HP 6289A power supply not outputting full current

[iroc86]

First time poster here but I've been following the forums and videos for many years. This is a great resource for learning electronics and I'm hoping to get some assistance repairing an '80s-vintage HP 6289A power supply. I think I've tracked down the problem but I have a few questions about the symptoms and diagnosis.

My 6289A will not output its rated maximum current of 1.5 A. If I connect a load resistor, max out the voltage knob, and try to dial up the current, the output peaks at around 950 mA. The same thing happens when I short out the + and - terminals as if I were setting the current limit. If the power supply sits for a while with a load connected, the output current will slowly increase beyond 950 mA, but when it cools down, it returns to the previous state.

First I checked all of the voltages and measured the ripple on the big filter caps. Everything seemed to be within spec.

The CC and CV regulation is spot on at lower current loads, so then I suspected that the regulator circuit was breaking down above ~950 mA output current. I swapped out many of the transistors and checked most of the diodes, but it didn't change anything. I also noticed that if I shunted the CC programming resistor R19 with a lower value, I could get the full 1.5 A output, but this didn't seem like an appropriate fix.

After some further diagnosis, I'm now thinking capacitor C16 may be bad. This is a 50 uF, 500 V ceramic on the pre-rectified 62 V rail presumably used for input suppression (there are no X/Y safety caps in this unit). It produces a fairly loud 60 Hz hum as the output current increases. Comparing the two output ripple measurements below, the waveform changes to a 60 Hz-modulated output when the supply hits a wall at 950 mA. Below this cutoff, the output seems fairly reasonable. If I manually raise the temperature of C16 with a heat gun, I can actually see the "bad" waveform change shape and become like the good one.

I haven't replaced C16 yet, but I wanted to get some feedback. Does this seem like a reasonable diagnosis? What failure mode in C16 is limiting the output current of the power supply? Or, am I off base and missing something else?

Here are a few pictures of the unit. I've also included a schematic for the 6284A, which is nearly identical except for the component values and voltage rails (I couldn't find a good schematic for the 6289A worth uploading). Thanks for the help.

[duak]

If I look at the ripple voltage waveform, I see the period is 16.7 ms corresponding to the line frequency of 60 Hz.  It ought to be 8.3 ms corresponding to 120 Hz.  The circuit should be balanced, but something on one side is not working well.  I would bet that either CR26 or CR27 or the transformer or one of the solder joints connecting these has gone blinky.  The buzzing noise tells me that the transformer is not happy and it could be caused by an imbalance.

I can't envisage any problem with C16 that could cause this problem.  If C16 had developed a partial short, it would get quite warm all by itself.  However, I've seen strange things that I wouldn't have thought would have caused trouble.

It's unlikely, but check the votages and waveforms on the other filter caps, C10 & C12.  These are floating power supplies not referenced to -OUT so take care.

If you don't find the problem, show us the waveform at TP 27 at full load.

I have a copy of the manual from the Keysight site, but the schematic isn't all too clear - not the best of scans.

[iroc86]

Thanks for the help, duak. You probably found the same (somewhat illegible) Keysight manual that I did. It's useful for comparing component values but I wish there was a better one floating around.

I agree that the 60 Hz waveform is a bit odd; the other filter caps all have a frequency of 120 Hz. Per the attached chart (from the 6284A manual), I checked all of the voltage test points a few days ago and everything was within spec. I also checked the voltages at various loads up to cutoff at 950 mA. The ripple across the capacitors remained the same except for the big C14 filter cap, which had almost 4 V of ripple at ~1 A output load. My other HP power supplies behave similarly, so I didn't think it was unusual.

If I have some time later I'll try to post a waveform of TP 27, which is essentially the voltage across C14. If I recall, it was a regular sawtooth wave (AC coupled) where Vpp was proportional to the output load. I don't remember seeing any distortion on C14 when the output went funky, so I originally assumed the problem was in the driver or regulator circuit.

From your thoughts about C16, I'll take another look at the transformer and CR26/CR27 rectifier diodes. It's possible that my heat gun test may have been hitting those components, although I tried to mask things as best as I could. I do believe that C16 is still making the noise because I can conduct the vibration through a screwdriver; it doesn't happen on the transformer... unless maybe the noise is resonating through C16 since it's mounted nearby.

