Author Topic: Teardown : Fluke 845A/AB/AR nullmeter/HZ voltmeter tweaks and mods (and repairs)  (Read 79350 times)

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Offline dietert1

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Yesterday i worked on a FLuke 845AR. It is working very well now and shows small movements up to about +/- 50 to +/- 100 nV in "ZERO" mode, which seems to be normal or lower than normal.

Renewed both neon tubes and changed the 10.5 V shunt regulator (meant as a battery charger) to a regulated supply using a LM7810 plus one diode offset. R202 became 10R and C201 gets 24 V now. I also replaced all carbon resistors as proposed by TIN. Most of them were off tolerance, especially those of 100K and above. One 680K had arrived at 790K. Removed the transistor sockets and renewed all caps, replacing 6.8 uF, 2x 10uF and 15 uF by film capacitors. Didn't do any adjustments yet.

I already noticed extra noise from mains/guard connections, so maybe the device needs batteries and/or some mains filter.
Should we replace R127 and CR103 by an AD587? What did others try to reduce noise?

Regards, Dieter
 

Online Kleinstein

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I don't think CR103 is a major noise source and thus not much gained from replacing it with a lower noise but expensive AD587. Zener noise can be quite different between types.  A TL431 + resistors could be an option too if the old zener should turn out too noisy. There is a change to get lower noise from 2x5 V zeners instead of 1 x 10 V. However this likely also comes with a different TC ( usually negative with 5 V and positive with 10 V).

Chances are other parts will dominate the noise: The resistors in the input filter, Q101 as a BJT is not the best choice for a high impedance signal and the neon lamps even if new can cause jitter when starting. The new neons and replacing some resistors could have already helped with slightly lower noise.

So to get really lower noise is would be more like a new design.
 

Offline dietert1

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Today i looked at the AC amplifier output (TP7) and found a tremendous noise level of about 12 Vpp with occasional limiting clearly visible on the scope. Limiting hurts. When running the device from an external power supply noise level went down to less than 50 %, no longer limiting. Output noise went down to about +/- 25 nV level, which is almost perfect considering the visual resolution of about 10 nV in the 1 uV range.

Will put two 10 KOhm resistors in front of primary side of mains transformer and see what happens. Such resistors are very effective RF blockers, too. Power consumption on the 10.5 V is 30 to 40 mA.

The photo resistors with their light guides are extremely sensitive to the slightest scatters/knocks, maybe even microphony. Don't know yet how to improve that.

Regards, Dieter

PS:
With R202 replaced by 10 Ohm, my LM7810 based 10.5 V supply runs perfect with 2x 4K7 in the mains connection. This is a 230V configuration and the instrument gets almost a factor 2 more quiet with this change. So there is magnetic coupling from the mains transformer into the guarded section.

The +/- 15 V guarded supply is one weak point of the design. The low voltage secondary of T202 has a resistance of 630 Ohms. The saturation driven 84 Hz oscillator shows strange changes of frequency and duty cycle depending on what the meter outputs. Apparently the isolated output circuit takes a good fraction of power, especially in out of range condition. For the time being i just put 15V zeners parallel to C117 and C118. This works well as long as there is no out of range condition.

I think what is causing noise is the missing 84 Hz filter before the sampler. In this type of synchronous detector one either uses a linear mixer driven with a sine wave or a narrow band filter followed by the sampler. Sampling into wide band noise like in the Fluke 845AR isn't the best design. Maybe changing some caps can make the existing AC amplifier more narrow band.

Also i don't yet understand the amplifier gains. The AC amplifier Q101..Q103 makes about 4700x (47K/10R). The AC amplifier Q104..Q105 makes about 220x (22K/100). The DC output amplifier makes 260x (3M9/15K). So in total there are 2.69 E8, which is 269x more than we need to get from 1 uV to 1 V. You want some negative feedback reserve, but maybe the instrument can work better at lower gain. Will try that, too.

And the sensitivity to mechanical touch/shock is from the light guides, which are free to move about 1 mm in my instrument, with terrible effects on the measurement.
« Last Edit: January 12, 2020, 10:31:16 am by dietert1 »
 

Offline dietert1

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Circuit revision revealed another problem. The operational amplifier formed by Q107 .. Q112 has the isolation drivers Q113 and Q114 as load. This load generates 200 to 300 mV peaks on the opamp output that go directly to the inverting input (base of Q108) via the integrator cap C116. The opamp can't perform well like that. The problem is easy to cure though. I used a bipolar 22 uF cap as buffer mounted close to Q113 and Q114. Another mod is a transistor gate limiting the integrator output to about +/- 1.3 V in order to avoid +/- 15 V supply breakdown.

