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PDP-11/44 Restoration

O

Old Computers

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I have a non-functional PDP-11/44 that does not power up (only the relay in the power strip in the cabinet activates). Upon closer inspection of the system I have found that a component (possibly the large capacitors) in the PSU (H7140) makes a high pitched whine after the power has been applied for a while, or after it has been removed from the power for a few minutes. Also one of the cards in the PSU has a burnt looking three lead capacitor on it. Could either of these things cause the computer not to function? After searching the internet for info on the computer and the PSU not much information has turned up so I thought I would come here for some help. Thanks.
 
Old Computers said:
I have a non-functional PDP-11/44 that does not power up (only the relay in the power strip in the cabinet activates). Upon closer inspection of the system I have found that a component (possibly the large capacitors) in the PSU (H7140) makes a high pitched whine after the power has been applied for a while, or after it has been removed from the power for a few minutes. Also one of the cards in the PSU has a burnt looking three lead capacitor on it. Could either of these things cause the computer not to function? After searching the internet for info on the computer and the PSU not much information has turned up so I thought I would come here for some help. Thanks.
Click to expand...

Have you found all the docs on the bitsaver website: http://www.bitsavers.org/pdf/dec/pdp11/1144/ Complete technical info, including theory of operations manuals and detailed schematics (including the power supply).

That being said, the 11/44 (and 11/24) later generation PDP11 systems implemented a much more power efficient, and much more complex, switching power subsystem architecture. The 11/44 supply can be debugged and repaired but it requires more advanced electronics knowledge.

Anything burned in a power supply is a cause for concern, that is not normal.

Don
 
Don,

Thanks for pointing me towards the Bitsavers archive.

Based on a cursory look at the field maintanance print set for the 11x44 system (http://www.bitsavers.org/pdf/dec/pdp11/1144/MP00897_11X44sys_Dec79.pdf), the board with the burnt out capacitor is the Bias and Interface board (5413605 See pages 165-171), and the burnt capacitor appears to be designated as C4 with a value 82uF 50V +100-10% Al El (which I suppose is aluminum electrolytic, and I'm not quite sure what the percentage measurement is for, although I think that it is some type of tolerance rating).

Also the notes on the schematic for that board indicate that there are signal lines that indicate the status of the PSU (like thermal shutdown and DC on, among other things). So could a fault on the board possibly cause one of those signals to be incorrect, preventing the PSU from starting up? Also should I look at other parts of the PSU right now in more detail as well?

Thanks,
Old Computers
 
C4 on p.168 is a filter cap on the 13V rail that supplies the bias startup voltage for the switcher. If this rail failed most likely the main supply won't start up.

+100/-10% is the tolerance. More modern devices are usually specified as +80/-20%, so replacing it with a 100uF/50V AlEl similar physical size cap should be fine.

I would also examine that board for any other burned and/or discolored parts, mainly resistors. Measure their values and replace if not within tolerance.

WHY that cap would burn is troubling, unless it just got old and developed an internal short ... apply a strong enough current source, and boom.

Also, be careful working on this supply. It converts the AC line into +300V DC for input into the switcher (main output is 5V at 120A IIRC ... enough to arc weld).

Don

PS: Here is another document on the BA11-A box and H7140 you most certainly want to access:

http://www.bitsavers.org/pdf/dec/unibus/EK-BA11A-TM-003_Aug83.pdf
 
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Before I check out the parts for their values, should I remove them from the circuit to get the most accurate readings, or will in-circuit testing be sufficient enough for my purposes?

I think your theory about the cap being old is probably true because it only looks burned at the bottom, and the metal casing looks like it has some kind of crust similar to corroded batteries near the burned area. It might have leaked.

Thanks for the safety tip. I'll be sure to be extremely careful; I always take a very cautious approach when handling high voltages and currents. No wonder there is a big red label on the PSU chassis that says to remove rings and watches.

Thanks for that link, haven't read it yet, but I am sure that it will help a lot.
 
Don't even bother testing the cap, just replace it.
When you have the cap out, you might want to test the CR1 bridge, as a starting point.
You might also want to rig up a dummy load for the PSU, so that you can test it without running risk of frying your motherboard.
patscc
 
But also when I test the resistors should I test them out of the circuit?

What exactly is the CR1 bridge, and where I can I find it on the board?
That's a good idea about the dummy load. How do I make one, or is it something I will need to find?

Thanks,
Old Computers.
 
