HP 6289A power supply not outputting full current

[duak]

Forward to the past!: https://www.eevblog.com/forum/testgear/nixie-display-for-hp3478a/msg2744792/#msg2744792

An idle thought - use a single chip instrumentation amplifier to drive the meter, like the http://www.ti.com/product/INA128   One resistor sets the gain.  Add a few diodes or LEDs across the output (after a series resistor) to clamp the voltage to protect the meter and you've solved the drift problem.

[iroc86]

Ha, very cool! +1 for that fellow's ingenuity with the Nixies. I have to say, I agree with him about the LCD display on those later model HP meters... probably why I'm still hanging on to my 3466A even though it's only 4.5 digits.

Nice idea about the instrumentation amp. I came across those when I was researching the amplifier used in these power supplies. I suppose HP's design is something like a discrete BJT version of the same thing? The resistor network across the input amplifiers is very similar, as well as the downstream amplifier (Q12/Q14?).

[iroc86]

The current limiting circuit with Q15 is continuing to catch my interest. Possibly not related to the glitchy amplifier, but who knows.

duak, can you check to see the orientation of VR6 on your 6284A? Was it per the schematic or the errata in my Reply #52 above? I've looked through several service manuals for HP supplies of a similar design and they show both arrangements (even with schematics that are clearly drawn by different illustrators/engineers). I suspect that the cathode to +12.4 V is the correct orientation since it's functioning as a clamp per the overvoltage condition you mentioned earlier. I'm fairly certain this circuit wouldn't work at all if VR6 was flipped around, but I'm just curious.

I tried a few other Zener diodes in place of VR6. I measured the Vce of Q15 and the meter current at maximum output voltage (50 V) in the low range meter mode (5 V). The higher values reduced Vce as well as the maximum current through the meter.

- 4.2 V VR6 (stock part), Vce=75 mV, 1.64 mA limit
- 5.1 V VR6, Vce=38 mV, 1.59 mA limit
- 6.8 V VR6, Vce=27 mV, 1.50 mA limit

This seemed strange to me. I could see how increasing VR6 would extend the saturation point of Q15, but with that, I figured the current limit would increase, too.

So, I ran some simulations in LTspice and found an interesting relationship between VR6 and Q15. At values of VR6 below a certain "knee point," the current through the meter is limited by Q15 acting like a transistor and throttling back. At values above the knee, Q15 is fully saturated; interestingly, raising the Zener voltage further will reduce the current even though Q15 is still in saturation. This relationship is illustrated in the plot below (output voltage = 50 V, meter on 5 V range). I've also attached a new LTspice model in case anyone wants to play around.

Based on swapping out VR6 with various values, I surmise that my circuit is behaving on the "right half" of the plot, beyond the knee: VR6 voltage goes up, maximum current goes down. This would also explain why I'm not seeing Q15 drop out of saturation. (I don't have any Zeners lower than 4.2 V to see if I can find the knee, but I can try to get some.)

This is still strange to me, since the transition point occurs around 4.5 V @ 50 V output. With a 4.2 V value for VR6, I should be seeing Vce rise to at least several hundred millivolts, as duak observed in his 6284A. All I can figure is that maybe some of the resistors in this circuit are out of tolerance and affecting the response in unusual ways. At the very least, I know that R62 is about 2.7% out (770 ohm vs. 750)... not enough to matter, but if enough of those variances stack up, who knows.

That all being said, I buttoned up the unit "as-is" and I'm moving on for now. I need the bench space back and this'll give me an opportunity to use the power supply for some other projects, so we'll see if the stability issue rears its head again. At the moment, I'm only experiencing slight drift (~0.25 V on the panel meter) over a 20-minute warm-up period, which is probably within the design spec and not much of an issue on the larger range. To recap, I replaced Q12, Q14, and Q15 with new 2N2907s and swapped the adjustment pots with regular resistors.

I may see about improving the meter circuit with either a modern matched transistor pair, the instrumentation amp duak pointed out, or perhaps a digital conversion with an ICL7107... all with increasing levels of time and effort.

[xavier60]

The 4.2V zener would be shunting some of R75's current to the 12V rail while still leaving just enough current for Q15's Base.
Higher voltage zeners allow more of R75's current to flow out of Q15's Base and through R62, increasing its voltage drop and reducing the current available to the amplifier.
It would have been better to have reduced R62 to rule out possible problems being caused by Q15.

[duak]

G'day y'all,

Xavier's pointed out what's happening with Q15 when VR6's voltage is increased.  In other words, higher VR6 voltages lead to the voltage at Q15's emitter dropping relative to the meter circuit's reference point  (+ Vout) and so it can't source as much current to the diff amp as before.

VR6's cathode is indeed connected to +12.4 V in the 6284A.  If VR6 were reversed it would act as an ordinary silicon diode and conduct with a forward drop of about a 0.6 V.  Q15 would turn on slightly and pass a far lower collector current, probably << 1 mA.