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Thread: PDP-11/24 in BA11-A with H7140 PSU

  1. #1
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    Default PDP-11/24 in BA11-A with H7140 PSU

    I collected a PDP-11/24 two weeks ago today. It is in a BA11-A with H7140 PSU. Cards installed are M7133, M7134 and M8743 MS11-PB. It was in very clean condition with some signs of corrosion. I had this weekend and last Saturday to start this project. I cleaned the inside, backplane, PCB's PSU etc. Need a solution to remove the white corrosion spots on the BA11 case.

    A.jpgB.jpgC.jpgD.jpg20201008_083656%20ed%20(Small).jpg20201008_084935%20(Small).jpg

    M7133 CPU has the following installed MMU, DAT/CTL and FPP, Both SLU's configured for 9600 Baud.
    M7134 Unibus Map has 767A9 (UDA50), 751A9 (RL01/RL02) and the CPU diag ROM installed. voltage monitoring switch pack E6 is set correctly BA11-A with only S3 ON. Oddly S3 was OFF and 1,2 and 4 where ON when I received it.

    After inspecting H7140 PSU for bulging caps, burnt tracks and any other obvious to the eye issues I powered up with no circuit cards installed. DC OK LED is illuminated and two of the three fans in the fan tray run. The fans are 35V. I cleaned the faulty fan and the bearings appear free enough but this fan still doesn't start even with a "nudge". Faulty fan requires further investigation, 35V fan is going to be like finding hen's teeth so may have to repair this one somehow.

    E.jpgF.jpgG.jpg

    PSU voltages

    Measured with Multimeter and checked with Oscilloscope

    -14.95V
    +14.95V
    +11.97V
    -11.98V
    +5.1V
    LTC 1.4v with a 50HZ square wave

    I removed the spare DD11-DK backplane (probably won't use this) and moved the M9302 terminator to slot 9 of the 11/24 backplane I also installed G7273 is slot 7, 8 and 9 (NPG grant not needed in slot 9 but I didn't have another G727) and G727 grant cards in slot 4,5, and 6.

    Inserted the 3 PCBS and powered on. DC OK on front panel illuminated, RED CLK LED on CPU is illuminated and Green battery power indicator is illuminated (not sure if this LED should be on, I don't have any battery connected) on MS11-PB.

    I connected a terminal and with system set to HALT powered it on and I get @ on terminal and a random series of numbers. when I press the space key the RUN LED on front panel flashes so I think the console SLU is working ok. If I type 0/ CLK LED goes out and I have to power cycle the box. looks like I can't address memory?

    Next time I powered it up the DC OK LED is flashing and green LED on MS11-PB is off with red Uncorrectable error LED now illuminated. Powered off reseated PCB's checked for any new obvious burn or damage. Nothing obvious.

    Checked voltages and +12V now at 2.86V (crowbarred?). White wire to P4. After lots of checks this is how it now powers up all the time.

    I have the drawings for the H7140 PSU but it doesn't have any test points from what I can see apart from 300V DC and at the power connectors themselves. The H7140 is a motherboard and daughter card configuration but I don't have any extenders or connectors to make any so I think I'm going to struggle troubleshooting the PSU issue. I suspect one of two PSU cards will be faulty either memory regulator board or memory inverter board. From the drawings it looks like 12V is generated here. Anybody repaired a H7140 successfully before and have any advice or pointers? I found one post on internet but for an 11/44 and his issue turned out to be the "ribbon" power cable used on the 11/44.
    Last edited by PG31; November 15th, 2020 at 08:17 AM.

  2. #2
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    Typo in my post " voltage monitoring switch pack E6 is set correctly BA11-A with only S3 ON. Oddly S3 was OFF and 1,2 and 4 where ON when I received it" should read as E6 was correct when I received it 1 on, 2 on, 3 off, 4 on.

  3. #3
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    Quote Originally Posted by PG31 View Post
    Typo in my post " voltage monitoring switch pack E6 is set correctly BA11-A with only S3 ON. Oddly S3 was OFF and 1,2 and 4 where ON when I received it" should read as E6 was correct when I received it 1 on, 2 on, 3 off, 4 on.
    I am totally confused. Was it correct, or wrong?

    Also, be aware that the nomenclature here in DEC documentation can be confusing. Sometimes you have "ON" and "OFF", at other times "OPEN" and "CLOSED". And what is equivalent to what is not always so clear.

