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Thread: DEC power supply 725 going hot in TU56H

  1. #11


    Quote Originally Posted by m_thompson View Post
    I wrote some simple diagnostics to help debug a TD8-E.
    Thanks for the mail!

  2. #12


    Quote Originally Posted by Al Kossow View Post
    A simple interface isn't difficult to make to read tapes. I built one about 10 years ago that just connected to a PCI parallel port card. You clock the mark and data tracks with the clock track.
    I just uploaded the dec and linc tape decoder code to
    This was written to interface to a PCI parallel port card in a Mac G3 running OS9, so there is a lot of code just for dealing with finding the card on the PCI bus.
    The main thing of interest are the guts of decoding the DECtape frames.
    Thanks, I'll look into that. My experience so far is that it's more difficult than you expect to copy the analog design and get it to behave as the original G888 module.

  3. #13
    Join Date
    Sep 2006
    Silicon Valley


    Quote Originally Posted by m_thompson View Post

    I asked because the PDP-12 tape controller inverts the clock signal when it is reading DECtapes.
    The TU56 has both polarities of clock signal available.
    Did you spin the LINCtapes in their normal direction, opposite of a DECtape?
    It has to be read in the reverse direction because LINCtape encoding isn't bidirectional.

  4. #14


    Quote Originally Posted by Al Kossow View Post
    No. One of your motor driver transistors is probably shorted.
    No it's not shorted. Because I'm testing the PSU standalone, without the drive connected. But the answer might be correct anyway.

    It's the marked resistor (R2) that gets hot. In the schematics it's specified to be "Fixed res. 25W 100Ohm". Since it's sitting in parallel with the bulk cap there will be 38V over it.

    P=U^2/R gives P=38^2/100 -> P=14.4W

    I'll guess that that will generate som heat at least.

  5. #15
    Join Date
    Feb 2011
    Salisbury Maryland


    If you are checking the power supply removed from the load R1 and R2 will be equal temperature. Do you have your transformer strapped for 240 Volts?

  6. #16


    R1 gets less warmer. It's on the 18V rail, power dissipation is just less than 4 Watts. The transformer is strapped for 117V, and I do have a 117V power outlet at my work bench. Makes life much easier when restoring old DEC machines.

  7. #17


    I got the drive in working condition, running nicely with a TD8E board in my PDP-8A. But I have used an external power supply until now. Now I have modified the power supply a bit, I put an external 50W resistor on a heat sink outside the drive (non destructive modification, it's possible to rebuild the drive in original condition). But when I measure the 38V rail I get about 43V! The 18V rail is just right!

    Is there anyone with a TU56 drive that can measure those two rails so I have something to compare with?

    I'm also looking for better documentation on the H725 power supply, in the TU56 maintenance manual there isn't much. But under the electrical specifications the supply voltages are 115/220V AC and +5V or +10V and -5V. On other places in the document it says 110V and I have also seen 117V.


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