HP 6289A power supply not outputting full current

[xavier60]

 The 60HZ signal at TP43 should be the same as TP12. I would like to know what it looks like upto TP22.
In CV mode,

[duak]

The floating series regulator is a flexible design that allows for remote control and ganging of power supplies.  hp has a description of the design concept here: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1962-07.pdf (hp Journal, July 1962 issue)    Just consider it to be positive ground with +S as the common reference and you're good to go.

An idea from way out in left field about the 60 Hz noise - perhaps the transformer's magnetic field is inducing some AC voltage in R54, the current sampling resistor.  R54 is a wirewound 0.66 ohm 20 W resistor and may have enough turns to develop enough voltage to affect the CC mode loop.  If R54 has a central hole, then putting a steel screwdriver in it should increase the 60 Hz noise.  If R54 is secured to the board with a steel screw, removing it might make a difference.

[xavier60]

I'm not sure what to make of this just yet, but I think I need to start probing some other areas to find where the 60 Hz interference is coming from. I actually disconnected the Voltage Clamp Circuit (via CR30 and CR32) in my testing a few days ago, just to rule out extra components, and the 60 Hz ripple was still there, so I suspect it might be coming from another place and just manifesting itself in the clamp circuit. I'm thinking the -4.3 V rail might be worth checking again now that the Current Input Circuit is fixed.

I suspect that the test points I mentioned earlier will have very little 60Hz on them or none. 
Because I don't know the value of CR3, I cant figure out the state of Q10.  So the 60HZ is from either the -4.3V, board leakage or just stray capacitive pickup and there is no real problem.
From my own design experience, some ripple in CC mode is expected because of the combination of BJT Early Effect and the lower loop gain  in CC mode.
Suspect areas of board leakage can be confirmed by directly probing the surface.

[iroc86]

Thanks for the PDF link, duak. That looks like an interesting read and should improve my understanding of these supplies. The ganging and remote control aspects make sense for the positive ground configuration.

Okay, so I made a silly mistake when I posted up those last scope captures--I had C20, the output filter cap, disconnected from the rear strapping points from prior debugging efforts. The presence of C20 doesn't seem to make a huge difference in CV mode, but it definitely affects CC operation (perhaps for the reasons xavier60 mentioned?). After installing C20, the output ripple went from ~6 mVpp to 660 µVpp .

That being said, I have a few more questions. I know that some output ripple is expected, but I want to be sure I understand things:

First off, the 60 Hz sine wave in CV mode at TP 43 was a result of poor probing and close proximity to the input transformer . However, when in CC mode, I could definitely trace the anomalous 60 Hz waveform from TP43 all the way to TP22. Again, I don't have the best probes for high-sensitivity measurements, but the averaging function on my scope got me close enough to where I could trigger on a waveform and be reasonably assured of a signal on those test points.

Out of curiosity, I compared the -4.3 V rail to the other references. It's definitely noisier and seems to correspond to the output ripple waveform. Even though the supply is working within its specifications now, could I assume that the output ripple is coming from the -4.3 V rail through Q3 and/or Q5? I would guess that some component of the waveform feeding the regulator (TP22) is also proportional to the sawtooth wave coming into the pass transistors, i.e. from CR26 and CR27.

Lastly, I have a question about probing VR4, the 4.3 V Zener diode. When I touched my scope ground lead to either side of VR4, the output of the supply went to zero, probably due to some kind of internal short. This seems strange to me, since I didn't have any other probes connected and the supply was essentially floating. I had to measure the -4.3 V by probing between TP41 and +S (not TP23). What would cause this? I didn't have any trouble probing directly across the 24 V, 53 V, and 62 V rails from the transformer.

[xavier60]

Lastly, I have a question about probing VR4, the 4.3 V Zener diode. When I touched my scope ground lead to either side of VR4, the output of the supply went to zero, probably due to some kind of internal short. This seems strange to me, since I didn't have any other probes connected and the supply was essentially floating. I had to measure the -4.3 V by probing between TP41 and +S (not TP23). What would cause this? I didn't have any trouble probing directly across the 24 V, 53 V, and 62 V rails from the transformer.