I also put LEDs instead of neon tubes and did some other mods mentioned in this thread, like tuning the capacitors for limited bandwidth. I also put a p-channel FET as sampler (replacement of Q104). The instrument is slowly becoming something useful.

Regards, Dieter
 

Offline Dr. Frank

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Hello Dieter T.,
please explain your use case for your improved 845AR.
I mean, it was intended for bridge nulling, with drift and noise parameters as is, so giving 200nV..500nV safe zero indication.
The undisclosed parameter was extremely low bias current in the fA range.
This instrument was built for alignment of 335D, 720A, 752A, but not much more, I think.

So I'm extremely curious what application you have in mind for an even improved 845A.

I fully agree with you, that this 1967 design has lots of disadvantages, which could easily improved, as you have described. Maybe you could more precisely describe, including a quick schematic, which components you have replaced, and how.
In particular, I didn't fully understand how you changed the 10.5V circuit, and where you placed this bipolar 22µF capacitor.

Thank you!

Frank
 

Online Kleinstein

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I would not expect leakage much below a few pA. Still quite good, but AFAIK not much information on this.

The 10.5 V part should be relatively easy: just replace the shunt type 10.5 V regulation with CR207, C201 and R202 with a filter cap and linear regulator. This should be easy and may reduce the heat generated.

Getting some pulses, interference from the chopper for the isolated output also makes sense. The simple buffer with Q111 and Q112 just has quite some cross over distortion and thus can not react fast to load changes. Not sure if just a cap at the output is such a good idea - I would at least include some series resistance.

The 845 circuit is old in many aspects, so not sure if local fixes can really improve the performance so much.

By modern standards there are several not so ideal points in the circuit: the neons (flickering, aging), a BJT input AC amplifier, the rather large caps at the input (and thus even with PP caps some input current from DA), the 10.5 V shunt regulator, the Q111/Q112 buffer with large dead zone, BJTs for chopping (need special types with high reverse UBE tolerance), C114 +C116 as electrolytic caps, rather high power consumption, combined power and sampling clock (this also has a few good points).

So it may be more logical to use a new design and keep the 845 close to original as an antique example of it's time.

P.S:  the gain of the amplifier without the final feedback needs to be high so that the final feedback has something to compensate errors. Only a factor of 260 extra is not that much. So less gain is not a good idea. With a better DC amplifier and demodulator, there could be less AC gain and more DC gain.
« Last Edit: January 15, 2020, 09:14:56 pm by Kleinstein »
 

Offline dietert1

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A very similar modern circuit can be found here, see Fig. 5:

http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdf

The author claims low noise (8 nVpp).
Or one can buy a Keithley nanovoltmeter for several thousand.
Or one uses a ADS1256 that includes a chopper stabilized amplifier.

My intention is to update the Fluke 845AR to a similar low noise level. Using the old Fluke there is a cabinet with multiple shields, a meter, a power supply, a range switch, low thermal EMF connectors, output isolation. I don't see any reason why it should not perform to todays standards when using modern parts. Power consumption isn't that bad, the whole device needs about 300 mW, so it can be run with batteries. And i think someone in this forum measured input current to be 15 fA (MisterDiodes?). Changing the input stage from bipolar to FET in order to reduce current noise should be a simple exercise. I got some 2SK369 FETs from another project, where 8 matched FETs where  selected out of 20. Yes, i can show my schematic when i'm ready.

Today i found that reducing AC gain just reduces the speed of the instrument (integrator gain). The needle will creep, no real improvement.

Regards, Dieter
 
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Offline valley001

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Hi all, great thread here.  Still absorbing much of it.  I have also been working on an 845AR, I hope it is ok to join in on this. 

I have recapped mine and installed new neon's that seem to be stable for the moment at least.  The unit seems to function correctly. 

