Fluke 8100A Nixie Tube DMM

[WaveyDipole]

I ran it with values as follows:

Small signal AC analysis
AC amplitude: 1V
Type of sweep: Linear
Number of points: 50
Start frequency: 10Hz
Stop frequency: 100MHz

Result attached.

Note: the regulation circuit on the LHS is not connected.

[Kleinstein]

Usually one uses a log frequency scale, not a linear one. So the relevant part (more 10 kHz to 1 MHz, but it is good to have also parts on both sides) may be missed.  The current source symbol looks a bit odd, but hard to see with the resolution.

[WaveyDipole]

The software has two symbols for a load: load and load2. The first has a symbol like a resistor with arrow through and is the one I used. The second is a symbol with a circle, similar to the voltage or current source. I have now changed this for the latter. As for the type of sweep, I followed an online video and selected 'Linear', but the default is 'Octave', then in turn 'Decade', 'Linear' and 'List'. Each one produces a different graph.

I have been thinking about why, when I power the meter in quick succession (i.e no more than, say, 5 or 10 mins between power ups) the rails stays at 7.01V, but when it is powered up after a longer time, e.g. an hour, or especially overnight, the voltage rises significantly. Typically, if I leave it for say, 15 mins, the voltage will have risen to maybe 7.5V and takes just a few seconds to drop back to 7.01V. After 30 mins its at maybe 8V and if left for an hour, its 8.49V typically by which time it takes maybe a couple of minutes to drop back down. If left overnight then first thing in the morning it will be over 9V and takes several minutes to level off to 7.01V again. What's the difference? Residual charge in capacitors? Then wouldn't the voltage drop rather than increase? If its capacitors discharging over time, then why isn't the effect reproduced by manually discharging all of the electrolytics before powering up?

On the other hand, if its because of oscillation (and I do not dismiss that possibility), then wouldn't I see something on the oscilloscope? Is the probe damping it?

[Kleinstein]

The oscillation may be quite high in frequency (e.g. maybe even 100 MHz range) and depending in where one probes (e.g. at the 7 V output) one may not see it. Probing at parts closer to the critical stage may dampen it enough so that it may not oscillate - if at all it is boarderline oscillating when cold.

For the things that change with a power up, it can be the temperature or some parts (e.g. the electrolytic capacitors as temperature effets the ESR). The are also internal changes in the capacitors possible (e.g. build up of oxide layer).  I agree that it is strange that the circuit behaves this way and as it look both with the original ref. and a makeshift replacement.

[WaveyDipole]

I took a step back for a few days but decided to have another look it today and I think I might finally be getting somewhere. When I previously removed the reference and replaced it with a transistor and 5.6V zener,, I found that the output stayed high at around 9.4V making it seem that the situation had been made worse. Replacing the reference seemed to improve although not solve the problem. However, looking back at my notes, rather embarrassingly I discovered that I had fitted the zener incorrectly. The orientation of the transistor was fine, but when soldering on the zener I had forgotten to take into account the fact that when fitting it to the underside, the connections on the flipped board would be in reverse. I had re-checked everything at the time, but somehow still missed this. Therefore the first thing I did was to remove the reference again and replace it with the same transistor and 5.6V zener diode that had been used the first time around, but making sure that the cathode of the zener diode was soldered to the correct pad.

On powering up, the rail started at around 7V, rose quickly to around 9V and then over 5-6 seconds sunk to 6.53V. I was initially disappointed, but then realised that this was different from previous attempts where the excessive voltage was present immediately at power on. A settled level of 6.53V was fine because this is approximately what the SPICE model had predicted for a 5.6V zener diode. I left it powered down for around 30 minutes and then tested again. The rail immediately came up at 6.53V. I have since left it powered down for over a couple of hours and have just tested it again and this time also, on the instrument being powered on, the rail started at 6.53V and stayed put.

Query: might the effect observed on first power up after fitting the alternative parts be down to residual charge in the smoothing capacitor? The zener initially clamps the rail voltage, but then becomes "saturated" by the residual charge held in the capacitor until it discharges to an appropriate level, whereupon it settles at the expected voltage?

Whatever the case, so far it hasn't done that on subsequent power-ups but has started at exactly the expected voltage. I will test it again after a couple more hours and again in the morning just to be sure, but if the rail still immediately comes up at 6.53V, then the fault must evidently be inside the reference. Incidentally, in the meantime, I have been experimenting with the SPICE model and found that severing the connection between the zener and the emitter on the transistor, or at the transistor base, results in an elevated output voltage of around the level being experienced. Could it be that one of the bonds inside the reference is being affected by the reference warming up? This did have the "feel" of a thermal problem, although one that has been rather elusive and inconsistent with whatever else I was doing on the board in order to trace the problem.

UPDATE: Noooooo! Its back to the same behaviour again this morning. Really thought I was on to something this time!

[WaveyDipole]

With a bit of help and the assistance of some air duster spray I was eventually able to determine an intermittent track between Q10 and R17. After this was resolved, the reference worked much better and the -18V rail was now being regulated and no longer high, even when being powered from the mains. However, the behaviour was not quite what it should be. The reference was now taking a lot less time (about 5 seconds) to settle. It turns out, rather embarrassingly, that the circuit around Q10 and particularly perhaps Q10b/Q11c is acutely sensitive to the cold damp air in my "workshop" shed, which is going to be particularly the case first thing in the morning. Once brought inside into the the warm and dry and left for a few hours, it worked perfectly OK, starting at 6.99 volts every time, no matter how long the meter was left before powering up again.

The reference can now be considered fixed and the voltage on 7VR has been adjusted to exactly 7V with the help of a 7 digit meter. The 15V and -18V rails are around 20mV each adrift from nominal, but I presume this is not that critical.

The display no longer flashes random digits and stays at zero, but still does not respond to input. I have started to look at the input buffer circuit but as yet I have no further observations to report as yet other than there seems to be no output from A1. Its output just seems to stay put at around 0.6V.