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PDP-11/44 Restoration

Old Computers said:
I have decided to explore one of the suggestions earlier in this thread. Tradde said that the power distribution box introduced some noise into his PDP-8 power supply. Would this be a good thing to check on my computer?
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I said it did with the one I had. It can't hurt to check to see if yours is causing a problem. If you have a scope you can easily check it's outputs. I compared
plugging my 8i into it and then without. It was amazing the difference. I never did find out what was causing the noise. I still have the power controller too.
 
I figured that it would not hurt to check on it. Right now I am stuck because I don't exactly know what to do, and I figured that I could do some checks that I can perform.

Is there any safety concerns or ways to connect the scope probe to the outlet when I check for the noise?
 
Sorry to bring back an old thread, but I want to start this project back up again. I have not been able to test the noise coming out of the power distribution unit mainly because I have conflicting information regarding whether I can test the power directly or indirectly with my 'scope. Any advice for testing this?

Besides that I am at a standstill with diagnosing the PSU. Before this project was put on hiatus I had managed to get the PSU to start working again by replacing 2 bad capacitors on the bias and interface board. With a proper load (it might be slightly less than the requirements) it outputs very close to the proper voltage on the main output, but after a minute or two it shuts itself off. I wait a minute or two and it will turn back on just fine. Since then I have not turned it on to preserve the components in case something were to go bad and take out a very hard to find part.

Do you guys have any advice for diagnosing/repairing this problem?

Thanks
 
I think that I might have discovered a cause for the shutdowns of the power supply.

I was reading the theory of operation section of the BA11A mounting and power manual. In section 3.2.5, it mentions that the logic inverter control module gets a +5v remote sense signal from the +5v at the backpane. I was also studying Fig 3-9 on page 3-21. It appears that the remote sense signal is also sent to a block called crowbar gen. I know that the crowbar circuit is used to stop over-voltage conditions. Could the lack of this remote sense signal cause the crowbar to activate?

It also appears that the lack of the signal could cause the output of the pulse width amplifier to not have a good wave form or change rapidly. The spikes in the oscilloscope trace that I posted earlier could be caused by this as well.

It also appears that the DC to AC conversion sends a current sense signal to a block that turns off the power. See the diagram to see how those interconnect to the pulse width amplifier.

The manual gives a much better explanation of the circuit than I can give.
What do you guys think of these ideas?
 
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Does anybody have any insight on my previous post? I want to try connecting the PSU to the backpane, but I am afraid of damaging something.
 
Man you have been working on this thing forever! By now you have way too much understanding of the control, start up and operation of vintage Dec switching supplies. My limited experience with this sort of thing would leave me to believe that you fixed the problem with the bad capacitors you have already changed and now the control circuits are giving you a fit because you are missing a return signal. But that’s just pure speculation. If you have +5, +15 and -15 or there about I would be for putting everything back together and trying it in the system. Power the system back up with the volt meter connected across the +5 volt bus and look at that first, if it’s between 4.8 and 5.2 then check the +15 and -15 Once again this is all antidotal but my experience with switching supplies and I have been doing this for a while is that the most common failures are filter capacitors go open or short, the switching transistors, the huge ones short or the diodes fail. In a number of years have never had issues with lack of regulation or any of the control or shut down systems as long as all sensing lines are connected. So I know other will say I am crazy but I am for put it back together and light it up.
 
Thanks for the advice. I have been thinking about just trying it out connected to the system to see what would happen because the more I thought about it the more it seems like that remote sense signal missing could cause problems.

I am guessing that I should probably remove all the other cards except for the console interface module to be safe. Would it be a good idea to get a card extender so I can measure the voltages off of the backpane?

I wouldn't say I'm an expert on DEC PSU's a lot of the theory behind how it works still escapes me. In my opinion it seems like they made it overly complicated.
 
I'm sorry I haven't been keeping up with this thread. It's quite long now, and skimming through it's 20+ pages I'm sure I'm missing a lot.

For what it's worth - You've followed the debugging method I'd have used myself for the most part. Light bulbs as dummy loads, scrutinizing apparently stressed components, and test-bedding as a troubleshooting platform.

From my quick read of the PSU TM and schematics, this supply is one of DEC's "Works of Art". Troubleshooting it, at some point requires all the feedback points to be hooked up in a meaningful way, and a mechanical environment [airflow and such] that simulates fully it's operating conditions.

You've come quite far even getting it running for more than a few seconds. You have gotten to minutes. I could easily agree that it's time to install it and see. However my experience with these types of Switched-Mode supplies (with so much monitoring and safeties built in) is that until it works on the bench - it's a risk to just put it together and light it up. Then again, that's what they did with the first Saturn V - and we rode it to the moon 3x later.

Mind you it may be fine, and I am undoubtedly being overcautious. I suppose it's the pessimist in me.

To answer your specific questions of late:

I know that the crowbar circuit is used to stop over-voltage conditions. Could the lack of this remote sense signal cause the crowbar to activate?
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My opinion is yes, an unterminated sense lead could fire the crowbar. Not having read every post in your thread you may have already addressed this.

