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PDP-11/44 Restoration

The breakers are held together with rivets. If I had to remove them, it looks like a bolt would be sufficient to hold them together again. Expect a picture later today or tomorrow.

@Qbus: I will keep those ones on ebay in mind.
 
Here are some photos of the breaker. The second one is a close up of the rivet that holds the breakers together.
IMG_0936.jpgIMG_0938.jpg
 
I removed the breaker from the power distribution box and I tested it. I found that if I slightly tap the bad breakers' switchs the bad ones will stay on. Another weird thing about this breaker is that the 3 switches must all be on to stay on. Moving less than three will not close the contact nor will the switches stay in the on position.
 
Try liberally spraying (trying to get as much into them as possible) them with contact cleaner. There's usually some lube inside them that can get gummed up with age.
patscc
 
I just tried using the contact cleaner and it has appeared to work! The middle switch seems to still be a little iffy, but I have put some more cleaner into it. Should I go ahead and try to clean the already working switch? I am kind of afraid to mess with it since it is still working fine.

I found a couple of car break lights. Patscc, I know that you said to connect 2 other bulbs of the same type in parallel. Since these bulbs are different, should I connect them in series?
 
Like they say, if ain't broke, don't fix it. I'd leave it.
No, series would increase the overall resistance, not what we want here. It's okay if they're different, all we're shooting for here is a load.
patscc
 
I re-installed the breaker in the power distribution box, and it works fine. I then connected the lightbulbs to the PSU, and together they draw around 6.7 amps. I measured the voltage and it registers around 5.31 volts. After a few minutes the PSU shutdown, and I smelt a slight electrical smell. I think that there is a little bit more to the problem than just those capacitors. I did hear a high pitched noise that came from the PSU. It seemed to peak in volume just before the PSU shut down.

Update: I think that it may be a thermal shutdown because after waiting a few minutes, the PSU was able to restart. The electrical smell worries me a bit, but that could be because it just got hot. The ammeter now reads 6.22A and the voltage still reads around 5.3v.
 
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No, you'll need to troubleshoot further. On a minimal load it wouldn't be shutting down unless there's a problem, and if you keep firing it up until something blows, it might take a bunch of other stuff with it, like the switching transformer, which would be next to impossible to find a replacement for. It sounds like maybe there's a biggish filter cap somewhere that's gone bad. I'll look at the schematics tonight and see if I can come up with some culprits to check. It's a good sign that the voltage is now at 5.3 v.
patscc
 
If it helps, I have noticed that the smell seems to be the strongest around what I think is the logic inverter board (the first board from the right on page 142 of the schematics). There are some large capacitors near there, but upon a brief inspection they do not look like they have failed (although that does not necessarily mean anything).
Update: I examined the PSU some more, and the smell seemes to be concentrated around the large heat sink on the logic inverter board with some large transistors on it. It is near some large capacitors, but they do not smell as strong as the heat sink does. The capacitors I replaced earlier still appear cosmetically sound, and there is no electrical smell around that board.

I think I have determined that the small transformer on the motherboard near the large blue capacitors is the source of the high pitched noise.

If things go well I will be getting an actual copy of the PDP-11/44 print set, but the seller says that it is missing a few pages at the beginning and end of the packet.
 
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Caps can be real weird if they're electrolyte is drying out, and switchers are a bit strange to begin with, kinda like working with a LW power amplifier. Can you post some pics of the suspect area ?
There's a snubber board in there. Can you disconnect it, and see if you get about 10 Ohms across each resistor, both polarities ( swap leads ), and not using diode mode, just plain resistance ?
Can you make out the exact model number on C1,C2,C3 ? I need to see if I can find specs for them.
patscc
 
I will check all of those things out this week. It might take a while becuase this power supply is confusing to take apart in some areas. I probably will get some pictures posted tomorrow though.
 
I am not quite sure. It was owned by an astronomy club, and they kept it in their observatory. I believe that it was heated but I don't know how well (maybe only during meetings). It might have gotten a little bit humid in the summer but that is pure speculation. There was no A/C from what I could tell. I think that it was right against a heater because the power cord coming from the power distribution box has a burn mark on it. I know that this was not caused by the PDP because of the appearance of the burn mark (it looks like an external source caused it and it is localized to a single spot), and upon checking for shorts none were revealed, and there were no instances of high resistance between the conductors.

The RK07 disks with it were pretty dusty and the machine cabinets had some dust on them as well. The president of the club said that it worked the last time that they turned it on (don't know how long ago that was). Before the club owned it, a nearby university was the owner. Some of the plastic pieces on the PDP, the Winchester drive, the DECWRITER, and the terminal have significant yellowing. The terminal had some fly excrement on one of the keyboards. I'll need to look but I might have some photos saved from the craigslist post.
 
Okay, I would suggest you treat every electrolytic cap as suspect. It's starting to look like a lot of them have dried out due to either the heater, or the sun baking the observatory dome, or both. The poor DEC...
Didn't you mention you had a scope ?
patscc
 
Yes I do. It is an older (circuit boards are dated 1968 or 9 at the latest) Tektronix one. I forget the model number right now. The second channel is non-functional. Other than that it works pretty well.
 
Wimp. Using a 'scope that blanks the retrace. :)
Good enough,though,even if it's a bit modern. What make/model is your multimeter ? Or, even better, a link to the manual if it's on line.
The fact it was stored working is great, that means we can focus on age/storage related issues. You won't be looking for the results of other folks attempted 'repairs'.
patscc
p.s. My bad. Shoulda had you hook the 'scope up earlier, to check for ripple, etc., sorry. I completely forgot you posted earlier about having one.
 
It's cool. I do have the manual for the scope, and I will see if I can find a link to post. (I do have some older tube based scopes, but they are in need of a tune up :) )

Another thing that I thought to mention is that there were two floors to the observatory, and the DEC was on the first floor, so it probably did not experience any heavy sunlight.

When the PSU shut down none of the breakers tripped, it just stopped outputting current and had the electrical burnt smell (no magic smoke though). The relay in the input assembly was still energized, and it did not de-energize until a few seconds after I turned off the breaker (I believe that this is a design feature because it has done that some other times).
 
The 'scope is Tektronix type 453.

Manual link: http://bama.edebris.com/manuals/tek/453/
The manual might be for an earlier revision of the 'scope because I cannot find any of the nuvistors from the PC board pictures. Transistors are where the nuvistors should be, and they are labeled with a Q instead of a V.
 
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That's more than good, the model would have been fine. And the make/model of the multimeter, if you please.
How are you set with a spare/parts junk box ?
patscc
 
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