That's nuts!. All I can suggest for now is to make the probe's ground connection via a 100Ω resistor to TP23. Then probe TP23, looking for DC or RF.

[iroc86]

That's nuts!. All I can suggest for now is to make the probe's ground connection via a 100Ω resistor to TP23. Then probe TP23, looking for DC or RF.

I can give that a shot. I did notice that I was picking up a ~1.7 MHz AC signal when the supply was shorted out. It would even pick it up on the ground lead only, with the probe itself floating. Maybe some kind of interference in the line?

[duak]

Iroc, you could cobble up a 1X probe good enough for low frequencies from a coax cable with a BNC connector.  Use a 1K resistor on the probe end to help isolate the cable's capacitance from the circuit.

Xavier pointed out some limitations in the pass transistors that allow AC ripple to sneak by.  A consequence of the Early effect is basically that increasing the collector to emitter voltage will increase collector current.  The feedback circuitry tries to counteract this but it has limited gain and can't eliminate it all.

The intrinsic series impedances of the capacitors (Xc and ESR) and zeners (Rz) limit how well they can reduce ripple.  The electrolytic caps are getting old and could be replaced with new ones with larger values.

I wouldn't think that grounding the other side of the current sense resistor would shut the supply down - but who are you gonna believe, assumptions or your eyes?  I wonder if there's a connection between the circuit somewhere and the chassis - maybe the pass transistor heat sink insulators.  I'd measure with an ohmmeter and expect something north of 1 Mohm.  Anything less indicates a problem and the possibility of future problems.  It might explain why the diode in the CC circuit died - maybe one of the back panel terminals is grazing the chassis.  I'd be extra careful with putting the ground clip anywhere but +S - it'd be easy to frap one of the hard to get semiconductors.

If you see a 1.7 MHz signal then what could be happening is a circuit  is being formed by the  various capacitances to the chassis & safety ground  and the inductances of the line cords for the 'scope and power supply and either forming an oscillator or is acting as an antenna and is coupling in some nearby RF.

[iroc86]

This is super informative. I'm learning way more about analog circuit troubleshooting than I ever expected by posting this issue. I really can't thank you guys enough for sharing your knowledge .

I'll try the resistor trick on the probe that you fellows mentioned. Those unobtanium semiconductors have proven to be especially resilient with my amateur probing, but I don't want to push it. I did pull out the pass transistors at one point to check them, and I was careful to put the mica gaskets and pin standoffs back in, but it might be time for replacement insulators.

New caps are next on my list. It'll be interesting to profile the supply before and after to see if there were any changes. I'm also going to pick up a variable power resistor and some current shunts to adjust the supply per the manual. I have two other HP supplies that seem to work okay but could use a thorough evaluation, so it'll be a good learning experience.

Regarding the new capacitors, how critical is the ripple current rating and ESR for these old supplies? For example, the Sprague 5600 µF, 25 V main filter cap in the 6289A is physically very large, presumably for high current and low resistance. Short of buying computer-grade caps, I was going to put a few smaller electrolytics in parallel to get a similar advantage. Is there anything to watch for in doing this? As an example, this is the cap I was going to triple up to replace the 5600 µF: https://www.digikey.com/product-detail/en/UHE1H182MHD/493-1627-ND/. Unfortunately, I couldn't find any specs on the original caps, so I'm kinda shooting from the hip with respect to ripple current and ESR. I suppose I could calculate the ESR and in-circuit ripple current using an LCR meter and the waveform from my scope, but it may not be accurate if the capacitors have degraded.

[xavier60]

It's a massive topic, electrolytic capacitors. For many of the rules there are almost as many exceptions.
Low ESR capacitors generally use a water based electrolyte which tends to be less chemically stable than conventional electrolytics.
Best not to unnecessarily  use low ESR capacitors. I would rather not indiscriminately replace electrolytics, mainly those that show physical signs of problems. For the rest  I do in circuit ESR tests.

[bob91343]

While ESR can be an important parameter in circuits, it also can be an indicator of capacitor health.  Low ESR capacitors can cause regulator instability in some cases.