As I work with the unit I notice it is sometimes difficult to zero the 1uv range, the zero pot is too sensitive and getting a good absolute zero is difficult due to under and over shooting the spot.  I have checked it against the specified "at least" +/- 5uv and see that in practice mine gives +8 and -6 or so.  I realize this is technically in spec but I would like to to be slightly less sensitive.  Is there a component I can change to slightly reduce the sensitivity of the zero pot?

« Last Edit: January 17, 2020, 11:27:47 pm by valley001 »
 

Offline dietert1

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If you increase R116, for example to 2M2, sensitivity will be less. I did this on my instrument and i think i will increase it even more. You loose the capability to compensate some microvolts of thermal EMF, but that doesn't help anyway. Thermal EMF isn't a stable offset to compensate.

The adjustment potentiometer R117 itself is a little suspect concerning noise. It has 5M, probably a carbon resistor and probably increased to 7 or 8M after all that time. Will probably renew it with a much smaller resistor. As recommended above, one should change the bias generation +/- 0.5 V to something stable, instead of taking it from the AC amplifiers. Currently i am using two resistor dividers to get the +/- 0.5 V from the +/- 15 V.

Regards, Dieter

PS: On my Instrument R117 just measured 8.26 Meg.
« Last Edit: January 18, 2020, 12:04:04 am by dietert1 »
 

Offline valley001

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Interesting so R117 is a problem. Original specification is 5 meg +/- 30% so well out of tolerance.  I hesitate to start measuring all the carbon resistors for fear of going through and replacing them all!   This brings up another question I have, I read back somewhere in this thread that using non carbon resistors (metal film etc) for replacements may not be recommended because carbon resistors are non inductive.  Is this correct?

So, 2.2 meg ohm replace for R116 will reduce how many uv sensitivity of the zero range pot, approximately? 

If you increase R116, for example to 2M2, sensitivity will be less. I did this on my instrument and i think i will increase it even more. You loose the capability to compensate some microvolts of thermal EMF, but that doesn't help anyway. Thermal EMF isn't a stable offset to compensate.

The adjustment potentiometer R117 itself is a little suspect concerning noise. It has 5M, probably a carbon resistor and probably increased to 7 or 8M after all that time. Will probably renew it with a much smaller resistor. As recommended above, one should change the bias generation +/- 0.5 V to something stable, instead of taking it from the AC amplifiers. Currently i am using two resistor dividers to get the +/- 0.5 V from the +/- 15 V.

Regards, Dieter

PS: On my Instrument R117 just measured 8.26 Meg.
« Last Edit: January 18, 2020, 02:45:34 am by valley001 »
 

Offline dietert1

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As far as i understand several people inserted metal film resistors and there was no report about oscillations. So i wouldn't think a lot about that remark of 2N3055.
The zero pot inserts current into the feedback circuit at R114. Near the center of the potentiometer its nominal output resistance is about 2.5 MOhm, so increasing R116 to 2M2 will change the total output resistance from 680K+2M5=3M18 to 2M2+2M5=4M7, so sensitivity in the middle will be down to roughly 0.66. The maximum + and - offsets will be down to about 0.3 with respect to the original circuit.

My instrument gets nulled in the +/- 1 uV range when the potentiometer outputs 1.27 mV. So the chopper has almost perfect balance all by itself and the bias of R117 could be +/- 20 mV - a factor 25 less than the original design.

Regards, Dieter
« Last Edit: January 18, 2020, 06:55:53 am by dietert1 »
 

Offline Dr. Frank

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Obviously, TiN has blown up his unit by or during replacement of all carbon resistors:

Doh, I replaced carbon (which were 5-10% high) resistors with modern ones in my 845AB, and now unit is dead :)
It was working before, just noisy, but now it's just overflow either way and just stays there, on any range.
Battery is ok +9.4V, +15V and -15V are ok too.

My own 845AR sometimes also shows signs of instability.
When I'm in the lowest ranges, i.e. 1 or 3µV, and have a probably noisy input signal, the 845AR just jumps into overload, and does not get back to zero, even if the bridge I'm measuring is probably nearly balanced.
So I have to range to 100µV or higher, and after the 845AR is displaying zero there, carefully go back to full sensitivity, where it really shows a few 100nV only.
I always assumed, that my unit is having problems with oscillations.
The story told by dietert1 now supports my assumption, maybe some carbon resistors also drifted too much.

