First steps testing and troubleshooting bench power supply

[guymo]

Hello,

I'm a relative newbie and have just got hold of my first bench power supply, an old HP 6205C which should be more than enough for my interests for now. I got it cheap and sold "as is" so I'm just starting to play around with it and see how well it works. I have a few questions!

At first sight it seems the output voltage drifts down quite a bit. This is very apparent on start up, but settles down after 30 minutes or so, though still noticeable even on my low res multimeter: maybe 10mV every few minutes. I have not tested under load, though -- would that make a difference? And is a rapid drift expected as the supply warms up? (The service manual does suggest a 30 minute warm up before making stability tests.)

The manual suggest using a 133 Ohm load resistor to test the supply at 20V/0.6A. Is it safe to use e.g. 12 x 2k 0.25W resistors in parallel? Each one should then be dissipating 0.2W. Do I need more margin than that? Or should I try something else entirely?

I have more questions but I'll leave it here for now. Thanks for any help!

[alm]

My basic tests would be:
- With knobs set to max, does it output close to its maximum voltage or somewhat higher? Check the output for ripple.
- If you turn down the voltage knob, does the voltage go down? Go all the way down to zero.
- Load it with something that will draw close to the maximum current. Ideally something that can draw 20 V / 0.6 A (= a fairly beefy 33 Ohm resistor or electronic load) and 40 V / 0.3 A (= the 133 Ohm resistor). If that is not available, then testing the maximum current at a lower voltage should also give you a decent indication. Measure ripple again under load (will likely be higher than ripple with no load).
- Test the current limit. Do not short the power supply before doing this, because it could blow the pass transistor if the current limit is faulty. For a 0.6 A current limit, I would use something like a 5 Ohm power resistor and set the supply to 2.5 V. 0.5 A should now flow. Increase the output voltage, and the current limit should kick in sometime before reaching 5 V. For 0.3 A you would use a 2.2 Ohm resistor.

Obviously for a dual power supply you would perform those tests twice.

I am sure in the manual you will find a much more complete performance verification that will verify every spec in the datasheet. This would be my off-the-cuff test. I would use an electronic load instead of messing with different power resistors, but resistors will work just as well.

Paralleling multiple resistors is fine, but try to have some air between them. Running them close to the maximum is also fine, but make sure you do not touch them or put them on something that might melt. They can get quite hot, in excess of 100°C.

Can you quantify 'quite a bit'? I turned on a fairly similar HP 6200B, and it drifted from 15.123 V to 15.067V in thirty minutes (most of it in the first ten minutes). Fifteen minutes later it is down another 6 mV (ambient temperature also went down). Drift like that is normal during warm-up. And even then, these are not precision voltage sources. 0.1 % + 5 mV drift at 20 V is 25 mV. Add to that changes in load (if any), ambient temperature or line voltage. Is your DMM stable enough to measure 10 mV change on a 20 V signal? What happens if you warm up, measure it, and measure it again after a couple of hours without significant fluctuations in ambient temperature, is it then more than 25 mV off?

[guymo]

Thanks for your reply.

The output voltage falls by over 1V in the first five minutes before levelling off. After that it varies +/-15mV or so over time. These measurements were made with no load.

On the 20V setting the output voltage goes full range with no load (40mV at the low end, up to just over 20v). It was only after reading your post that I realised I hadn't tested on the 40V setting yet. Bad news here: setting the switch for 40V output makes very little difference to the actual output and the maximum voltage with no load is 20V on one channel and 22V on the other. Should the output voltage jump up immediately when the 20V / 40V switch is pressed?

It gets worse with a load connected. I wired up 15 2k resistors in parallel to make a 133R load. On the 20V setting, the maximum outputs on the two channels were 10V and 12V, and on 40V setting, 12V and 14V. So, overall I conclude that it's not working very well.

The manual has a fairly detailed procedure for checking out the regulation circuits. I've made a start and the first batch of reference voltages are all ok. Going further will involve some fairly detailed work shorting out various transistors in the feedback loop.  However, the fact that both channels are defective in the same way seems suspicious to me. Could the transformer be to blame? That's all that the two channels have in common. Or is it reasonable to imagine that the regulation circuitry has gone wrong on both sides in the same way?

The first thing I had to do when receiving the unit was re-jumper the transformer for 240V operation so that the two primary windings are in series rather than parallel. Could I have damaged something in doing this? It was a very quick and easy job. 

[bd139]

Nice power supplies. Setting to 240 shouldn't have harmed anything.

I had a 6236B front the same series for a number of years. The main filter caps were shot. This was causing ripple bad enough that the voltage reference wasn't working properly. This got worse as the supply warmed up. This is probably the first thing to check. Either an ESR meter or observing voltage across them with a scope will do the trick. You can probably get away with a DMM on AC settling to get an idea of the ripple as well.

