[time-nuts] HP5065A C-field current is temperature sensitive

[timeok at timeok.it]

Hi ,

I think the Regulator Board should still be questioned.
To calibrate the +20 V,it is necessary to use an Extender Board because the calibration trimmer is not accessible with the card fitted normally.
I have used the extender HP built for service model 5060-0049. After alignment I could see that the +20 Volt  measured between the card A9 and A14(or other positions) have a variation of +50mV removing the extender.
I repeated the test several times with the same result.
The solution is to adjust the voltage with the extender to 19.95 volts and then remove the extender but a better solution will be to have a muti-turn trimmer accessible without removing the board.

Luciano

On Thu 20/08/15 10:55 , "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:

> --------
> In message ,
> Bill Byrom writes:
> 
> >I don't have an HP5065A and can only see the portion of the schematic
> >you copied.
> 
> The manual is on K04BB if you want the full monty.
> 
> >Here is
> >what I can surmise from your measurements (assuming you don't have
> >any ground loops or measuring instrument or test lead thermoelectric
> issues):
> 
> The measurement setup isn't optimal, I can only do one measurement
> every five seconds on each point and the HP34972A is only a 6½ digit
> instrument, but otherwise the setup is solid.
> 
> >The change of the 20 V power supply when you lock is roughly 400
> >uV (20 ppm).
> >
> >The change of the C-coil current isn't really repeatable. I see a big
> >500 nA (111 ppm) drop at 1,000 seconds, but no such clear change earlier
> >in the test.
> 
> It didn't actually lock until 1000 seconds, so one difference is that
> the logic "continous operation" logic didn't reach final state and
> its lamp didn't turn on in the previous attempts.
> 
> >But the actual voltage changes you are measuring seem to be
> >roughly correlated between the 20 V power supply and C-coil current
> >sense resistor changes.
> 
> As they should be, because both are derived from the same A15CR5 zener.
> 
> >I'm guessing about these values, based on converting your current
> >numbers into voltage based on a perfect R10 || R11 parallel combination
> >of 691.6 ohms. I see that R10 has an * asterisk, and I wonder what is
> >shown for that note. R10 might be a selected value at manufacturer, or
> >it might have a specific temperature coefficient.
> 
> The asterix means "selected". I have not been able to figure out
> what criteria it is selected for.
> 
> >The temperature coefficient of the resistors may be much more important
> >than anything else, especially for an old product.
> 
> All the important ones are wire-wound, probably for exactly that reason.
> 
> >those old electrolytic capacitors (C4, C6, and C7 for example) are still
> >OK, or whether they are showing any changing leakage currents. You might
> >want to change them with new capacitors just in case.
> 
> Good point.
> 
> >So I'm not convinced that the time curve is showing a correlation based
> >on the 20 V power supply affecting the C-coil current.
> 
> No, the main correlation is via the common zener, but the step at 1000s
> is *not* present in the zener voltage, which means that the current
> generator has really bad supply sensitivity.
> 
> >It's possible that measuring system errors (such as where you connected
> the
> >measurement system ground) might cause some of these changes. So you
> >might want to check your setup and the instrument and test lead
> >computed accuracy.
> 
> I think the way I've done it is OK. The 34972A has floating inputs
> and I measure from a local GND for all six points. I'm not seing
> any noise-artifacts.
> 
> >I also disagree with your estimate of the CR5 zener current. [...]
> >[...]
> >the CR5 diode current must be (12.2195 - 4.2456 mA) = 7.9739 mA.
> 
> Good point.
> 
> >That's
> >pretty close to the zero temperature coefficient current for the 1N938
> >you describe. So I do not recommend changing the diode current.
> 
> The 7.5 mA optimum is from a much later data-sheet, and may be for
> a particular "high performance" variant of the 1N938, so there is
> no guarantee that there even is a zero-tempco current for the one
> in my HP5065.
> 
> Either way, fixing the zeners tempco is only half of the solution,
> it looks to me like the "real" solution involves an entirely new
> C-field current driver.
> 
> --
> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG | TCP/IP since RFC 956
> FreeBSD committer | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by
> incompetence.
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