Up to now, I only replaced several electrolytics and the neons, and didn't want to touch a running device, because I only want to get the alignment of my bridges done, at ~ 200nVpp noise level, and <<1pA bias, which is fully sufficient for me.

Maybe I will have a closer look now, how the different circuit parts are behaving.

Frank   
« Last Edit: January 18, 2020, 09:06:23 am by Dr. Frank »
 
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Offline dietert1

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To solve the instability under overload conditions
- i increased the power supply from 10.5 V to 12 V. I am using LEDs, neons will get higher voltage, too.
- put 15 V zener diodes over C117 and C118
- made a limiter for the integrator that keeps the output of Q111/Q112 within +/- 1.3 V.

Attached sketch also shows the buffer cap close to C113/C114 i mentioned before.

Regards, Dieter
 
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Online Kleinstein

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Taking the reference for offset compensation or some  +-0.5 V from the amplifiers is not that bad. The voltage is essentially a diode drop in both cases. If the needed offset compensation is so small the value does not need to be that stable.

How much offset there really is to compensate, can be different between units (e.g with the balance in the chipper and neons). If the range is sufficient I see no problem increasing R116 to make the range smaller. One could alternatively use series resistors and a lower resistance pot (may be easier to get than a 5 M pot).

The suggested modifications on the supply make absolute sense.  The divider for the base voltage reduces the need for transistors that can tolerate a high reverse base emitter voltage.

If overload to the output stage that effects the supply voltage, it is possible that the circuit gets a kind of second stable point in overload. So the suggested modifications of the supply could fix the problem of the amplifier getting stuck in overload.

One weak point could still be the buffer with Q111/Q112 with a large dead zone and C115 in front.
 

Offline Dave Wise

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I am reminded of my Fluke 887A AC/DC Differential Voltmeter.  Some years ago I noticed that if the instrument saw a transient of the right size/shape, the amplifier in the AC converter would break into oscillation, which would not stop except by turning off the power.  In troubleshooting, I discovered that several transistors were saturating during the oscillation cycle, and their charge storage time was so long that it introduced more than 90 degrees of phase shift.  This turned the overall negative feedback into positive feedback.

I fixed it by preventing saturation, by means of a "Baker Clamp".  Just a 1N5711 Schottky diode antiparallel to the base-collector junction.  When the transistor approaches saturation, the diode absorbs current that would otherwise become excess base charge.

I don't know if your 845 is performing the same perverse trick, but I recommend you check it.

HTH,
Dave Wise
 

Offline dietert1

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The 845AR does not have an AC mode, only DC. Its AC amplifier sits behind the chopper and is well protected from fast transients.

Kleinstein's hint concerning C115 was correct. The DC amplifier (Q107 .. Q112) remains stable when removing that capacitor (at least with my 22 uF buffer cap in place) and of course it works much better. To be safe i put in a 1 nF in series with a 100 Ohm resistor.
Also i added two 22 uF buffer caps for the collectors of Q111 and Q112. The spikes on the output of the amplifier vanished (< 1 mV).
When nulled i measure 0.25 V across each of the 1 KOhm resistors R158 and R159. The whole amp takes about 1 mA.

What worries me are the mains cable that "tunnels" the guarded section and the Null switch located near the mains switch without shield. I think the null switch mount lacks proper grounding. Also i would like to check R114 for thermal EMF, maybe i can find something better. Amazing how well the instrument worked with all these little issues.

Another thing i will try is doubling the chopper frequency, the photo resistors are fast enough for that. The modern circuit i mentioned before supposed to reach 8 nVpp has a 1 KHz chopper.

Regards, Dieter
 

Online Kleinstein

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A higher chopper frequency is mainly needed if the AC amplifier has 1/f noise. The old BJT based version should have very low 1/f noise. The other point would be filtering at the input and possible aliasing. Here a higher frequency helps to filter with smaller caps.
The down side of a higher chopper frequency is that one gets mode charge flow from Charge pumping of the chopper. So the input bias tends to go up. 

The main idea with such a chopper amplifier made from separate parts instead of a ready made chopper stabilized OP is that one can use slow chopping, as one is not limited to small capacitors and a CMOS amplifier with high 1/f noise.

