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PDP-11/44 Restoration

We'll try to help you make the best of whatever you end up with.
 
I now have 4 of the 0.1 ohm resistors. I couldn't get 8 of them, but I still have the other smaller power resistors too.

What should I do now?
 
Looking at your inventory, the best I come up with is to make a high power load for the 5v out of the four 0.1Ω resistors. That will be to put two 0.1Ω in series making a 0.2Ω load that will draw 25A@5V. Do the same thing with the other two 0.1Ω, for a total load of 50A@5V (250W).

To mount the resistors, I'd make up a paddle board (1/4 ply?) with machine screws through it (countersink, maybe 2 inches long?). Put nuts on the screws all the way down to the board, turning them into studs (3 in a row) at the hole spacing of the resistors. Another set of nuts to act as stand-offs. maybe 1/2 inch from the end of the screws. Mount the two resistors onto the studs. (two resistors share the center screw) Apply a final nut to the center screw.

For load wire, get some #12 or heavier stranded insulated wire. It would be nice to have multiple colors, but if not come up with a tagging method. Cut wire sections into convenient lengths and crimp lugs on the ends. You'll need to size one end to be mounted where you've been attaching your loads previously. The others will go to the remaining ends of the mounted resistor pair.
​

Repeat that entire thing a second time. You might do something clever like use both sides of the same paddle and place it in front of a cooling fan.

Hopefully, that all makes sense. (I almost lost that post to an expired token. Got it back via auto-save)


That 4 resistor array will keep each resistor well under it's power rating, and get your supply over 40%. Maybe the other resistors can play a part later on, loading other power supply sections. Should more resistors become available later, they can be added in the same manner.

Speaking of other sections - since the 15V supply seems to be the source for the reference voltage, it would be a good idea to measure it whenever the DC ON light indicates a problem. You might even do that once with the real system boards again if the opportunity presents itself.

Is this thinking clear for you? (3-4am isn't my best time)
 
I would start with your new 25A load and see where that gets you in turn of voltages; the load will be across the 5 volt bus to the backplane. That’s the two big straps. Hook it in there and see where your 5.0 and 15 volt supplies are and maybe keep the one board that’s the con interface so you can see what the DC on light is doing.
 
Thanks for the circuit ideas. So what I would want to do is wire two resistors in series, then do the same for the other two. After that I will connect them in parallel to make the 50A load. Then to make the 25A load I would just have one set of 2 resistors in series.

So a fan would be enough to cool the resistors sufficiently?
 
Big fan! What’s the wattage of the resistors that you have? I usually mount power resistors to something like a heat sink being they won’t dissipate lots of heat by themselves. Back to the idea of gutting an old UPS for parts. Too bad you’re on the other coast from me. Have plenty that I would give.
 
Yes, I think you understood me.

Each resistor would be running at ~60 watts. (140W rated, right?)

You'll need to monitor them, but I think I saw they're ceramic spool types. If so they can get quite hot. (200 degrees F should be ok) Mounted as I've outlined and exposed to forced air I expect you'll be fine for quite a while if not continuously.

But it is a concern.
 
The resistors are 140W. They are a ceramic tube, with a rather wide, flat wire that is wrapped around it. The wire is perpendicular to the surface of the tube, and is slightly wavy.

I don't want to burn my hand or the PDP. 250W is a lot of power. So I will definitely be careful not to touch them.
 
I am in an exceptionally similar situation to Old Computers. I am in Ohio as well with a PDP-11/44 in the middle of restoration. Apologies for any thread hijacking.

However: I lack a power controller (so I need to bypass the 3-pin molex stuff there), the PSU does NOT power on at all...and it sporadically makes a clicking noise that sounds unusually random and inconsistent to be a relay. I do not however see smoke, arcing, or any immediately visibly damaged components.

Backstory: all the cables in the rear were cut. (No big deal, I can rig up a TU58, console, power, and other cables easily). I got the mains cable connected and my Kill-A-Watt shows 13W with the 11/44 "on". This is far as I have managed to get. Flicking the breaker has it spike to anywhere between 20W and 80W depending on how long it's been unplugged.

I then decided to try the "jumper pins 1 and 3 or 2 and 3 or 1 and 2" to see what would happen. The result? Nothing! I then decided to measure the voltages using pin 3 as the reference. 1 (power request) and 3: -0.01VDC (it's positive before flicking breaker so it's not caused by the multimeter itself), 2 (power inhibit/emergency shutdown) and 3: 2VDC, 1VAC. This strikes me as...bizarre. Should I have AC there or is my multimeter/my my multimeter skills at fault? (I'm assuming it's USER ERROR of an old-school Fluke)

I proceeded to follow this logic: "Hmmm. Emergency Shutdown is 2V...that could mean the emergency shutdown state is asserted. I notice there's an "airflow sensor" board in the PSU. Let's remove it in case it failed somehow." No change there.

Any further ideas? The clicking confuses me...it's fairly quiet...something I'd expect from something arcing internally and not a relay. The patterm in't "click-click-click-click" like a
relay toggling...it's more random and sporadic with no reproducable cause. It seems to come from the lefthand side (with the power cord on the left).

Note: I don't have a scope and my electronics skills aren't the greatest, so I'll need a bit more of an explanation than you gave Old Computers.
 
Well after having to put this project on hiatus for a while I have decided to try to start it back up.

So pretty much right now I am going to assemble the dummy load for the PSU and see how it responds to that.

Gewtnet, good luck with the restoration of your PDP-11. Is the clicking noise coming from where the cord enters the PSU, or from the opposite side? There is actually a relay that is inside the system. It prevents an inrush current. If I remember correctly it is near where the cord enters the unit. Before doing anything else I would recommend that you remove all the cards from the chassis, and mark where they all go and what their switch settings are. That way you don't damage any of the cards if the PSU is outputting any bad voltages.
 