[bob91343]

I would first suspect the 'current input circuit' and see if it's working properly.  I seriously doubt that the ceramic capacitor you are looking at is causing trouble.  Either the power supply isn't putting out enough voltage under load, or, more likely, the current control circuit isn't allowing sufficient current.  Look at the current sensing resistor(s) and the error amplifier.

[ArthurDent]

I agree with duak that either CR26, CR27, or a connection from the transformer through one of the diodes to C14 is open which accounts for the 60 hz instead of 120 hz ripple. With only one good diode or ½ of the transformer providing power, the D.C. voltage across C14 will droop too much at high loads.  Check the voltage on C14 compared to the output voltage meter at high load and see if the C14 voltage is too close to the output voltage causing that to limit the max current the supply can output.

I also see no way C16 could possibly hum and I’d wager that if you lift one leg of that capacitor and retest you will find that it’s the magnetic field of the transformer that is causing the hum. I have one supply that is quite loud at high loads. 

[iroc86]

Sorry for the delay in posting an update; I was out for a few days. I made some new observations based on all of your feedback:

• CR26 and CR27 turned out to be okay. For good measure, I swapped in a pair of 1N4004s, but the problem was still there.
• The voltage across C14 is 65 V at no load and 60 V at full load. The output is only around 10 V at full load with a 10 ohm resistor, so that rail is still 50 V higher than the output. The waveform at TP 27 is attached; there is no discernible change in the shape of the curve when the supply hits the wall. Only the ripple changes.
• I pulled one leg of C16 and the hum went away. It didn't affect the problem, though.
• I reflowed all of the solder joints coming from the transformer. No change, but I knew this was a shot in the dark.

Next I watched the output waveform while adjusting the current and voltage knobs. When the current knob is maxed out, the output is normal until the wall at 950 mA (adjusting voltage). When the voltage knob is maxed out, the output has the 60 Hz ripple at all loads (adjusting current). This seems to suggest that the current limiting circuit isn't operating properly.

From a diagnostic standpoint, I'm seeing two separate issues here, but they're probably related: 1) the current is limited too early, and 2) there is 60 Hz ripple on the output when the current is limited.

So, I thought that perhaps the output transistors Q6 and Q7 were being fed a wonky 60 Hz signal. This is true. TP 22 and TP 26, which are connected to the transistor bases, exhibit the 60 Hz waveform when the supply goes into current limiting mode. I think this makes sense as to why the output rails have the same kind of ripple.

Then, I checked the ripple on the regulator rails (referenced to +S) with varying output load. The -6.2 V, +6.2 V, and +12.4 V rails were all rock solid, which tells me that the 24 V between TP 34 and TP 37 is probably okay. However, the -4.3 V rail exhibits the 60 Hz ripple when the supply goes into current limiting mode. This rail is used in the Driver & Error Amplifier circuit. Might the -4.3 V rail be picking up interference through the transformer and passing it along to the output transistors? (The base of Q7, in fact, is wired into the same transformer tap as the -4.3 V rail.) I also swapped out VR4, the 4.3 V Zener diode, but it didn't make a difference.

I also tried to troubleshoot the Current Input Circuit, but I'm not sure how to interpret the output of the OR gate at CR3 and CR4. If I probe TP 17 relative to +S, I get a constant 2.9 V output regardless of the voltage and current settings on the front panel of the supply. How does this (and the Voltage Input Circuit, for that matter) drive Q3? Should I be referencing against a different rail?

I think we're slowly getting there. I appreciate the guidance very much.

[duak]

Iroc86 - sure, I could see that something in the -4.3 V supply would affect current limiting.  Most likely CR24 or CR25 or a solder joint in that circuit.  C12 could also be bad - if so, anything between 470u to 680u, 50 V or greater should be just fine.
 

[iroc86]

Well, I just swapped out CR24 and CR25, along with C12 from another "good" HP power supply that I have. Still a no go!

I also checked all of the test points that are connected to the -4.3 V rail. They all exhibit the weird behavior, especially TP 12 and TP 42 from the Voltage Input and Voltage Clamp circuits, respectively. At those locations, the output waveform hits almost 1 Vpp at the cut-off point, but then it quickly falls back down to ~60 mVpp with the 60 Hz modulation. Screenshot attached.