Experience is the best source of perspective regarding this.  I tend to discard old capacitors that have ESR much above 1 Ohm.  But smaller units often exhibit higher values.

Capacitance readings are important.  If a unit has less than, say, 75% of marked value, I toss it.  If its value is much above rating I won't use that as criterion for trash; tolerances are often sloppy.

Leakage can be important; if too much it will cause internal heating and even thermal runaway.  Especially when an increase in applied voltage causes more than proportional leakage current increase.

Old capacitors often recover after a period of use.  But they can fool you by suddenly deciding to fail.  There is no measurement of which I am aware that can predict this.

Certainly replacing all the electrolytic capacitors in a unit is a good idea sometimes but it sets you up for making errors such as bad solder joints, reverse polarity, and miswiring.  Plus it's time consuming and sometimes expensive.  The dimensions of modern capacitors prevent an easy replacement due to the mounting differences.  The modern cans are much smaller and have different mountings.

And I have mroe than once found a brand new capacitor defective.  I have one here rated for 100 Volts that had excessive leakage above about 75 Volts.  It was fine up to that stress but any more and it ran away thermally.

In summation, there is no simple answer and any course of action has its pitfalls.  Electrolytic capacitors in particular have many shortcomings and it's sometimes better to replace with some other type.  The most reliable are probably ceramic, in spite of their often sloppy tolerance.  Mica is good but even then, age can take its toll.

Since electrolytic capacitors and paper/plastic capacitors are usually wound in rolls, they can have annoying series inductance as well.  That's why you often see a small capacitor in parallel with a large one so that the combination has better characteristics than either one alone.

[xavier60]

Regarding the new capacitors, how critical is the ripple current rating and ESR for these old supplies? For example, the Sprague 5600 µF, 25 V main filter cap in the 6289A is physically very large, presumably for high current and low resistance. Short of buying computer-grade caps, I was going to put a few smaller electrolytics in parallel to get a similar advantage. Is there anything to watch for in doing this? As an example, this is the cap I was going to triple up to replace the 5600 µF: https://www.digikey.com/product-detail/en/UHE1H182MHD/493-1627-ND/. Unfortunately, I couldn't find any specs on the original caps, so I'm kinda shooting from the hip with respect to ripple current and ESR. I suppose I could calculate the ESR and in-circuit ripple current using an LCR meter and the waveform from my scope, but it may not be accurate if the capacitors have degraded.

With regard to large filter capacitors, there should be no need for anything out of the ordinary. So long as the capacitance value is scaled properly to the output current, any respectable brand of capacitor would naturally have an adequate ripple current rating also.
If space permits, upgrading the  voltage/size of the replacement capacitor usually improves durability.
I did a search to find how the DC load current relates to ripple current and got conflicting results.

[xavier60]

Needing some capacitors for myself, I found it difficult to find standard electrolytics.
Epcos/TDK, a trust worthy brand, has a series of standard type that claim to have an extremely long life span under low stress conditions,
http://www.farnell.com/datasheets/2367538.pdf?_ga=2.260007049.2004012069.1559655723-1727119234.1557827711&_gac=1.89793001.1558609709.CjwKCAjwiZnnBRBQEiwAcWKfYuGHUTqfk8jkJaF0-WFSzt7elG_M6tpqgaVBEIab-sNwByC66YENWBoCGCQQAvD_BwE

[iroc86]

I appreciate the feedback on replacing the caps! There are a lot of parameters to consider. I do remember reading that sometimes these new super-low ESR caps can spell bad news for old designs. It seems like there is no single answer to knowing when/if an old cap needs to be replaced. Perhaps the best course of action regarding my situation is to pick up an LCR meter and just see what I've got to establish a baseline. For all practical purposes, the power supply seems to be operating within its specifications, so maybe I shouldn't fix what's not broken.

I won't have a chance to get back on this project again until late next week, so I may have some delays in responding, but I'll continue to post updates for everyone.

[iroc86]

I wanted to post an update on my progress with this power supply. The unit is working pretty well these days. I conducted a full performance test per the service manual and the unit passed admirably. The output ripple is a little on the high side compared to my 6236B supply (which is nearly flat), but the 6289A is still within its 1 mVpp rating. Eventually I will get an LCR meter and test the caps individually, but for now, the performance is more than good enough for my use.