It also appears that the DC to AC conversion sends a current sense signal to a block that turns off the power.
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My reaction is that any such protective command - to shut down, can be tested to see if in the final moments it's occurring.

Even the crowbar acting, has to leave an over current sensor elsewhere that asserts the shutdown. (although in your case - you won't know the current is an accidentally triggered crowbar unless you have a current meter on it's leads -[even a "clamp on" ammeter might show this])

If you think the shutdown might be thermally induced - make certain the supply is in airflow conditions like those of the operating unit. Such a shutdown also has a hot component somewhere. I use an IR thermometer to find these. It's inexpensive enough a tool and has proven invaluable over the years. I have often wanted to invest in an IR camera, but so far haven't. If you find a cheap enough one - go for that instead.

So much of this work is constructing a meanful test jig. I know it seems overburdensome to build a test rig for one troubleshooting session you may never have to repeat. But you don't know it will be the last time. A PSU this complex could go a few rounds before you get all the marginal parts out of it. Might you have to help someone else with another system in the future? You might buy a spare supply and want to check it out before trusting it? [come to think of it - this isn't a bad method to verify the one you have either - if two do the same thing] Can others learn from your posts here, and build on your methods to troubleshoot their systems and make rigs of their own?

All these would be on my mind. An 11/44 is a lot to risk on a questionable supply that is well capable of melting down the whole shebang if she really blows. However, if we were talking about something even rarer than an 11/44 - say a KA10 - I wouldn't even consider the risk until the supply had demonstrated it's reliability for hours of 100% output.

How would you feel about your risks if it were to go wrong not having taken every precaution?


Well, I guess I've offered you a little counterpoint to think about. I hope it's been helpful and not discouraging. You're doing a great job and it's not an easy one. In the coming days I'll try to better inform myself of your posts and these documents. [have to spend the next month or so in a hospital with a relative so I can load up the MP and TM in my laptop for some quality time] I apologize if my suggestions are redundant or inappropriate because of effort you've already expended. My speed reading isn't what it used to be.

BTW - I saw the question posed, but didn't see an answer - Where in the world are you located?
 
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RSX11M+, thank you very much for your insights. None of the suggestions and insights you made are redundant. Before this point most of the troubleshooting has been with getting it to turn on and figuring out what is causing the bad trace, along with fixing a problem with the breaker in the power controller unit.

I have briefly looked into the remote sense signal a little bit earlier, but not too indepth. The idea just dawned on me the other day after thinking about the problem. So this problem has not really been discussed much. As for detecting the current going through the crowbar, I am unsure if that is feasable. I am a bit afraid to totally dissassemble the PSU because I cannot figure out how to do that properly.

As for the thermal aspect, here is a picture of my cooling setup if you have not seen it when looking at the previous posts:
View attachment 10503
The test setup for the PSU
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Ignore the connections to the multimeter and dummy load, I am working on a better setup for them.

Maybe the air is flowing the wrong way. Do you have any suggestions to improve this setup. I think I may need to re-post it, but in one of my earlier posts I have a picture of an airflow sensor board. Maybe that has something to do with the shutdown as well. I like your advice about the IR thermometer. I just obtained one today.

You advice is far from discouraging. I am very nervous about connecting the PSU back to the system because this is my first minicomputer, and I have had this system for about 3 years now, and I would hate to see it go up in smoke. I am more inclined to take the more cautious route, but at the same time I am inclined to just connect it becuase I am unsure how to properly connect the remote sense signal without connecting the PSU to the backpane. I am also encouraged because my theory about the missing remote sense signal seems to be sound.

I am located in Ohio.

Thank you for helping me with this, and I hope that your relative will have a speedy recovery.
 
I have read that an old web cam can be rigged as a IR camera? and as the capacitors may have been cooked by the heater and age, maybe it would be a good idea to take one out and test its ESR to see if its dried open?
 
Don’t think air flow is an issue, thermal sensing on those old power supplies was usually a matter of a thermistor on or by a heat sink and when there is no load on the supply they develop very little or no heat. If anything think you’re missing the returns from the backplane that provide the +5 volt sense line. The crowbar may be firing due to the output of the power supply rising above its normal level and that will happen if there are no returns to the power supply. I would not attempt to try to operate the power supplies outside the system myself because of all the stuff that goes between the backplane and the power supplies. If your concerned pull all the cards from the system and then try it with just the back plane and you can always try jumping your load for the +5 volt supply across that bus. What part of Ohio are you in? The Dayton hamvention is coming up in a couple weeks and I will be there. I have one or two bad unibus cards that will suck down current that you can have to stuff in the system to provide a minimal load if you want them.
 
Qbus said:
... I would not attempt to try to operate the power supplies outside the system myself because of all the stuff that goes between the backplane and the power supplies. If your concerned pull all the cards from the system and then try it with just the back plane and you can always try jumping your load for the +5 volt supply across that bus. ...
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This is a good intermediate approach. If the problem is related to not having feedback on sense leads, this should help. Picking a load point off the chassis should simulate card loading nicely.