At this point, I'm kinda thinking that the -4.3 V rail is the culprit, but I'm not sure how else it could get 60 Hz interference except from the AC line. Since CR24, CR25, and C12 all seem to be okay, do you think the next logical step is to investigate the transformer? Is there some way I could test the windings to see if something is breaking down internally? I have a Variac--how about injecting a separate AC voltage to drive just that part of the circuit?

[duak]

Other than what we've discussed, nothing jumps up and says "i'm bad!".  It's kind of a complicated circuit with floating supplies and non-obvious interactions and now we're getting into subtle part failures.

You could check the windings for continuity - each winding should be, say, 10 ohms or less.  Shorted windings are harder to find - usually the transformer gets unusually warm with no load.

I've attached a multipage scan of the schematic, but it doesn't seem to be all that clear.  It has some voltages to check for.  The voltage across C12 is supposed to be 53 V so my advice above that it was OK to use a cap rated for less than 75 V is clearly wrong - I was reading the 6284A schematic.  A 63 V cap is OK but right on the 15% margin.

I'd check the parts around Q6 and Q7, in particular R36 and C6.  R81-R83 look like they should have a combined resistance of 40 ohms.

I'd like to see the waveform across VR4 (TP 41 to TP 23) as the load current is varied.  Note: TP 23 is pretty much +Vout so this measurement has to be taken carefully if the 'scope is grounded.

[xavier60]

The voltage control loop could be tested on its own by removing CR4 to disable the current control.

[duak]

xavier60 - that's an excellent idea to test the voltage loop.  Have to be careful with the load resistance though - without current limiting a 10 ohm load could easily draw too much current and kill a pass transistor.

[iroc86]

These are all really good avenues for diagnosis, guys. I'll look into this again in more detail over the weekend and try out some of your suggestions. Good reminder on the current draw vs. load resistance--I have a 100 W adjustable power resistor in my Digi-Key shopping cart but I'm waiting on some other components before I place the order. I might want to hold off testing the control loops individually until I'm sure I won't blow up the supply.

Yesterday I began systematically lifting "non-essential" components from the board in an effort to rule out as many items as possible. It's not my preferred way of troubleshooting, but this one has me stumped. Of note was the removal of Q7, which is apparently only there to reduce the load on Q6 at higher output voltages. With Q7 gone, current still flows through R81-R83 (which tested good). I thought that maybe the 53 V rail was passing some interference through Q7, but it didn't make a difference. Aside from the -4.3 V reference voltage, there's really nothing else from the 53 V tap used here, so maybe I'm chasing a ghost. I'll post up a waveform of VR4 soon. FWIW, R36 tested good and C6 was one of the components I lifted.

Thanks for posting up the 6289A schematic. No worries on the cap voltage mixup; it's hard to keep things straight when referencing similar service manuals. I found another schematic that might be the best one yet. It's for the 6255A, which is the dual-output version of the 6289A. Near as I can tell, the schematics are identical because the '55A simply uses a separate PCB for each channel. I've attached the new schematic, which is all one page. It should be a lot easier to reference and has the correct component values.

[xavier60]

Much of the Base current for Q6 comes from the Emitter of Q7 via R35. VR5 sets the Base of Q7 at some voltage above the Q6's Emitter voltage.
This area would be worth while checking. I don't know what VR5's voltage is.

[xavier60]

It might be useful to know what voltage change there is on the Base of Q4 while going from the good state to bad with increasing current setting.

[iroc86]

I decided to put all of the components back in and start from square one again. I also changed my troubleshooting approach: instead of checking the transition between CV and CC modes by turning the control dials, which may affect other parts of the circuit, I am now changing the load to alternate between states. I only have 10 ohm power resistors on hand at the moment, so I set the CV/CC switch point to ~7.5 ohms using the chart below from another HP manual. With the panel dials set to 1 V and 130 mA, I can run one 10 ohm resistor for CV operation and add another resistor in parallel for CC operation. The bad waveform only appears in CC mode, regardless of output load, so I can troubleshoot at much lower power outputs that I originally thought.

