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

PG31

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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:

PG31

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Sefton, Meresyside, England
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.
 

bqt

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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.

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. :)
 

Qbus

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Feb 23, 2011
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Salisbury Maryland
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!
 

PG31

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Messages
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Location
Sefton, Meresyside, England
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 :confused:) Also I'd like to compare my switch setting on the CPU, Unibus map and MS11-PB memory boards.
 

bqt

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Sep 15, 2019
Messages
506
Location
Zurich, CH
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.

View attachment 64826

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 :confused:) 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...
 

PG31

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Sefton, Meresyside, England
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.
 

PG31

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Sefton, Meresyside, England
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:confused:

Power On.jpg
 

bqt

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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:confused:

View attachment 64851

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.
 

PG31

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494
Location
Sefton, Meresyside, England
So after Christmas and a few months away from this restoration I decided to take another look. I built one of Jorg Hoppe's G7273 diagnostic cards with the small volt meters. I also bought an Emulex CS11 controller so that I had at least one controller in the Unibus slots. The G7273 card showed voltage as 4.92V (5V), +15.09V and -15.04V, the +5V was a little low. Looking at the circuit diagram for the H7140 I couldn't see any adjustment. When I returned to the 11/24 the fans where still running but I now have no voltages displayed on the G7273 and the DC OK LED is no longer flashing. So I'm now going down hill with the PSU and not sure where to start. Part of the PSU is working because the fans are running and they run off +35V I think? I've examined the PSU cards physically and can see no burnt components. This for me is a really tricky PSU and I can't find any info about a successful repair. I did find another similar post on here from several years ago so I'm going to post on that also to see if the original submitter had any success.
 

PG31

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I've replaced the electrolytic capacitors on the Bias and Interface PCB and for good measure and considering it only had two electrolytic capacitors on the Air Flow Sensor PCB I have replaced them also. I tested the removed capacitors and all of them seemed to have a correct reading with my cap meter. I still only have the fans running with no other voltages present :(. No idea where to troubleshoot the PSU from here.
Recapped Bias and Inerface PCB.jpg
 

PG31

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May 31, 2011
Messages
494
Location
Sefton, Meresyside, England
I've spent several weekends now trying to troubleshoot this PSU with no joy, I will have top ut the 11/24 on the "to do later" shelf or sell/swap to someone with the knowledge to fix it. :-(
 
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