The 845 chopper can never reach the very low noise levels of he circuit here:
http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdf
as there is already way more noise from the 300 K of resistance at the input.
The noise comparison off some 200 nV for the 845AR  and 8 nV for the CMOS chip chopper is in addition using a larger BW for the modern circuit. So the ratio is even larger. The two circuits are kind of different ballparks: the 845 is low bias (though I still have some doubt in the < 1 pA number), but high noise. The low noise circuit should have a higher bias current (more like 20-200 pA), but a much lower noise (e.g. 50 times less at the same BW).
 
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Offline valley001

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Dieter, did placing the resistors on the primary side of the mains transformer have any effect?  I am interested in some of these less invasive modifications for somewhat improved performance. 
 

Offline dietert1

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Yes, soldering in the guarded section is a bit tricky. The board is two-sided and has several vias not plated-through. For example when you unsolder C115 and insert nothing, the instrument no longer works because one of the wires of that capacitor is needed as a via. The board does not have solder masks. Quite easy to rip off traces when unsoldering parts or to produce shorts.

When i installed the resistors into the mains connection (on the other board), noise at the output of the AC amplifier came down from 1.5 Vrms to 0.8 Vrms and it stopped clipping. This was with the two upper covers open. Low frequency noise on the DC output should be down proportionally.

@ Kleinstein: I was proposing exactly that: choosing a chopper frequency. Above 100 Hz the device may work better, with some distance from the 100 Hz fundamental present in the power supply currents (magnetic coupling).

Regards, Dieter
« Last Edit: January 20, 2020, 09:13:33 am by dietert1 »
 
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Online Kleinstein

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Dieter, did placing the resistors on the primary side of the mains transformer have any effect?  I am interested in some of these less invasive modifications for somewhat improved performance.
Resistance at the transformer primary acts a little like R202. So one may have to/can reduce R202 to compensate. Resistance at the primary side could reduce the magnetization peaks in the transformer a little and this way slightly reduce the stray field. However otherwise it is rather similar to R202.  If used, the resistors should be well behaved on overload, e.g. fusible resistors.
 

Offline Dr. Frank

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Yes, soldering in the guarded section is a bit tricky. The board is two-sided and has several vias not plated-through. For example when you unsolder C115 and insert nothing, the instrument no longer works because one of the wires of that capacitor is needed as a via. The board does not have solder masks. It is quite easy to rip off traces when unsoldering parts or to produce shorts.

..

Regards, Dieter

@ TiN: I suspect that's why your 845 does not work anymore (since 2016) after replacement of all carbon resistors!
Maybe you take the wrecked instrument out of your repository and solder on the upper side, also.

Frank
 

Offline dietert1

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Maybe in a lab with multiple 3458As you can use one of them as a null meter in case you need one. Even our 3456As perform almost as well a native 845AR.

Regards, Dieter
 

Online Kleinstein

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The 3458/3456 should be lower noise (over the same waiting time), but chances are it would have a higher input current and more current noise.
Also battery operation is not really practical with the 3458/3456  :(.  AFAIK battery operation is not included with the 845AR  :-//, but it sounds very doable and a point I would really like with a null-meter.
About the only really good point of the 845 is that is can have very low bias and low current noise (not so sure about this - it would need to increase the 1 M resistor originally present at the input, as this alone causes some 100 fA/Sqrt(Hz)).
 

Offline valley001

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I notice my unit will drift 300-500nv over night, either to negative or positive.  The specification for "stability of zero" (drift?) is less than .15uv/hour and less than .3uv/day, so I think I may be slightly out of that specification.   At first I thought this may be temperature related but as the house warms up into the afternoon the needle does not travel back to zero.  Where would I look to improve this? ...or perhaps I am too picky.  :-DD
 

Offline Dave Wise

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The 845AR does not have an AC mode, only DC.
<snip>
Regards, Dieter

I'm quite aware of that.  My point is that any amplifier that has feedback and whose transistors can saturate is susceptible to the phenomenon I described.

Can you support your claim that it's possible to run the 845A successfully with 2x chopping frequency?  I have two HP 740B DC Voltage Standard / Differential Voltmeters, and both instruments' photocells are insufficient at 1x frequency.  Yes, the neon lamps are good.  I even tried driving the cells with high-brightness LEDs, tuning intensity and pulse width individually for each cell.  It was not possible to pass the tests for Input Resistance and Efficiency simultaneously; making one property adequate made the other unacceptable.

Regards,
Dave Wise
 


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