HP Power Supply help

[budm]

Re: HP Power Supply help

I would check that silver part with 2 screws( Don't know the name on English) that look like it may be the culprit those are something like a thermal fuse or thermal something

That is TO-3 case is 5V Regulator IC U12.
See attached of PDF for the SCH of the board as shown in post 1.

[budm]

Re: HP Power Supply help

Page 39 schematic.

[pentium]

Re: HP Power Supply help

You literally do not require additional photos or readings from components NOT on this board. As we've already tested every other board in the PSU the fault is exclusive to the Low Voltage board. It's not a damaged winding. It's not a weird regulator. Everything but passives have been changed at this point. My next step after this is a full replacement of all diodes, resistors and solid-state capacitors unless someone else knows what would cause somwthing like this.

[Drack]

Re: HP Power Supply help

The one on the second pic

[kc8adu]

Re: HP Power Supply help

bet its the tantalum upper right screw of reg.
little silver can.

[Longbow]

Re: HP Power Supply help

You literally do not require additional photos or readings from components NOT on this board.

Well, you may be correct, but you have not given US sufficient evidence to come to that conclusion.

As we've already tested every other board in the PSU the fault is exclusive to the Low Voltage board.

Not so. It probably IS on that board, but assuming so is incorrect if the 2 LV boards are not the same.

The U6 in your print above is a dual monostable which looks like it is a power-on reset circuit. Once again, we only get a small piece of the design which makes everything more difficult.

My next step after this is a full replacement of all diodes, resistors and solid-state capacitors

HP did not arbitrarily throw just any part into its designs. I have worked on lots of equipment of this vintage and found 50 year old tantalums (for example) that had lower ESR's than new ones. Transistors and other parts that HP used were not off-the-shelf. They were individually tested and selected to meet HP specs. Personally, I would not approach this problem with the shotgun technique (if you can call it technique), but if you insist then let us know what the problem was.

[Longbow]

Re: HP Power Supply help

Just from experience with circuits like this, I would have a close look at the photo-thing near the connector. First generation part. Early ones used CdS cells which age, become intermittent, and go away. They typcially use a filament lamp inside the package which also ages, or fails. Some people call them Vactrols, but this one might be a newer design with a phototrans. Still suspect. You won't find the original HP part, only an approximation, so I would not pull it out unless I was real sure it was #$%#.

[Longbow]

Re: HP Power Supply help

And another thing, as budm has already pointed out...a hot transistor means it is conducting lots of current. That current is going somewhere, not just out to space. If there is a short to ground, you will measure very low ohms to ground. If some part like a capacitor has failed as a low resistance, you can measure that too. Generally aside from a direct short to ground, some other part will be running hot also. That other part could well be on some other board. Or not. It is pretty easy to insert your ammeter right at the output of a regulator and see exactly what the current draw is. What is the current draw?

[pentium]

Re: HP Power Supply help

Well, you may be correct, but you have not given US sufficient evidence to come to that conclusion.

I can make such a conclusion simply by using donor components from the loaned power supply to verify everything else works. If one of the other boards were the source of the problem it would of been found through the board swapping.
Simply I put the LV board from the working donor into my PSU, it works. I put my LV board into either PSU, neither work.

Also yes, the donor boards and my boards ARE marked so I have not mixed up my boards.

Not so. It probably IS on that board, but assuming so is incorrect if the 2 LV boards are not the same.

They ARE the same. The schematic however is of a different revision.

Transistors and other parts that HP used were not off-the-shelf. They were individually tested and selected to meet HP specs. Personally, I would not approach this problem with the shotgun technique (if you can call it technique), but if you insist then let us know what the problem was.

That is a rabbit hole that arguably I'm not going down.

Just from experience with circuits like this, I would have a close look at the photo-thing near the connector. First generation part. Early ones used CdS cells which age, become intermittent, and go away. They typcially use a filament lamp inside the package which also ages, or fails. Some people call them Vactrols, but this one might be a newer design with a phototrans. Still suspect. You won't find the original HP part, only an approximation, so I would not pull it out unless I was real sure it was #$%#.

As stated in the OP the optoisolator modules have been swapped between the good and the bad board. If it was bad the fault would of moved with it.

f there is a short to ground, you will measure very low ohms to ground. If some part like a capacitor has failed as a low resistance, you can measure that too. Generally aside from a direct short to ground, some other part will be running hot also. That other part could well be on some other board. Or not.

There's nothing suspicious. There is no obvious short or other components roasting away.

It is pretty easy to insert your ammeter right at the output of a regulator and see exactly what the current draw is. What is the current draw?

This is something I'll admit I considered but it's not going to be easy. Because of how the power supply is assembled it's impossible to actually reach the regulator unless you partially disassemble the unit at which point you cannot operate it.