    But apart from that, the PS most likely need some minimum load, which you might not have right now. But you could also have a cap that just don't work to spec. I am not good at power supplies. Especially not switched ones. Too much black, analog magic for my taste.

  4. #4
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    It was correct, set for BA11-A. Only SW3 OFF. Couldn't read my own notes

  5. #5
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    Don’t run the power supplies without a load, bad things can happen! Both the 12 volt and 5-volt bus have huge crowbars that protect the cards in your system. Capacitors will tend to decrease capacity over the years and the power supplies without a load will look good with only needing to produce very short duration switching pulses but under load they cannot produce the wide pulses necessary to put out the current required. The HV supply can suffer the same issue and look good under a minimal load but die at a normal service level and last but not least beware that a lot of the later power supplies used HOT primary sides so if you don’t understand working on that sort of non-transformer isolated power supply read the manual and remember that both sides of the primary power supply can be at line potential!

  6. #6
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    I had a few hours to spend troubleshooting this weekend. +12V is at 2.86V . This is the white wire to P4 in the photo. This is under load with 3 hex height PCB's (CPU, Unibus map and memory) grant cards and Unibus terminator. I pulled each board one at a time to make sure one of them wasn't pulling the +12V line down. Scanned over the memory regulator and memory inverter boards in the PSU for bulging Caps and lifting one leg of the large diode's for shorted/open diodes. None to be seen. Not even sure I'm looking in the correct place. Checked large electrolytic's on these boards with capacitance meter all are either in or above spec.

    E_LIa.jpg

    Does anyone have a running 11/24 in a BA11-A that can check the voltage on P4 white wire to double check it should be +12V? (pretty sure that it should, as I made a note of it but I've already found 1 error in my notes ) Also I'd like to compare my switch setting on the CPU, Unibus map and MS11-PB memory boards.

  7. #7
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    Quote Originally Posted by PG31 View Post
    I had a few hours to spend troubleshooting this weekend. +12V is at 2.86V . This is the white wire to P4 in the photo. This is under load with 3 hex height PCB's (CPU, Unibus map and memory) grant cards and Unibus terminator. I pulled each board one at a time to make sure one of them wasn't pulling the +12V line down. Scanned over the memory regulator and memory inverter boards in the PSU for bulging Caps and lifting one leg of the large diode's for shorted/open diodes. None to be seen. Not even sure I'm looking in the correct place. Checked large electrolytic's on these boards with capacitance meter all are either in or above spec.

    E_LIa.jpg

    Does anyone have a running 11/24 in a BA11-A that can check the voltage on P4 white wire to double check it should be +12V? (pretty sure that it should, as I made a note of it but I've already found 1 error in my notes ) Also I'd like to compare my switch setting on the CPU, Unibus map and MS11-PB memory boards.
    CPU, Unibus map and memory... I'm not sure any of them put any load on +12V at all, so I would wonder if the +12V is completely unloaded here. DEC had special cards to be used if you didn't have any normal load on the power...

  8. #8
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    Thanks bqt, it is odd though that the chasis DC OK LED was solid and the green "battery power" LED on MS11 was on when I started and then after power cycling several times I now have DC OK LED flashing and no green LED on the MS11 now. I don't currently have any other Unibus boards to plug in and increase the load.

  9. #9
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    Gave the none working fan a good clean out today and now all three fans are running. One thing I did notice is that when it attempts to POST/Self test the characters all run down the left side of the screen until the final output as if it was sending a <cr> after each character. I think this may be something in the terminal settings <cr> <lf> etc. I've tried different setting (VT100, VT420, VT520 and different "handshaking" etc)but it always appears the same. Strange

    Power On.jpg

  10. #10
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    Quote Originally Posted by PG31 View Post
    Gave the none working fan a good clean out today and now all three fans are running. One thing I did notice is that when it attempts to POST/Self test the characters all run down the left side of the screen until the final output as if it was sending a <cr> after each character. I think this may be something in the terminal settings <cr> <lf> etc. I've tried different setting (VT100, VT420, VT520 and different "handshaking" etc)but it always appears the same. Strange

    Power On.jpg
    You definitely have something wrong in your serial settings. Either speed, parity, stopbits or combinations of all of them. If you look, the characters printed are not any characters the machine should be printing, and most of them have the high bit set, which is not correct.

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