[guymo]

I had a 6236B front the same series for a number of years. The main filter caps were shot. This was causing ripple bad enough that the voltage reference wasn't working properly. This got worse as the supply warmed up. This is probably the first thing to check.

Thanks. For the avoidance of doubt, you mean the large capacitor which is connected across the output of the bridge rectifier, right? There's a resistor in parallel which is pretty accessible for my scope leads so I will give that a look later today.

[bd139]

Yep that's the one. Check the rail voltages against the service manual as well while you are there. That's usually a big indicator of where failures are.

[guymo]

What do you mean by rail voltages here? Sorry for being dim. Feeling very glad I posted in the beginners' forum...

[bd139]

No worries. We've all been there

There are a number of voltages defined in the service manual. These are usually the reference voltage which the output is compared against, the positive and negative supplies for the opamps etc and the main regulator input supply. These are generally referred to as "rails" as in power distribution rails because they are constant voltages which are used to supply power to utility functions in the device. They usually have a defined voltage to expect with a tolerance. Best to look at the service manual for what these should be. You can mark up the schematic in the manual with "expected" and "actual measured" voltages. This tends to lead to clues when things are wonky. I've fixed many a thing over the years purely from inferring the failure from these measures voltages.

Edit: before I forget, I'd just like to add that a broken bit of kit is the best education you can get

[alm]

Sounds definitely like a bad filter cap to me. The ripple gives it insufficient headroom to go beyond 20 V. As you increase the current draw, ripple will increase. It would probably be more stable at very low output voltages. Measuring output ripple with the supply set for 40 V should show a very large ripple.

If this is not the case, then I would suspect a bad range switch contact or rectifier diode before suspecting the transformer.

[guymo]

Thanks again.

That was my guess as to what you meant The manual has a good table of reference voltages and test points. I've checked them and they all seem good.

I'm with you on the education point, and enjoying the process despite feeling a bit in the dark for now.

@alm: I've just had a quick look at the voltage going into the bridge rectifier and the range switch has a convincing effect on that side, more or less doubling the voltage as expected. So I think the transformer is likely ok. Will get the scope on the filter caps later.

[guymo]

Hmmm, so I scoped the output ripple of the filter capacitors, and I think they look good.

With the 133R load connected, the power supply was able to deliver 10V, and the scope showed a skewed triangle wave (steeper rise than fall) of 2V amplitude at 100Hz, riding on a DC offset of 35V or 50V depending on the range switch.

I think that is what would be expected, isn't it? The mains power is 50Hz so 100Hz ripple makes sense. The filter cap is 490uF so I think the 2V ripple is the right order of magnitude. So I'm thinking that the bridge rectifier and filter cap are ok. Does that sound right?

The service manual's clues for troubleshooting low output voltage are, first the rectifiers and filter cap, second the front panel meter (are they just suggesting that the readings are wrong, or that the mete itself is somehow spoiling things?), and then the regulator feedback loop. Does this mean it's time to get stuck in to the loop circuitry?

[bd139]

That's a good set of measurements. Transformer, rectifiers and caps are likely good then.

Id ignore the meters for now and check with an external DMM.

Regulator/feedback it is. Start with voltage reference. Monitor that over time. That determines half of the output stability over time. Other half is output sample voltage. Monitor that over time too. I don't have the schematic here but there's usually a voltage divider across the output. Might be dodgy pot as well you see.

[alm]

That sounds fine. One thing to check that I believe was not mentioned is the strapping on the terminals on the back. Especially the sense and programming terminals. The manual should have the correct strapping for front-panel operation with local sensing.

Did you also measure the output ripple with a scope on both the 10 V and the highest voltage setting? Will probably be negligible since the cap looks fine, but it would also show if something is oscillating.

[bd139]

Good ideas!

[guymo]

Thanks again. I'd checked the strapping before but I had another look just in case and it is correct for ordinary front-panel controlled operation.

I scoped the filter cap on both the 20V and 40V settings, and I varied the control pots a bit to see if there were any differences. The overall ripple decreased when I lowered the output voltage, but overall looked the same.

It seems to me that the low level of the output is more of a worry than stability at this point, and presumably there's a good chance that whatever is wrong that keeps the output voltage low might also be causing the drift. So is there anything to look at first to diagnose the output voltage?

The front panel pots measure ok as far as I can tell, though I suppose I could eliminate them from the picture by setting the unit up with a programming resistor instead.