[Longbow]

Re: HP Power Supply help

Ok, thanks. Whenever I have been at this point in the road it turns out I have made an incorrect assumption somewhere along the way, even though everything seems obvious. One possibility that has not been considered is pc board contamination - things that should not be connected ARE connected via hidden contaminants. Since you have a working board, that possibility could be checked by making resistance measurements and noting any differences.

[Longbow]

Re: HP Power Supply help

Just for the sake of completeness, here is the data sheet on the regulator:

This is an 8 volt regulator, with added protection by CR11 CR12 and CR13. Generally, CR13 is added to protect the part from damage at turn-off. Current foldback starts at about 2.5 amps and the part also has thermal control. You would expect an 8 volt regulator to have 15 to 20 volts input, also based on the rating of the input filter cap.

My theory is that the part is latching (protection) at turn on very predictably due to a circuit fault on the output side. Or, if CR13 is leaking or turning into an 18 volt zener, then the input voltage would appear at the TP terminal. So, a question. Is the mystery 2 volts d.c. appearing at the actual output terminal 3 of the regulator, or is 2 volts measured at TP? I'm thinking along those lines because the regulator can test fine out of the circuit, but you can't duplicate the distributed constants found in-circuit.

Lots of 3-terminal regulators require a low impedance right at the in-out terminals. Possibly hanging temporary 1 uF tantalums - pin 1 to 2 and pin 3 to 2 would give you added information. Also, it would be very interesting to look at terminal 3 and TP with a scope as the power is applied. If you can get a capture at those points, please post.

[pentium]

Re: HP Power Supply help

All components (aside from resistors, which all read good in-circuit and YES, that includes those tants) using P5R on the Low Voltage board have now been swapped between boards with no movement in the fault.

Last thing that is left before I start drinking more than usual is everything on the other side of the optoisolator (and using P5L). This portion of the schematics, the layout diagram AND both my boards match. According to the schematics this area consists of the 5V regulator and the POP circuit.
In a way, this is good, this means that any signals entering on this side are KNOWN TO BE GOOD, simply because if we pop in the good board everything works. Glancing over the schematics that means if the output from this side through the optoisolator is bad it will cause the other side to not function and if I'm reading this, the voltage rail that mysteriously goes into a short does so to prevent the power supply from coming up at all.
This also means that forcing that portion to work using an additional 5V source would be a bad idea as it would effectively defeat the interlock.

[pentium]

Re: HP Power Supply help

I got it.

U1 and U2 are a pair (I assume they are spec matched, so I can't mix them for testing) of OP-Amps. One of them is bad because when I move the set between the boards the fault moves with them. They are stamped with HP part 1820-0477 which I can buy locally from Sphere Research. The non-HP part number is LM301.

[Longbow]

Re: HP Power Supply help

That's great. I'm trying to figure out why the symptoms were so confusing. I would not spend a lot of money on matched HP parts. I would use a generic part. I don't think you need a high gain precision op amp here. Interesting to see all the non-standard and hand-selected resistor values even in the supply. So typical of HP. In the past, most of the problems I've had with earlier HP gear have been from carbon resistors that have changed value. I still can't understand why the shutdown mechanism zeros out the 5 volt regulator...were they counting on the current foldback feature of the regulator itself?

[Longbow]

Re: HP Power Supply help

I'm still not sure I am looking at the correct schematics. In any case, your post #1 indicates that the IC's on the board had already been replaced. Are we talking 2 dead IC's? Can the problem be completely explained by the op-amp failure alone?

Suggestion on power supply tweaking. If I have the right drawing, CR4 is THE reference for the whole supply and is almost certainly a temperature compensated zener (an HP favorite). Should the supply reference need to be tweaked, I have used an LM431 3-terminal adjustable shunt regulator in this exact spot with good results.

[pentium]

Re: HP Power Supply help

I'm still not sure I am looking at the correct schematics. In any case, your post #1 indicates that the IC's on the board had already been replaced. Are we talking 2 dead IC's? Can the problem be completely explained by the op-amp failure alone?

Suggestion on power supply tweaking. If I have the right drawing, CR4 is THE reference for the whole supply and is almost certainly a temperature compensated zener (an HP favorite). Should the supply reference need to be tweaked, I have used an LM431 3-terminal adjustable shunt regulator in this exact spot with good results.

Here's a diagram to explain what happened:



No idea why it took me so long to realize the optocoupler was going the direction opposite to what I was thinking in my head.

As for adjustments my dutch connection says that I should not need to fine tune the PSU once new op-amps are installed. The circuit is so simple in the eyes of HP and the op-amps are so general purpose that whatever variation the chip swap does add is within the threshold. Ordering from Sphere won't be much more expensive than ordering ones through ebay. The advantage is that they are the next town over so I can order tomorrow and they'll be here by Monday instead of next month.

[Longbow]

Re: HP Power Supply help

Great, thanks. I agree the op-amps tend to swamp out any little variations in other components. Part of my difficulty is seeing a larger part of the whole circuit instead of just a couple of parts pointing outward into space. Hope the restoration project is successful.