By varying the load, I figured that I should be able to watch the output of the current and voltage comparators change when the supply switches modes ("error voltage" per the manual). This would occur just behind the OR gate on the anodes of CR3 and CR4. If I trigger off the output of the Voltage Input Circuit at CR3 (TP 12), I can detect a clear falling edge as the supply transitions from CV to CC mode. I presume that the fall time is dictated by the speed of Q6 and Q7 to adjust their output as necessary to accommodate the change in the load.

I expected to see a similar (if inverted) situation in the Current Input Circuit at the anode of CR4 (TP 16), but this was not the case. The output of Q2B is pegged at 3.6 V regardless of what the supply is doing. Consequently, the output of the OR gate (TP 17) always reads about 3 V. In all of my probing, I'm using +S as a reference per the manual.

I don't think the output of the current comparator should be a fixed value; frankly, I'm not sure how the supply is operating under these conditions. If I trigger off CR3 and watch TP 17 (output of OR gate), there is no change, AC or DC. Yet, if trigger the same way and watch the base of Q4, there is a small amount of ringing and then a definite change in DC voltage, suggesting a change in the output of the power supply. I'm not sure how the downstream amplifiers can respond to a change that doesn't exist. Maybe I'm probing incorrectly?

I also checked the voltage at VR4, the -4.3 V rail, to see if that was causing any of the output ripple. I'm not sure that's the case anymore. The DC amplitude is around -4.6 V and the ripple is less than 10 mV, which is within spec (the manual states normal ripple at 20 mV). It holds while varying the output, too.

[xavier60]

With the CC control at a high setting and no load, the Base of Q2a should be much more negative than the Base of Q2b. Q2b should be passing the full tail current pulling its Collector to something below 2V.
Low tail current maybe, R22.

[duak]

The way I think of this circuit is that the Voltage Input and Current Input circuits each have vetoes that override and reduce the voltage/current applied to the load.  Diodes CR4 and CR5 'OR' the veto signals from their respective comparison circuits.  I think this is a current operated circuit, so the voltage change at their cathodes may not be that large.   I can't think of any correct operating condition where at least one of the vetoes is not active, ie., in control.  There might be a short time when the voltage or current mode switches back and forth and neither is in complete control, but it takes a short time for one to take over and keep output within the set limits.

Does the anomalous waveform change much at lower current limit levels?

Are all the jumpers on the back terminal strip in place and tight?

This unit is in middle age.  What happens to the output if you tap on the variable resistors with the plastic end of the screwdriver?  Maybe one has gotten a bit blinky with age.

[iroc86]

Thanks for sticking with me, guys. I finally have some promising news below.

duak, if I remember correctly, the irregular waveform seems to lose some of its amplitude and shape at low current levels, but not much. If I dial it in "just right" at the CV/CC switching point, then the ripple goes through the roof at several volts peak-to-peak.

All of jumpers are good and tight. Tapping around on the board didn't seem to affect anything, either.

I took xavier60's advice and pulled CR4 to disable the Current Input Circuit, essentially turning the supply into a "CV only" unit. I made sure to put a high-value resistor across the output terminals to avoid any current surges. I was able to verify operation up to 40 V with no issues. I also tested the current output with a 10 ohm power resistor and was able to output the full rated current (!) by adjusting the voltage control. The output ripple was completely normal and did not exhibit any of the 60 Hz modulation as when in CC mode. So, the next step is to start troubleshooting the Current Input Circuit. xavier60, I'll check Q2 per your analysis and report back.

duak, I think your assessment of the CV/CC mode switching logic is correct, but I don't think the circuits are controlled by current. I tried lifting CR3 and putting an ammeter in series to measure the current, but it stayed fixed around 145 µA regardless of the output voltage and current. I read through the theory section in the manual again. HP describes a transient scenario to explain the way the feedback loop regulates the output. I highlighted the sentence that suggests it's a voltage-operated loop after all. What do you think?