If the system were to remain powered in this configuration (rather than turning off in a couple minutes) I would be inclined to put it all back together and let her rip.

If not - it's clear you'll have to test bed it because you need access while troubleshooting components you can't reach now.
 
I believe that I am going to go ahead and try reconnecting the PSU without the cards in the chassis. Is there any special procedure I should keep in mind while doing this (besides carefully noting where each card goes)? The only card I have removed from the system so far is a small flip chip, and that was just to look at it. What would be the best way to monitor the voltages off of the backpane?

As for rorypoole's suggestion, I would like to try that. Do I need a special meter to measure the ESR or can I check it using the multimeter?
 
Old Computers said:
... Is there any special procedure I should keep in mind while doing this (besides carefully noting where each card goes)? The only card I have removed from the system so far is a small flip chip, and that was just to look at it. ...
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High resolution digital flash photos is a good way to document how the system was. They will refresh your recollection as well as be sharable with anyone trying to assist you remotely.


  • Images of the entire card cage with board numbers legible [prior to removal]
  • Compare card placement with any stickers in the box or lid - note discrepancies
  • Include detailed shots of any cables and connectors paying close attn to ribbon placement and polarity.
  • Add witness marks to cable ends as required [some ends are best not swapped]
  • Backplane images [from the underside]
  • Be sure to note all power supply connections to the backplane and elsewhere
  • As each card is removed, try to get a shot that shows the settings/position of any DIP switches in the cards

These steps are particularly helpful for novices who've never configured a system before. It's much more difficult to return a system to it's original config than one might imagine until you have to do it.
NOTE: Be particularly watchful for Duplicate Card Types. [multiples of the same card] Mark them to assure correct replacement.
​

Don't forget to get snaps of the continuity card placement for any un-populated slots, and the card orientation.

Basically, you can't have too many snapshots.

While I don't doubt that instruments like ESR meters and other specialized tools can be helpful, my training is in voltage-drop troubleshooting techniques. These tend to be in-circuit power-on where many specialized tools require component removal. Power-on troubleshooting can be exasperating when components are near failure [smoke and explosion] or where systems won't stay powered. Having documentation greatly enhances the troubleshooting effort.

Access to place probes and leads on circuit points is essential, and this sounds like it will be one of your challenges.

My best advice on troubleshooting is to not rely on too temporary a connection. Leads that "pop-off" and clips that come undone can themselves present risk of damage, often catastrophe. A roomy surface on which to work, comfortable seating, good lighting, visual aids [magnifying glass, loop, hand lamp, etc.] are indispensable particularly for an end-stage hunt like yours. Of particular help will be a location that can be left as-is between sessions, not requiring cleanup and without any danger of being disturbed. [this can save many hours]

Aside from photographic reminders, I make notes. These help me in protracted sessions where I can lose track of what I've tried and what results I saw. [especially when it was weeks ago that I last made an attempt] These notes are often on working copies of schematics which I make for more involved sessions. A notebook or computer file is also useful.

This is an outline of my "loaded for bear" stage, which should be where you are about now after all this work. This is characterized by a level of caution intended to "not allow things to go wrong". I never kick myself for over-preparing. It's the opposite that ends in regret.

I hope this is in line with what you wanted in the way of advice, and is timely enough to assist.
 
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Thank you very much for this advice. It helps a lot and I should be able to start removing cards from the card cage. I am very much hoping that the only problem with this power supply is the lack of the remote sense signal. I will let you guys know when I have removed all of the cards.
 
After being busy for a bit I have started to remove the cards. I am hoping to have the cards removed and the PSU re-installed by this weekend. To be able to load the PSU and monitor voltages what pins on the connectors should I look at?
 
So far I have finished removing the cards from the first backpane, and the first slot of the second backpane. I have been putting a piece of paper with each card that notes where it goes and the position of the switches if the card has any. I have also been taking photos of the switches as well, and noting how cables connect to the cards.

The only trouble I have been having with this process is removing ribbon cables from the boards. They are the Berg connectors, and there is no way it seems to pull the cable out of the connector without pulling on the cable itself. Do any of you have suggestions for removing these ribbon cables safely?
 
Old Computers said:
The only trouble I have been having with this process is removing ribbon cables from the boards. They are the Berg connectors, and there is no way it seems to pull the cable out of the connector without pulling on the cable itself. Do any of you have suggestions for removing these ribbon cables safely?
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Can you use the point of a sharp knife or screwdriver blade to wiggle it out slowly? Once you've got it out, you could tie a loop of dental floss around the connector part to get a better purchase on it next time it's removed. This is what I do with those little circuit board jumpers used on IDE hard drives, to set master/slave as it's a lot easier to slip a finger through the loop and pull, rather than gripping it with a pair of needlenose pliers.
 
Thanks for the tip. I will be sure to do that when I get the cable removed.

As for removing the cable with a screwdriver, I am unsure where to place it. There is a small ridge on the blue part, but I am afraid that the driver will slip and damage the cable there. There is a white part of the connector that I could use, but I think that that is the part that crimps the cable to the connector. I will just keep trying to remove it gently.
 
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