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PDP-8/L restoration - PSU problems.

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  • Bitly
    replied
    Originally posted by MattisLind View Post
    I have now tested the idea changing the zener value in the real world (well almost - with dummy loads).

    I replaced the zener with a 1.3W 4.3V zener BZX85C4V3 and the input network for the matched pair was changed into a voltage divider consisting of a 560 ohm to the 5V and a 2.4k to ground. I tested with dummy loads loading the PSU with 2.5A, 3.3A and 5 A. I got no overshot in the output voltage when ramping up the input voltage. At 2.5A the output was at 5.17V and a 5A it was at 5.08V. The crowbar triggered at around 5.7V. No false triggering occurred.

    I am happy with this solution and will put it in use in my other PDP-8/L and BA08. Possibly even the PDP-12.
    It would be interesting to see the waveforms when powering up the actual hardware. It is possible the overshoot doesn't occur in reality due to inrush current. It's possible your change will slow the slew rate of the supply when powering up the computer, but I can't think of any reason this would be bad.You might see higher currents due to the long transition through the undefined state.

    CW

    Leave a comment:


  • m_thompson
    replied
    Good idea!

    Leave a comment:


  • MattisLind
    replied
    I have now tested the idea changing the zener value in the real world (well almost - with dummy loads).



    I replaced the zener with a 1.3W 4.3V zener BZX85C4V3 and the input network for the matched pair was changed into a voltage divider consisting of a 560 ohm to the 5V and a 2.4k to ground. I tested with dummy loads loading the PSU with 2.5A, 3.3A and 5 A. I got no overshot in the output voltage when ramping up the input voltage. At 2.5A the output was at 5.17V and a 5A it was at 5.08V. The crowbar triggered at around 5.7V. No false triggering occurred.

    I am happy with this solution and will put it in use in my other PDP-8/L and BA08. Possibly even the PDP-12.

    Leave a comment:


  • m_thompson
    replied
    The crew at UMN Duluth had a crowbar problem with H-724 power supply in their PDP-12. Reforming the output caps for about a week, and putting a significant dummy load on the power supply reduced the voltage overshoot at power on and stopped tripping the crowbar. The power supply has been working fine for the last few weeks.

    Leave a comment:


  • m_thompson
    replied
    Originally posted by DDS View Post
    Since he had already shown that he had no idea how to work a wet/dry bulb hygrometer, much less what it was for, I just took it all with a grain of salt.
    I have a sling psychrometer at home. Doesn't everyone?

    Leave a comment:


  • DDS
    replied
    I've been told that it's the thermal cycling that stresses things. Note that incandescent light bulbs usually blow when first turned on. When devices go from room temperature to powered on temperature, their component materials expand rapidly and at different rates. This stresses the points where one material is physically connected to another material and over time those connections fail. When things are powered off, they typically return to room temperature at a much slower and therefore much less stressful rate.

    You've probably heard of this lightbulb:

    https://en.wikipedia.org/wiki/Centennial_Light

    The key here is probably that it has remained powered on for most of it's working life.

    Leave a comment:


  • DDS
    replied
    Originally posted by MattisLind View Post
    I am aware of the abs max rating. My thinking is that it might be a bad idea to expose 50 year old TTL chips to voltages at 5.7V at above which is what the PSU will output momentarily at startup. Or maybe it is a quick way to weed out marginal chips?
    Don't laugh!

    I was once told by our HVAC SME (Subject Matter Expert) that our buildings were allowed to heat up to 90F at night to expose marginal circuit packs.

    Since he had already shown that he had no idea how to work a wet/dry bulb hygrometer, much less what it was for, I just took it all with a grain of salt.

    Leave a comment:


  • anders_bzn
    replied
    Originally posted by Al Kossow View Post
    very strange. not a DEC PCB part number and almost no US manufactured components.
    The machine does not have the regular text on the front panel, it's says "READING, ENGLAND" instead of "MAYNARD, MASSACHUSETTS". Might explain something.



    More on the restoration: http://www.pdp-9.net/pdp-8-l

    Leave a comment:


  • Qbus
    replied
    Question: what’s more stressful? Operating vintage electronics at lower then originally rated voltages, at rated voltage or slightly higher voltages? On very old radios with tubes I tend to run them around 10% below the original line voltage keeping in mind that the modern line voltage is slightly higher then what was around at the time of their manufacture and want to try to extend tube life. Others run sets at way reduced voltages to keep from stressing the sets. But a vintage computer is something from maybe the late sixties or early seventies when higher line voltages were common so I run all the vintage computer stuff at regular line voltage with the idea of reduced voltage may produce more stress on power supplies then the design voltage. Something about reduced voltage will require more current to accomplish the same power and its current that’s where the real work occurs in both power supplies and distribution. It may be antidotal but I have noticed that most damage occurs when turning on systems and would speculate that perhaps this occurs because of reduced voltage and increased current for a short period of time and perhaps that’s what pushes devices into failure? Or maybe it’s just the insulation of the gates that fail due to the application of voltage at startup? The real question is what pushes gates into failure upon startup?
    Attached is a picture of my 1938 Navy HRO receiver, at almost eighty years old it’s still working great. My two PDP-11 systems are half that age and would like to think that they may be around just as long.


    38hro.jpg

    Leave a comment:


  • Chuck(G)
    replied
    I recall that a friend who worked as a DEC CE used to grumble about "transistor-protected fuses".

    Leave a comment:


  • MattisLind
    replied
    Originally posted by dave_m View Post
    The absolute max supply voltage for the Vcc bus on TTL is 7.0 Volts. It is recommended that it operated between 4.75 and 5.25 Volts.
    I am aware of the abs max rating. My thinking is that it might be a bad idea to expose 50 year old TTL chips to voltages at 5.7V at above which is what the PSU will output momentarily at startup. Or maybe it is a quick way to weed out marginal chips?

    Leave a comment:


  • Al Kossow
    replied
    Originally posted by anders_bzn View Post
    I noticed that there are at least one more version of the regulator board. I have reverse engineered the one in my 8/L #905
    very strange. not a DEC PCB part number and almost no US manufactured components.

    Leave a comment:


  • MattisLind
    replied
    Originally posted by Qbus View Post
    Maybe it’s just my paranoia but I have had nothing but misery with SCR being every time they somehow get the gate turned on it manages to short out the device if there is nothing on the cathode. Like a three legged self-destruction device!
    I happily replace SCRs every time the crowbar trips if it saves tens or hundreds of TTLs or other components.

    Leave a comment:


  • dave_m
    replied
    The absolute max supply voltage for the Vcc bus on TTL is 7.0 Volts. It is recommended that it operated between 4.75 and 5.25 Volts.

    Leave a comment:


  • anders_bzn
    replied
    I noticed that there are at least one more version of the regulator board. I have reverse engineered the one in my 8/L #905 schematics here and pictures in a zip file.

    I wander if the crowbar is designed for tripping at 6,3V to 6,5V to avoid tripping if the 5V rail reaches 5,7V at startup. Looking at the schematics it looks right that is should trip at this voltage.

    Leave a comment:

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