To check this behavior, I set up the following test: CR4 removed (per above), 10k resistor across the output terminals, voltage control set to 10 V, and a 10 ohm resistor ready to short the 10k. I'm externally triggering my scope off the floating 10 ohm resistor lead and measuring the collector of Q1A, which is essentially the CV input to the OR gate. I'm using a second channel to observe the output voltage of the supply. When I connect the 10 ohm resistor, the scope triggers and captures the regulation happening in the Voltage Input Circuit to maintain 10 V despite the 1000X increase in output current (1 mA to 1 A). This all happens so fast that I couldn't see it before, and I assumed something was messed up because the OR gate was always at a stable value. As shown in the picture, the output voltage drops slightly due to the instantaneous load change, but Q1 alters its conduction to stabilize the voltage again... and then stabilizes itself.

I still think something is wrong with Q2 and/or its associated circuit based on the above, but I agree that perhaps it's normal for the base of Q3 to appear like it's sitting at some fixed value; if the regulation is working properly, I suspect that the supply will drive Q6 and Q7 at whatever level is necessary to maintain a zero change on the driver amplifier input.

[duak]

Iroc, you're right about this being a voltage operated circuit too.  You've clearly shown there are some significant voltage variation during CV/CC mode transistions and that should make fiinding the problem easier.  My experience with diode OR gates and transistors in general is that it doesn't take much of a change in voltage to cause a large change in current and I understood from your observations that the voltage variations were quite small.

I'd like to see what's happening on TP 14, 15 & 16 when you're getting the anomalous output during CC mode.  I would think that in CC mode, there should be almost no variation in any of these TPs.

The latest 'scope shot shows an interesting double pulse where the second pulse is of lower amplifude.  It'd be interesting to see where it's coming from.  I'd start with TP 11.  Also, the Voltage Clamp circuit (p 4.5) seems to operate in CC mode.  Perhaps there's something interesting on TP 42 and 43.

I have a Kikusui power supply that probably uses a similar design to hp's.  It would be clean at low and no output current but would put out a weird AC ripple at mid to high currents, but not at all voltages.  It turned out the capacitor on the output terminals was bad.  Your supply has a 490 uF cap across the output.  May I have you try putting 100 to 500 uF across the output, vary the current limit and see what happens?

[xavier60]

With the CC control at a high setting and no load, the Base of Q2a should be much more negative than the Base of Q2b. Q2b should be passing the full tail current pulling its Collector to something below 2V.
Low tail current maybe, R22.

The Base of Q2a should be close to -0.66V with the current control set to 1 amp and not loaded.

[iroc86]

It'll probably be a few days until I can get on this again, but I wanted to post back with some comments.

duak, I think I did check those TPs earlier on, but not with triggering the scope. Maybe we'll learn something if I set up a similar test as with the CV circuit. Also, regarding that second pulse, it may have just been a loose contact when shorting the output with the 10 ohm resistor leads, almost like switch bounce. I'll run through the triggering a few more times and see if it's repeatable.

That's an interesting story about your Kikusui supply. I don't have an LCR meter and can't really test any of these caps, but I do plan on replacing them down the road for good measure. I've been able to rule out the 490 µF output cap C20 by disconnecting jumpers A9 and A10, although I was hoping for an easy fix like that! In so doing, I saw the output ripple double to around 2 mVrms, so I guess the capacitor is still doing something useful. I have a spare 490 µF I can put across the back and see what happens when tweaking the current limiter, as you suggested.

xavier60, I'm impressed with your analysis of the circuit and being able to provide details about its operation. Can you elaborate on how you've come up with the numbers? The dynamics of the power supply operation are still a bit fuzzy for me, especially with the negative voltage rails and ground reference to +S. I'm also not entirely sure how the summing points at A5 and A6 are supposed to work.

[xavier60]

This is the Harrison or floating type of design that you might have noticed others also mention.
Having the + output or +S be the ground reference for all of the functional blocks including for the control rails, allows the blocks to be connect directly together and control the Base of the pass transistor with respect to its Emitter.
Most linear lab and bench supplies are based on very similar principles.

Some voltages given are approximate.
The Base of Q2b is tied  to 0V so assume that the Emitters are -0.6V.  R22 goes to the -6.2V rail putting a drop of  5.6V across it resulting in 0.9mA of tail current for the differential pair.