Out of curiosity, how much drift should I expect with the analog output meter? I've been chasing down an intermittent issue where I can't get the panel meter to calibrate properly. I think it might be related to thermal drift, but I haven't really been able to isolate the conditions. The meter will periodically read about 100 mV lower than the output voltage. It tends to drift around during use. The offset is consistent across the entire 0-40 V output range. At higher voltages, the meter is well within its +/- 2% accuracy rating, but at lower voltages it's obviously not. The output voltage is always rock-solid within 1 mV, either loaded or open circuit, regardless of warm-up time and temperature.

If I try to re-calibrate the supply after the meter has drifted, the adjustment that always goes out is the "meter zero" setting via R63. As soon as I reset that trimmer, the accuracy returns. I've attached the adjustment procedure from the manual and the meter circuit below. The full schematic is a few posts up if you want to see the whole thing.

I thought the drift might be related to the reference voltages, so I measured the +6.2, -6.2, and +12.4 V rails at initial turn-on and again after 30 minutes of warm-up time. The +/- 6.2 V rails didn't change at all, but the 12.4 V rail increased from about 11.98 V to 12.05 V. I can't imagine that 70 mV would make a huge difference, but I'm not sure. This rail has a +/- 1 V tolerance, so it's still within tolerance, but the manual doesn't provide stability specifications.

Any thoughts on this one?

[duak]

If there are reversible changes I would first suspect the parts with contacts in them.  If you twiddle S2's knob (not enough to take it out of the current range) or the variable resistors slightly, does the meter jump a bit?  If so, try cleaning its contacts. Make sure the soldered joints in this section as well as the mechanical connections to the meter are good.

If you can get some freeze spray, you might try components one after the other.  A heat gun can also work, but is less selective and it's easier to frap (or flambe) a part.

 

[iroc86]

If there are reversible changes I would first suspect the parts with contacts in them.  If you twiddle S2's knob (not enough to take it out of the current range) or the variable resistors slightly, does the meter jump a bit?  If so, try cleaning its contacts. Make sure the soldered joints in this section as well as the mechanical connections to the meter are good.

If you can get some freeze spray, you might try components one after the other.  A heat gun can also work, but is less selective and it's easier to frap (or flambe) a part.

The switches were all fairly sticky when I started repairing this unit, but I cleaned everything with contact cleaner a while back. Twiddling the knobs doesn't seem to make any appreciable difference in the meter reading. For good measure, I also pulled out all of the trimmers and thoroughly cleaned them, too. Unfortunately, it didn't have any effect. I also tried poking a few of the components and solder joints with a spudger, but didn't notice anything unusual.

I'll pick up some freeze spray and see if I can isolate the issue, perhaps to the zeroing circuitry or the voltage references.

[duak]

Here's a really goofy thing: are you in a dry environtment?  I remember that one analog meter I had was sensitive to the static charge built up by dragging a finger across its face.  I found this out when I cleaned it with some IPA and the pointer followed the swab.  Totally weird and unexepected.  Also, many analog meters are sensitive to magnetic fields.  With the number of super magnets around these days, is there something nearby that might contain one?

There may be another explanation.  There two types of meter movement pointer suspensions - taut band and jewelled.  Jewelled movements can get a bit sticky over time that often manifests as a zero setting that doesn't stay put.  I don't know what type hp used.

Science news from the lab!:  I have an hp 6284A and the pointer is indeed sensitive to rubbing a finger across its window.  It takes a few passes all going the same way to move it.  A Lambda supply meter does the same but is even more sensitive.  Both are sensitive to a super magnet moved rapidly but seem to come to their original zero positions.  Hope I didn't demagnetize them.

[iroc86]

Here's a really goofy thing: are you in a dry environtment?

Actually... yes. I live in New Mexico.