Old Computers said:
Well after having to put this project on hiatus for a while I have decided to try to start it back up.

So pretty much right now I am going to assemble the dummy load for the PSU and see how it responds to that.

Gewtnet, good luck with the restoration of your PDP-11. Is the clicking noise coming from where the cord enters the PSU, or from the opposite side? There is actually a relay that is inside the system. It prevents an inrush current. If I remember correctly it is near where the cord enters the unit. Before doing anything else I would recommend that you remove all the cards from the chassis, and mark where they all go and what their switch settings are. That way you don't damage any of the cards if the PSU is outputting any bad voltages.
Click to expand...

The original H7140 was totally dead. I got another one...that appears to also have issues. ;)

This new one has the bias/interface module from the old one, the relays click, transformer hums, DC OK light blinks...however: no fans! It took me awhile to figure out where to connect them (and in the process...the wires got cut and shorted together. Thankfully I had the label on the wire to cover that up *grumble*). I have since repaired the wires...but still no fans. 0VAC on the fan power rails and I don't dare put any load on until I have working fans.

I saw no signs of arcing, current didn't go beyond anything too excessive (between 40 and 90W) and it wasn't on for long...I'd be surprised to not see some sort of overcurrent protection there...so I will assume that didn't damage anything and any damage was already present.
 
If you can find some fans like I have in a picture in one of my earlier posts you should be okay. When I tried running the power supply without those it shut down and I smelled the hot electronics smell. Thankfully nothing seemed to be damaged. With enough air flow you should be okay. Are you measuring the output to the fans with your multimeter set to DC or AC? I believe that the output of that is an 11V square wave. Also when you turn it on again be sure to draw at least 6A from the 5V line. The system needs a minimum load. Several car headlights in parallel should suffice. I will take a look at my schematics and technical manual and see if there could be anything obvious that might be at fault.

Also the maximum output rating is 120A on the 5V bus so you should be fine with the current that you are drawing.

For my 11/44 I am still working on it. I have not had much time at all to work on this project lately. I have everything almost in place to turn it on again with the larger load. I am thinking that I will turn it on again with the smaller load first just to see what it will do. Right now I am trying to think of an easy way to connect it since I will be drawing 50A. I am good in terms of the wire gauge but am uncertain about connecting the wires to the outputs. Any suggestions?
 
Old Computers said:
If you can find some fans like I have in a picture in one of my earlier posts you should be okay. When I tried running the power supply without those it shut down and I smelled the hot electronics smell. Thankfully nothing seemed to be damaged. With enough air flow you should be okay. Are you measuring the output to the fans with your multimeter set to DC or AC? I believe that the output of that is an 11V square wave. Also when you turn it on again be sure to draw at least 6A from the 5V line. The system needs a minimum load. Several car headlights in parallel should suffice. I will take a look at my schematics and technical manual and see if there could be anything obvious that might be at fault.
Click to expand...

It seems that the fan signal is generated on the Bias/Interface board...the board I don't know the functionality of. Apparently the fan wires being shorted would've taken out an H-bridge...which shouldn't be too difficult

Also the maximum output rating is 120A on the 5V bus so you should be fine with the current that you are drawing.
Click to expand...

Ahhh.

For my 11/44 I am still working on it. I have not had much time at all to work on this project lately. I have everything almost in place to turn it on again with the larger load. I am thinking that I will turn it on again with the smaller load first just to see what it will do. Right now I am trying to think of an easy way to connect it since I will be drawing 50A. I am good in terms of the wire gauge but am uncertain about connecting the wires to the outputs. Any suggestions?
Click to expand...
 
It seems that the fan signal is generated on the Bias/Interface board...the board I don't know the functionality of.
Click to expand...

That is correct. The Bias and Interface board is the brains of the PSU from what I understand. It does a little bit of everything. From what I have learned it generates bias voltages to feed the other parts of the system. In addition to that and the fan voltage it also handles all of the signals such as the Power Good, Thermal Shutdown, Etc.

If you haven't found this yet here is a good collection of documents http://bitsavers.trailing-edge.com/pdf/dec/pdp11/1144/
http://bitsavers.trailing-edge.com/pdf/dec/unibus/EK-BA11A-TM-003_Aug83.pdf This manual describes the Power Supply very well including the Bias and Interface board.
 
Old Computers said:
That is correct. The Bias and Interface board is the brains of the PSU from what I understand. It does a little bit of everything. From what I have learned it generates bias voltages to feed the other parts of the system. In addition to that and the fan voltage it also handles all of the signals such as the Power Good, Thermal Shutdown, Etc.
Click to expand...

So, if I replace some transistors I identified that didn't test as expected...I might be good. ;)

[qupte]If you haven't found this yet here is a good collection of documents http://bitsavers.trailing-edge.com/pdf/dec/pdp11/1144/
http://bitsavers.trailing-edge.com/pdf/dec/unibus/EK-BA11A-TM-003_Aug83.pdf This manual describes the Power Supply very well including the Bias and Interface board.[/QUOTE]

Does that have full schematics? I have schematics...but as a bunch of multi-image .tif files.
 
Yes MP00897_11X44sys_Dec79.pdf in the 1144 directory contains the full schematics in PDF form.

Replacing the transistors might fix it. Did you try doing an out of circuit test on them as well just to be sure that the other components in the circuit might have caused a bad reading? Also you might want to look at the other parts as well in case there is a different component failing that caused the transistors to fail.

An update with my system. As I thought the DC light was still flashing. I was drawing about 6.6A according to the meter. I measured 5.15 volts from the backplane. So nothing much has changed since last time. I think I can safely conclude that it is not an intermittent problem. I am looking forward to testing it out with a bigger load.
 
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