The voltage on A5 doesn't change much, actually the bit it does change by doesn't matter at all.
The voltage across the Current Programming resistor and so its current remains constant.
This current causes a negative voltage on A5 in proportion to the resistance of the Current Control. -0.66V for 1 amp setting.
The -0.66V applied to the Base of Q2a causes it to be cut off causing all of the 0.9mA of tail current to flow through Q2b.
The voltage drop caused across R24 results in 1.9V at Q2b's Collector. It looks like 1.5V marked on the schematics?

The increasing voltage drop across the Current Sampling resistor that an increasing load current would cause, makes A5 change in the more positive direction.
As the voltage at Q2a's Base approaches 0V, it begins to conduct, leaving less and less current for Q2b causing its Collector voltage to rise, and eventually taking control of the output from the CV loop.

CR5 is a likely fault suspect.

Extra: Just occurred to me that Q2a's Base will not quite get to 0.66V because of CR5 being forward biased, and assuming it isn't faulty.
Anyways, a voltage reading of  Q2a's Base will be useful.

 

[duak]

Iroc, I probably can't improve on xavier's description of the Current Input circuit.  I'd probably overcomplexify and confusicate it just trying.  These days, I seem to confuse desktop stuffing and stovetop publishing all the time.

It's important to look at the waveforms on TP14 thru 16 while in CC mode.  I'd also suggest looking at +6.2 V and -6.2 V at high sensitivity with AC coupling.  When I think about it, any ripple on these supplies is coupled coupled directly into the veto signal and thus affect the output.  hp used a fairly elaborate regulator for the reference voltages to minimize noise because of this sensitivity.

If there is ripple on the reference voltages, I'd check CR22 & CR23 and C10.  A quick test is to connect your 490 uF cap across C10 (check polarity!) and see if there's any difference.  Since you're seeing 60 Hz, i'd suspect a diode or connection.

[xavier60]

I have been ignoring the ripple because I am hoping that it is the result of the power supply operating open loop because of a fault in the Q2 area.
The 60HZ component could be due to primary to secondary capacitive coupling although it does seem to be more than would be expected.

[iroc86]

Thanks for explaining the circuit theory, guys. I think this makes a little more sense now. Would I be correct in stating that the floating +S ground allows all of the functional blocks to shift their common references "automatically" as the output voltage changes? How does -S, the output "common," play into this? The operation is different than other control circuits I'm used to, where the reference points are fixed. What's the advantage of a floating design?

As for the troubleshooting, we are making some progress! I probed the test points in the Current Input Circuit while in CC mode and discovered that TP 14 (base of Q2A) was not behaving as described. It turned out that CR5 was indeed faulty (shorted) as xavier60 suggested. I presume that this short was causing Q2 to conduct constantly? I noticed that the OR gate now seems to work as expected, with the inputs at CR3 and CR4 switching according to CV or CC mode.

After fixing CR5, the current limit now reaches the full rated amount (1.8 A in over-range) . However, I am still experiencing the weird 60 Hz modulation in CC mode .

I took duak's advice and checked all of the voltage references again. I admit that I don't have the best probes for high sensitivity measurements, but I couldn't really detect any 60 Hz ripple on the rails. The 6.2, -6.2, and 12.4 V rails were all within 1 mVpp, too.

For confirmation, I also added the extra 490 µF cap across C10. There wasn't any change in the output ripple, although the ripple on the 12.4 V rail dropped from 4.5 mVpp to 1 mVpp. Either way, it's in spec, but at least I know my extra cap isn't toast. I also put the capacitor across the other filter caps, C12 and C14, and did not see any significant change on the output ripple. So, I don't think the ripple is a filtering issue.

So, thus far, I had only been able to observe 60 Hz ripple on the output. I couldn't find it anywhere else in the circuit. That may have changed now, but more probing is required--I found 60 Hz in the Voltage Clamp Circuit at TP 43. I think this circuit is only used in CC operation, so it makes sense that it would affect that operating state.

I have three waveforms below that show different states based on the position of the voltage control knob, relative to the CV/CC transition point, with the current limit set to 1 A. In full-on CV mode, the output ripple is normal and there's an obvious 60 Hz sine wave on TP43. At the transition point, the 60 Hz waveform changes, but it curiously corresponds to the 60 Hz output ripple (though inverted). In full-on CC mode, the 60 Hz waveform goes away entirely, and yet the output ripple remains.

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.