That's really interesting about the static charge. It's not something I would have considered for this little HP supply, but your anecdotal evidence definitely provides some food for thought. I'll have to take a closer look. When I restored the supply, I polished the meter face with plastic compound and a special cleaner, so it's possible a charge may have built up from the rubbing. I also hadn't considered magnetic fields, but if that were the case, I'd expect my 6236B to act funny as well, since it's sitting right next to the 6289A on my bench. All good ideas, though.

It seems that the tolerance of my meter has actually gotten worse, or at least returned to its original state that I thought I had fixed: it's now inaccurate by about 1 V. When I was troubleshooting a few days ago, I may not have powered off the supply long enough to stabilize at a "cold" state, hence the 100 mV offset I was seeing. I don't think there's any significance to the magnitude of the offset, just the fact that it's worse than I thought.

I can try to measure the voltage at the meter terminals and see if it changes over time. I think that would at least indicate if the problem is with the calibration circuitry or the meter itself.

[duak]

Ayup, if you have a sensitive enough meter, look at the actual meter voltage - that'll tell lots.

I live in Vancouver, BC and with the humidity here, you couldn't build up a static charge even if you rubbed two nylon cats together.  Yet it seems I have a telekinetic abilty, at least on meter pointers.

I've travelled around NM a couple of times and I'll say its my favorite SW state.  My wife and I bumbered around Santa Fe the longest - a neat little city.  Away from the cities and towns the various parks, natural monuments and just things you find outside are quite unlike what we have here.

[iroc86]

That's cool! I'm actually up in the mountains, so it's not quite as desert-y as SF or even Albuquerque... more like what you'd expect in Colorado. Lots of tall trees and snow-capped peaks. The isolation can be pretty intense, but I agree that the parks and natural sights are quite impressive. I visited Vancouver a few years ago on my way to a cruise in Alaska, and I was super impressed by the city. It was clean, refreshing, and the people were very welcoming. I have a few friends who are infatuated with Victoria, so that'll probably be a stop in the near future.

I'll report back on the meter measurement soon. This is shaping up to be a busy week, but we'll see how things go!

[iroc86]

Quick update. I had a chance to set up some repeatable conditions to better define the meter drift in this power supply. Throughout all of the characterization, I left the supply adjusted to 20 V output voltage. Regardless of temperature, output current, or meter switch position, it always regulated to within +/- 10 mV of the setpoint, suggesting that the regulation circuitry is working great. I'm actually surprised that it's so stable considering the analog potentiometer controls.

My test involved a load resistor set to 26 ohms, per the service manual, which loads the supply to 0.77 A at 20 V. This is about half of its maximum rated output, so this seemed like a good test point. I then connected a DMM to the terminals on the panel meter and measured the voltage as the unit warmed up. After about 10 minutes, the meter input voltage increased by about 1 mV, from 36.8 to 37.7 mV. The meter seems to have a sensitivity of about 1.8 mV/V on the 50 V range, so 1 mV of drift would be quite perceptible--about the width of the needle itself.

Interestingly, I noticed a fair bit of fluctuation at the 10-minute mark, jumping around +/- 0.5 mV. The output voltage was solid at 20.00 V throughout. After about 30 minutes, the meter voltage seemed to stabilize again at the 36.8 mV mark.

I don't currently have a means of datalogging with a DMM to take a bunch of points over a long period of time, but it appears that the meter circuitry is going through some kind of warm-up jitter before it fully stabilizes back at the original level. Admittedly, the HP manual does suggest performing all calibration steps after the supply has been turned on for at least 30 minutes, but I would have expected a little better stability on the metering, especially for a lab supply that may be frequently turned off and on.

My next step is to get some freeze spray and start isolating components in the metering circuit...

[cbutlera]

Many of the symptoms described in this thread, including the buzzing, are consistent with a leakage path on the PCB between the mains voltage area and the current input circuit.  I would focus particularly on the track between the current control potentiometer R16B on the front panel, and the A3 terminal on the rear.  On my somewhat similar HP Harrison 6111A, that track (green arrow in the photo) runs frighteningly close to the tracks connected to either side of the dropper resistor for the neon indicator (red arrows in the photo).  At the closest point next to the resistor pad, the creepage distance is just 0.6mm!  On a 50+ year old PCB with no solder mask, it would be a surprise if there wasn't significant leakage between these tracks.

