Two HP6253A Powersupplies - Restoration + repair (and modifications)

[TheUnnamedNewbie]

Hi guys - This will manly be a repair/restoration log. I bought two HP6253A powersupplies - two times 20V/3A output (but it will go higher, manual just goes "we don't guarantee it's noise performance"). One of the units I received was broken (I was aware of this, and it's the reason I bought it - an fully analog, old school PSU seemed like a great first big-boy repair project. In fact, the thing doesn't even have any IC's, at best a matched diff pair!

The powersupplies are basically two fully isolated devices - even have their own fuses and transformers. The only thing they share is the power cord and the big clunking switch on the front.

One of the two supplies (the one with a broken channel) seems to have almost no use. I suspect either it was damaged in the first few hours of using it, got shelved and never got repaired, or it was a backup unit in a production test setup and it never even made it out of the box. It's clean inside, which seems to confirm this. The datecodes inside of the device are all 1989, suggesting it was one of the later models. It has the nice HP terminals, only problem I have is that it's (in my opinion) got non-sensible colors: Red for both positive and negative, black for earth. This is one of the things I'm thinking of changing. I'm already using this supply to power some LED's I'm running as a desk light. (the broken channel's fuse is removed, so it doesn't turn on)

The other supply is a bit older and has clearly seen use. It's got quite a bit more dust going on inside. Both channels work. This supply is also outfitted with the crowbar, which is something I'll have to play around with.

A few things I want to look at doing:

• Clean up the supplies. Check if there are components that need replacing (capacitors etc.). Give some of the components a clean (the powerresistors in the back have a good amount of dust stuck to them that smells when you push the supply. The resistors are rated to 350 degrees C so they will burn whatever biological residue is on there)
• Look at changing the front terminals. On the old supply, the terminals are not the nice HP type. I am thinking of replacing them with the HP style ones. The new supply also has red for both positive and negative terminals. I'm thinking of changing that to be red for positive, black for negative and green for earth.
• Add power sockets. Currently they have (or at least one of them does, the other I already removed it) fixed power chords. Since they are imported from the states I had to replace the plug on the one I'm using. I would however like to get IEC14 style mains sockets on there. Requires a lot of file work so I will see how it goes.
• Output on/current set switches. Currently, the supply will turn on it's output as soon as the devices is powerd on. I would like to look at changing this, so it has an output enable and I can set to voltage without the output being turned on. The supply is quite roomy so I should be able to fit in a mosfet switch (or something similar) to do this. Similarly, a switch to short out the output for setting the current limit would be interesting.
• Bring sense terminals to the front. The sense terminals are at the back currently. I think it would be usefull to see if I can bring them to the front as well.
• Consider changing the fine/coarse adjustment to a 10turn pot. Currently it's using a fine/coarse style adjustment for voltage and current. I'm going to look at exchanging them for 10 turn pots. However, as 10 turn pots cost quite a lot, I don't think I will (8 ten turn potentiometers are not going to be cheap)
• Lighting for the meters. I think it would be usefull if I could get some kind of lighting in the meters so I can easily read them even in poor lighting conditions. I'll have to see tho, I don't want to risk damaging one of the meters.

Now ofcourse, some important pictures!

The "old" supply. It's missing a frame around one of the meters but it still works fine. I've already taken the cover off because I decided to start this "log" only after starting to tinker. You can clearly see it's been used, it has a big scratch on the front, and you can tell by the terminals and the potentiometers that they have been handled. It's come out of a rack, as evidenced by the marks of the mounting hardware.


One side of the old supply's internals. I removed the power resistors already as I started looking at how to place a IEC socket.
Transformer seems to have a datecode of 71, latest datecode I could find is 73.


The newer supply. Notice it is very clean from the front, no scratches or marks of wear. It has rubber feet as well, something the other supply does not have. In this picture it's powering the LED's I use to light my desk while studying (exam period, I'm doing this in my breaks, twice a day I take half an hour to work on these things).


The insides of the new supply. It really is very clean!


And the damaged section of this supply. The two burned components are CR19 and R35. A quick probe yesterday seemed to indicate that CR18 is also measuring a short. I'll have to take a look at this. Current suspicion is that CR18 being a short allowed current to be injected into CR19 and R22 from Q7, bruning them up. I've not spent much time on it though, this is something I'm waiting to do after my exams.


For those who want a schematic, the manual can be found on Keysight's website: http://www.keysight.com/en/pd-1000001530%3Aepsg%3Apro-pn-6253A/power-supply?cc=US&lc=eng

Feel free to give tips, ask questions, etc...