To check my understanding of the operation:  is the regulation supposed to deliver a certain voltage regardless of the 20V / 40V setting, so that on 20V the output would max out (and presumably stop being very well regulated) as you turn the control pot up about half way? Or should switching from 20V to 40V double the output voltage for a given setting? It's easier for me to figure out how circuits work when I know what they're meant to do

[alm]

With output ripple, I meant ripple on the output connector (i.e. after the regulator). It is an easy thing to check and could give you some information.

On my 6200B, which is fairly similar, the output voltage stays the same regardless of the range setting, unless it is close to 40 V (on the 20 V setting it goes up to about 38 V without load, less under load). So basically the 20 V mode is only using half of the range of the pot.

I would start with the steps in table 5-5 of the manual.

[guymo]

Ahh, sorry for my confusion and thanks for your patience. I'll have a look for oscillations etc, probably tomorrow. And thank you for explaining the expected operation.

I've been eyeing Table 5-5 with a little apprehension all day, and was hoping you wise folk might have another test or two to suggest first, but ok, I'll jump in. This probably means a rapid reduction in the number of questions I ask because I imagine it'll take me a while, so that's good news for you at least

[alm]

Not sure if you already did it, but measuring the reference voltages and ripple (table 5-2) would probably be my first step (thou shalt measure voltages) after a visual inspection for burnt components, loose connections, blown traces, etc. Measuring the supply voltages looks easier than table 5-5, and an unstable +12.4 V rail could easily cause issues with regulation, since the reference voltage is derived from that.

[JXL]

Your no-load voltages before for the regulating transistor measure OK.  I suspect the supply may be current limiting.  Did you check the current limiting circuit as suggested by @alm in reply #1?  Turn the current limit pot to both extremes and check the loaded output voltage.

[guymo]

Did you check the current limiting circuit as suggested by @alm in reply #1?  Turn the current limit pot to both extremes and check the loaded output voltage.

I haven't done that, because I don't have an appropriate load resistor. But it does make sense to see what current it can deliver. I'll get some appropriate resistors, and also have a poke around the current limit circuitry for voltage measurements. Thanks!

[bd139]

I tend to just stick my DMM across the output in mA range with current limit set to around 100mA and compare the supply readout / metering to the DMM output as you ramp the limit up and down. Saves smoking resistors then. Occasionally costs a fuse however. Basically the shunt in your meter is a very small resistor. The power supply voltage readout will drop line a tone to under 1v in this circumstance.

[Vtile]

I'm reading this from phone so I'm not 100% sure if I have seen everything related. If the voltage controller circuitry is negative feedback type using integrator the offset error might come from blown integrator cap if the parasitic capacitance is not enough to pull it to set value. But this pretty far fetched I think.

Edit. C5 in Driver & Error amp. section is I guess it is also integrator of the feedback loop. I'm not particularly good with analog implementations though so I might be totally wrong. Look at it when all other thing is checked and if the offset error is still there.

[guymo]

The current limiting seems to be ok: I got hold of two 2.2Ohm 10W resistors and hooked them up in series to the supply. It was able to deliver 360mA on the 40V setting and 720mA on the 20V setting, just a little bit above the advertised .3 and .6A, which is what the manual told me to expect. I confirmed the current with my DMM as well as the panel meter. So that looks ok.

It seems there is nothing else for it but to get stuck in to the feedback loop. Thanks for your replies so far.

[bd139]

Good luck!

[guymo]

Progress of a sort.

I worked through the feedback loop troubleshooting procedure from the manual, saturating and shorting various transistors, and everything checked out: each step sent the output voltage to maximum (38V on the 20V setting, higher on the 40V setting). I skipped the step of disconnecting the current limiting section because it seemed like hard work for a long shot. Hmm.

I started to wonder about the output sampling section. After poking about a bit and getting suspicious of some of the voltages, I decided to try disconnecting the panel control and using a resistor to program the output voltage. According to the manual, each 200 Ohms should give 1V of output. I tried a 4k7 and a 6k8 that I had on hand, and lo and behold, 22V and 32V output -- not exactly what is expected but close, and I think I had been tinkering with the programming trimmer, so no big deal -- and stable with my 133Ohm load.

With the panel controls connected, that programming resistor is replaced by the panel pot and a 5uF capacitor in parallel. This reminded me of the suggestion to check for oscillation, and sure enough, the output is oscillating: there's a downward dip of about 4V every microsecond. (It is possible this is caused by capacitance in my scope probe but it was the same on 1x and 10x setting.)

So I am wondering if this oscillation is messing up the output sampling because it passes through the 5uF cap. Does that sound plausible?

If so, there's a capacitor to check out and a trimmer to adjust, though I lack the equipment to test as suggested in the manual (para 5-72 if anyone is reading!). But any other thoughts you might have would be most welcome.