This is clearly unsafe by modern standards, and I'm amazed that it was ever regarded as acceptable.  The troublesome live tracks carry the connections between the transformer primary, the power switch, the neon indicator and its dropper resistor.  On my unit, I made a number of 10mm long cuts to isolate these tracks, and reconnected the switch, neon indicator and resistor to the transformer primary with flying leads.  I then connected two of the now floating tracks (indicated by the red arrows in the photo) to ground, to act as a guard.  The transformer primary terminals are still connected to the PCB, but there are now no live tracks going anywhere else on the PCB, except to the immediately adjacent pads, which I used to terminate the flying leads.  As can be seen in the photo, I made a small transparent shield to cover the remaining live area of the PCB and I also put clear heat-shrink on all of the exposed live terminals.

As a side note, if you are trying to achieve high stability, make sure that C1 has a very low leakage current. Even a few nanoamps is too much, and will give rise to tens of microvolts of drift.  My 6111A used a Sprague 1 µF film capacitor in this position, and replacing it with a new polypropylene capacitor improved the stability by about a factor of 50.   It can now hold 10 volts to within about +/- 20 µV for an hour or more, once warmed up.  Although bear in mind that the 6111A has an ovenized Zener reference and a switched precision resistor network to set the voltage.

[iroc86]

Well, it's taken me a bit longer than I expected to go back and try to tune up this old power supply!

Thanks for your insight, cbutlera--apologies on the very late reply. I see what you mean about the possibility of leakage with the unmasked PCB traces. My 6284A doesn't seem quite as tight as your 6111A. The trace from R16B to the A3 terminal actually jumps between top and bottom a few times and has at least 1 mm of clearance to the adjacent traces. Compared to the photo you posted, my supply also has a bit more clearance between parallel traces when they neck down to squeeze between pads, etc. Even so, this is good knowledge for repairing other vintage gear. I like your solution with the flying leads and improvised guard traces. Also, thanks for the tip about the leakage current of C1 (is this the same as C4 on your 6111A's schematic?). My 6284A will drift about 10-20 mV at 20-30 V... not a deal breaker for my use, but a little better performance is always nice.

Now, about the meter drift I've been seeing: I applied some freeze spray to the various components in the meter circuit (the schematic is a few posts up) while measuring the meter voltage across TP48 and TP49. Everything was thermally stable except for Q12 and Q14. When Q14 got cold, the voltage went down; when Q12 got cold, the voltage went up. This makes sense since these BJTs are in a differential amplifier arrangement and are just jellybean PN2907s per the service manual.

In an earlier post, I noted that the meter voltage fluctuated up and down until the supply reached a stable temperature. I wonder if I'm seeing the effects of these two transistors coming up to temperature at different rates and "pushing/pulling" each other until they stabilize. In the photo below, you can see that these are two discrete components with no thermal interface. Interestingly, HP chose to use a matched pair for Q11, the other differential amplifier in the meter circuit (TO-39 can, bottom right of photo).

Just for kicks, I soldered in two new PN2907s to see if they track a little better. I don't have any data yet, but I'll report back. This may not even be the issue I'm chasing, but it's interesting nonetheless.

[duak]

Iroc, could you make up an aluminum bracket that thermally ties the two transistors together?  I envisage something that looks like a staple for paper after it's been crimped.  hp often uses an 'S' shaped bracket to tie matched devices together.

If you have two new devices with long enough leads, maybe stick two together head to head inside a metal tube and then bend the leads to fit the holes.

I see that there are what look like 1/2 W resistors below the transistors.  Do they get warm during operation?

[iroc86]

Hey duak, I haven't had a chance to try the thermal bracket yet. The 1/2 W resistors nearby don't get appreciably hot, maybe 5 degC over ambient. There is a rather toasty (100+ degC) wirewound resistor nearby, but I don't think it's causing any thermal gradients. I tried putting some FR4 board in between the warmer components to act as a barrier, but it didn't make any difference.

I think I still need to run some more tests and determine if the drift is being caused by the amplifier circuit or the meter itself. The two transistors seemed promising, but I'm not so sure anymore...