[alm]

My $0.02:
Binding posts: the old style is perfectly functional. Changing would be purely cosmetic, so that would the first thing to skip if you have a limited budget. As for the red/black/red, that was the HP standard at the time. Agreed that it does not make much sense.

Shorting switch: make sure to disconnect the load at the same time (so use a DPDT switch). Shorting a load with stored energy (like a large cap) or even a battery if you are so careless to charge batteries without series diodes could result in fireworks. Also make sure the switch is rated for > 3A DC (DC current rating is often much lower than AC current rating because the arc is not extinguished as quickly).

Putting sense terminals on the front: make sense if you have the front panel real estate. I would try to put them at 19mm/0.75" from the force terminals so you can use standard shorting bars. Not sure if there is space either under the meter or below the knobs.

Changing the pots: if eight multi-turn pots are over your budget, consider changing only the voltage pots. I share your preference for 10 turn pots over coarse/fine, but needing to set a voltage to some precise value is much more common than setting the current to a precise value. Also, 3 A is not that large that you often need a current limit < 1% of full scale, so just using the coarse pot should often suffice. Having a fine/coarse (or ten turn pot) for voltage and a single turn pot for current was actually fairly common in linear power supplies from e.g. Lambda and Tektronix from that era. I have also bought used brand name (e.g. Spectrol, Beckmann) ten turn pots from eBay for much less than what Digikey charges. Only received one with an open spot. Finding them between the tons of counterfeit Chinese ones may be tricky, however.

About the defect: could be that CR18 failed short: this is a protection diode, so would not impede functionality. Subsequently the positive terminal was raised well above the emitter voltage of Q7 and the base of Q6, putting a lot of current through CR19 and R35, for example by turning the power supply off with a battery / large cap connected to the terminals. Or by paralleling two power supplies and turning this one off first. In that case it would not surprise me to find more broken components in the circuit that drives the base of Q6 (where did that amp or so that killed R35 go?). According to the manual you linked CR18 is rated for 3A, so using that in a 3A power supply is cutting it a bit close (especially since these power supplies may go a bit higher). I would replace it with a diode with a higher current rating.

If CR18 were killed by an active load, then that would mean the reverse voltage would have exceeded 200V (rating according to manual). Then I would have expected much more expansive (and expensive) damage.

[TheUnnamedNewbie]

Quick update - will add pictures tomorrow night, after my exam.

Took apart the older of the two supplies. I was originally planning of just taking off the front panel but I quickly noticed that there was a lot of grease coating the inside of the supply. I suspect that it's either from the plastic degrading over time (plastic wrapped around cables to keep them together) or the supply was used in an enviroment where it was exposed to "greasy" fumes. As the grease just kept coming I decided to go all the way and completly dismantled the supply. I also removed all the cables connecting the boards together, as the documentation is quite clear and I have a second supply I can look at to figure out where the cables go in the first place.

I've decided that it would be too much work for now to implement the front-panel sense terminals and the output enable switch. For now these supplies are fine, I'm not really working on projects where the startup performance of the supply is vital (most have their own regulation and such anyways). One of the boards is cleaned, but I've noticed the 3A diodes seem to have quite some corrosion going on. Also, they are only rated to 3A, and as I would like to push the supply a bit further every now and then, I might replace them with modern 5A or 6A rated diodes with similar specs. Haveing also looked at the cost of ten-turn potentionmeters, I think I'll just order 1 and see if I prefer that over the current setup (let me rephrase: think the difference is big enough to warrant the investment of doing it for each supply).

I'm also having a look at the transformers. The ones in the old supply really seem like they have seen a lot of wear. I'm thinking if I could find some way of repotting them, but I worry about thermal performance. I'm gonna start by looking at mounting them different from how they were: they had little cardboard tubes around the screws for what I assume is damping vibrations. I'm thinking about getting some very soft rubber or silicone washer-like things to put in between and try and (more effectively) damp vibrations that way.

There is also the matter of capacitors: I was thinking about replacing the three big electrolytics, but the problem is that (esp. the biggest one, which is a screw-terminal style capacitor) modern equivalents are far smaller and require very ugly things to be done regarding footprints. I'll have to see what I do.

Here is a picture of the board after I gave it a wash. I removed the 3 big capacitors before doing so.

[Zucca]

cool picture... what you can find here is always a surprise...

[Armadillo]

That is the way to wash the board. Fine example, piece of Art. The backdrop not to shabby either.
If you are not gutsy OVER the board, you will never master it! 

[Zucca]

actually a hair dryer can also do the job...

[TheUnnamedNewbie]

actually a hair dryer can also do the job...

Sure, but a hair dryer is loud. The weather is nice out these days around here, might as well put it to use, don't you think!