Image Map Image Map
Results 1 to 9 of 9

Thread: PSU: pdp11/10; 5v on regulator board (not the H744) damaged

  1. #1

    Default PSU: pdp11/10; 5v on regulator board (not the H744) damaged

    This idiot thinks he shorted the Anode (red lines in the circuit diagram in the link) to ground.
    Dont ask - scope lead and careless and and..
    Anyway what was a nice pulse (but with a bit of ring) at that point has now flat-lined.

    I have some skill with an iron, but little skill in understanding analogue power-supplies. Can anyone suggest what I am most likely to have blown - so I get an order of what I should attack first!

    see http://www.scotnet.co.uk/iain/DEC/psu5v1.jpg for the circuit in question.

    All input appreciated - except the one that says "dont put a scope lead there"

  2. #2
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    1,981

    Default

    Been there, done that, got the tee shirt! Although this was 30 years or so ago - and I have never done it again...

    I would have thought Q6 was the most likely candidate to have died - but can you provide me with exact details of the module you have please. I don't seem to be able to find a PDP11/10 on bitsavers. The more information you can give us, the more help we can be.

    Dave
    Last edited by daver2; September 3rd, 2018 at 09:40 AM.

  3. #3
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    1,981

    Default

    I think I have found the full schematic at http://bitsavers.trailing-edge.com/p...ings_Apr74.pdf.

    First thing, is fuse F1 (15A) intact?

    If so, do you get approximately 39V d.c. across C3 when measured with a multimeter?

    Another possibility (other than Q6 being duff) is that the thyristor Q11 is inadvertantly operating - thus crow-baring the supply.

    Let's take it in steps. First the fuse, then the 39V d.c. measurement.

    Dave

  4. #4

    Default

    I will recheck, the fuses were the first things I checked and I believe they were all A OK. I have not taken the crowbar off. I am not in circuit so I can footer like that.

    The board is an H740 (dec drawing 5409728-0-1) I have my own engineering drawing and (as you might expect) they are for a PDP11/05. I have not found identical ones on bitsavers. The PSU they have feeds a negative voltage (diodes reversed) and uses FETs instead of darlington pairs in place of Q13 and Q14.
    OK I read more carefully - there are two circuits in that link. The 2nd one has the FET and the "upside down" diodes. Mine is the board above that. (I am rev T the one on bitsavers is rev R). Superficially they seem he same.

    Near the end of http://www.bitsavers.org/pdf/dec/pdp...ings_Jul76.pdf is a slightly newer version. My paper seems to be about 12/74. The PSU is the long-thin one that runs vertically backwards, not the PSU that is bolted on the rear.

    My kit is a 115v run off a variac at present - I used that to try to bring the capacitors back. Success? I dont know but all the cap I have tested in-circuit were within spec.

    This is not the first time I have screwed up a board ;-( When I stuck the probe in I said to myself "take care of that probe, the earth ring is exposed. There was no bang - been there done that - so a fuse is likely rather that a smoke emitting diode!

    I will go measure again. Thanks for the heads up. I have a Q6 on order already. I can unsolder the beast and see if it seems to transistor OK.

  5. #5

    Default

    OK, you were right. Fuse F1 blown. Measured it twice (late at night). Your input appreciated.
    I guess I need some 15 amp picos, and some more care and attention

  6. #6
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    28,268
    Blog Entries
    20

    Default

    A friend who worked as a CE for Digital many years ago used to joke about DEC's "transistor-protected fuses". That might be something to keep in mind...

  7. #7
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    1,981

    Default

    Chuck beat me to that joke...

    The 'late at night' is the clue. Go to bed and try again when you are fresh. It's always the safest way.

    From a human factors point of view - telling yourself 'not' to do something whilst you are doing it is exactly what you will do! My way of working would be to attach the measuring leads etc. to the power supply whilst it is switched off - and then turn the beast on. You can then take the measurement and turn the unit off again.

    If you replace the fuse - and there is a transistor fault - you will only 'blow' it again...

    I would be inclined to use a variable voltage power supply with a current limit and try to bring the +5V supply back to life again with the fuse actually removed. Use the current limit on the dc power supply to prevent any further damage, and you can bring the voltage up slowly whilst monitoring the output voltage itself. That's the way I would proceed at any rate (assuming you have a suitable variable voltage power supply with in-built current limit of course).

    I am restoring my 11/45 power supplies using the same technique - removing the multitude of voltage regulators from the ******* big transformer and feeding them from my variable voltage, current limited supply. This method seems to be working well for me. I remove the large capacitors and reform them externally. I am, however, looking for a fairly cheap variac to bring the whole lot up when I have finished refurbishing the regulators themselves. I have my eye on a suitable candidate at work (!) when they decide to sell off the contents of the lab when the lab technician retires next month...

    When you have the regulator running successfully, you can refit the pico fuse.

    Dave
    Last edited by daver2; September 3rd, 2018 at 11:11 AM.

  8. #8

    Default

    A lot of words of wisdom - but sometimes late at night is all I have - it is relaxation!
    Thanks.

  9. #9
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    1,981

    Default

    Yep, know how you feel !

    Dave

Tags for this Thread

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
  •