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Thread: PDP-11/44 Restoration (part 2)

  1. #11
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    For the Q-Bus "The maximum allowable current per pin is 1.5 A" according to http://bitsavers.informatik.uni-stut...ndbook1979.pdf.

    I cannot find the Unibus specification but as the basic electrical/mechanical interface is the same, I would not exceed this limit.

    Jerry

  2. #12

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    Thank you for that piece of information.

    Do you all think it would be safe to just not load the other voltages if the mail 5V rail is the only one that needs a minimum current load? I don't want to burn out the backplane with too much current. I was thinking about buying an extender card or two so I could easily connect wires for the voltmeter.

    I have gotten some parts to make the dummy load for the +5v rail. I will target it to draw about 24~25A. It has two 0.1 ohm 140W resistors. So each individual resistor would dissipate about 65W of energy. There are two brackets with screw holes open on the 5V rail on the PSU. I am going to try to connect the wires to those using a screw. Once I finish building it I will let you guys know.

  3. #13
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    Quote Originally Posted by Old Computers View Post
    Thank you for that piece of information.

    Do you all think it would be safe to just not load the other voltages if the mail 5V rail is the only one that needs a minimum current load? I don't want to burn out the backplane with too much current. I was thinking about buying an extender card or two so I could easily connect wires for the voltmeter.

    I have gotten some parts to make the dummy load for the +5v rail. I will target it to draw about 24~25A. It has two 0.1 ohm 140W resistors. So each individual resistor would dissipate about 65W of energy. There are two brackets with screw holes open on the 5V rail on the PSU. I am going to try to connect the wires to those using a screw. Once I finish building it I will let you guys know.
    According to EK-BA11A-TM-003_Aug83 the H7140 has a minimum draw of 6A on the +5.1 V supply and 0.0 on the rest. So you could reduce the load a bit, say 10~12A if you have the parts. Bolt the load to the PSU with short lengths of 12 gauge wire.

    An extender card to aid measurement is a good method. Do not use it for the load device.

    Heed the cautions on Page 2-9 in the manual. No rings, watches or contact with HV!

  4. #14

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    I will not use the extender card for the load device. I initially thought that I might do that, but it seems kind of risky. I wish there was an easier way to access the other voltages.

    I will go ahead and order an extender card. I might even buy one without connectors so I could solder the test leads on to it directly and test it from there.

    I actually went ahead and got 10 gauge wire just to be on the safe side. I could also add another resistor I have to make it draw 12.5A. In the first round of troubleshooting I did, one of the people giving me advice were suggesting that I draw closer to 50A. From my understanding that amount was to more closely approximate a fully populated system.

    As for the warnings on the PSU, I will definitely heed them. When I obtained the machine I saw that warning and it made me uneasy working on the power supply at first. I am glad this PSU has all the fail-safes built in, but at the same time it seems to be way too complex. Too bad it's not a simple linear PSU.

  5. #15
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    A linear version would weigh about 5 times a switcher.

    This is definitely a complex Switching PSU. A mid 1970's design, it had to deal with overvoltage shutdown, current overload monitoring, temperature monitoring, battery backup for memory, and control sequencing. It was built with discrete parts, op-amps, voltage regulators, TTL and 555 ic's. Hopefully the problems are minor, like faulty electrolytic capacitors. You should still be able to source replacement parts or equivalents.

    Compare this to preceding generation, in which "transistors where used as fuses" in many of comments on the web!

  6. #16

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    Oh wow. That would be really heavy then! That thing already weighs about 50 pounds as it is. I have heard about the transistors as fuses comments. That is pretty interesting.

    I am really hoping that the problem is something simple too. I am going to get the extender board ordered soon. I am in the process of building the dummy load and will give an update once it's ready.

  7. #17
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    Here's the dummy load I made for the H777 PSUs in my 11/04 & 11/34, might give you some food for thought:-
    http://www.vcfed.org/forum/showthrea...PSU&highlight=

    No messing about with the backplane, it just plugs straight into the PSU.
    You can still get the mate-n-lock connectors from DigiKey (brand: "TE Connectivity AMP Connectors")

  8. #18

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    That's definitely a good idea for the dummy load. I will keep that in mind for building the dummy load for the other voltages. Thank you!

    I did manage to get the dummy load built for the +5V rail. I am still just trying to think of a good way to mount it on a board so it won't burn anything. I's about 0.2 ohms, and I calculated it will draw in the ballpark of 25 amps. Each resistor will dissipate about 64 watts of power (that's just above half of each resistor's rated power). Since the resistors aren't going to be dissipating their fully rated power, does that mean I probably won't have to worry about a time length for running those, or would it still be good to not run them for too long when testing the power supply?

  9. #19
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    I prefer to run resistors at no more than half their power rating, because they still get hot. But with the oil, it can obviously go higher.
    At half the rating, I suppose they *should* be able to go continuously (with adequate convection cooling conditions). Add forced air cooling I would expect continuous operation to be no problem.
    Then again, 15 minutes should be enough for most testing.

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