In the EK-BA11A-TM-003_Aug83 doc, look at page 3-04.
It's the diode bridge that feeds C1 & C2 on the input side. Physically, I'd need a picture.
Sometimes, if the bridge fails, too much AC ripple is presented to the cap, causing to fail prematurely.
Power resistors often discolor the area around them due to the heat, but can still be good. I'd wait with unsoldering them until you've checked the bridge. Depending on the resistor, you might be able to test in-circuit, at least for a shorted one. For ones that are open, you should be able to check the voltage across the resistor and draw a conclusion from that.
You'd need to rig something up for a dummy load. Basically, something like a small lamp for the 12v & 5v legs to present a reasonable load to the switcher, since switchers can be a bit wacky without a reasonable load. If you need help figuring the dummy load, just let us know.
patscc
 
You don't need to test all the resistors, only ones that look suspicious (discoloration, etc). Most times you can test them in circuit. You'll have to look at the schematic to see if there are any other parallel components that will affect the resistance reading (diodes, transistors, inductors, resistors, etc). Many times you can orient your multimeter leads such that you can measure the resistance without forward biasing any parallel semiconductor junctions. In some cases however there is no alternative but to temporarily lift one lead to break a parallel resistance path.

The 'CR1 bridge' is a reference to the full wave diode bridge rectifier that produces the intermediate DC voltage from the AC input voltage. You'll find one of these on the 'input module' page of the H7140 power supply schematics. However, be careful as the H7140 bridge produces 300VDC on its output. If this is nonfunctional then the supply obviously won't work at all.

A 'dummy load' is just a simple passive resistive load that is used to apply a known current draw on the supply, used during testing. Since your power supply is non functional, I would have removed all the logic boards from the backplane before doing any debug/testing, in case something goes wrong. At least you won't destroy your systems logic boards.

Typically for a dummy load for 5V I'll use a 5ohm 5W power resistor to draw a 1A test load. I have several mounted on an old DEC plugin breadboard that I use just for this purpose. Picked them up at an electronics surplus store.

Don
 
Oh, and the whine might be from the hf-transformer, and not the cap.
I take it the breaker on the box didn't pop ?
AK6DN, I took a quick glance at the schematic, and seem to remember seeing a thrystor in there somewhere. Presumably part of a crowbar circuit ?
Old Computers, what do you have in the way of test equipment ?
patscc
 
Here is a quick summary of what I have done so far. I have inspected the Bias and Interface board. No other components seem to be bad, except for possibly a capacitor (one end looks rounded). There are a couple of places where a component looks to have been added later. I'll check the schematics for them, but they do look to be factory installed.

The breaker on the PSU is fine and hasn't tripped, but the cabinet's power strip's breaker needs extra pressure on it constantly to close the circuit (I have determined that this is not because of the current being drawn because it does that even when the PSU is not plugged in. I think that it's just gone bad over the years).

The test equipment I own are an oscilliscope, a frequency counter (if I need to use this can you tell me how to use one), and a typical multimeter. I hope to get some pictures of the Bias and Interface board soon. Also before I get around to testing the CR1 bridge, is a screwdriver across the giant caps' leads sufficient to discharge it, or is there a special technique I need? The PSU hasn't had power applied to it since about late last year.
 
I just checked the rectifier (CR1) on the input assembly. All the diodes showed about 0.5 on the diode checker, and when reversed biased, they showed infinity. I also checked the breaker, and line voltage select switch. They check out fine as well. The relay makes contact when I close it by hand, and all the wires in it also show continuity. The resistor across the relay is 3 ohms, but I am getting a reading of 3.6 (it's out of tolerance, but it appears that the probes have a resistance of .6 ohms so i think it's okay). The only thing I didn't check was the line filter. Do I need to check that?
 
Can you post a picture and part number ?
Is it a actual "two caps in one can", or just a single cap with an extra lead for additional support ?
patscc
 
I'll get a picture up in the next day or so. It is a plain electrolytic cap with an extra lead. 82uF +0-50 VDC. There is a 2 with a circle around it, and then the number 672D USA 3208H. I have determined that a company named Barker MicroFarads makes this style of capacitor. I can only find the capacitor when I search the net on parts suppliers that need requests for prices. I'm guessing that they sell to businesses instead of individuals.
 
Yes. It appears that that is the exact form factor. I tried to get the specific number from the data sheet with the table on page 2, but I couldn't get the tolerance figures. It would be great to keep it as original as possible, but getting it to work is my first priority.
 
Well, since caps are are a dime-a-dozen, like you said, I'd just focus on getting it to work first, then deal with originality. That said, I'd just clip the old cap off close to the board, and temporarily tack on the temporary new cap onto the remnants. This way, you can avoid stressing the pads and vias until you do the actual 'proper' replacement.
I think BM changed their part nomenclature a bit. I wouldn't worry about the tolerance; if you can only find a cap with a larger tolerance, test it to make sure it's measured value falls within the range of the old cap. If you're a bit over, that's okay.
patscc
 
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