan 2014
    Location
    Freedom City
    Posts
    6,070
    Blog Entries
    1

  3. #3
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    6,962

    Default

    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.

    m

  4. #4
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,495

    Default

    Quote 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.

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

  5. #5

    Default

    Quote 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, http://www.zimmers.net/anonftp/pub/c...8032029-11.gif

    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.

    http://www.hammondmfg.com/pdf/5c007.pdf

    The original transformer is/was a beast.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •