Teardown : Fluke 845A/AB/AR nullmeter/HZ voltmeter tweaks and mods (and repairs)

[Kosmic]

And yes, cadmium solder have huge difference on stability.
With normal 60/40 solder I had poor thermometer instead of nanovoltmeter. See second graph. 

So did you used the Sn50Pb33Cd17 solder bar you got recently ? I was wondering if a little bit of cadmium would help.

Nevertheless, If Wikipedia is right, Cd70Sn30 is the real thermal free solder. I don't think it's still made though.

[TheSteve]

TiN any testing done with silver solder?

[antintedo]

TiN any testing done with silver solder?

See this message: https://www.febo.com/pipermail/volt-nuts/2016-October/005023.html. "Silver solder" described by Keithley is Sn96Ag4, they use it in their short for 2182A. According to that post it is not much different from PbSn alloys. I tested samples of numerous alloys at lower dT with unsatisfactory results.

[e61_phil]

Does anyone know if any cadmium solder is used in the Keithly 182?

@TiN: Thank you very much for the measurement!! I was searching exactly that to compare it to my Keithley 182. Unfortunately I don't have a short for my meter.

[notfaded1]

Actually what happens to the neon tubes is called cathode poisoning.  Over time the cathodes sputter material and it collects inside on the glass.  This is VERY common for nixie tubes and other cold cathode tubes.  You're assessment is 100% correct... the tubes were worn out.  Replacing them is easiest and best solution here (they're cheap).  For nixie tubes with cathode poisoning we often crank up the current to exercise the cathodes and burn off built up material sputtered from one cathode to another... in these little NE-2 tubes that won't help much because the glass inside is covered with sputtered material.  Nice to hear the new NE-2 tubes made a significant difference... you're right 1968 technology... pretty neat!  I feel the same way about Nixie tubes, Pixie tubes, Dekatrons and other cold cathode display tubes.  You can totally see from your pictures how the difference is visually obvious.

Bill

[Dave Wise]

READ TOGETHER WITH LATER UPDATES

Here is my take on Fluke's chopper replacement board.

So it doesn't get lost, I'll start with my biggest doubt.
In an original-construction instrument using neons, when T202 terminal 9 goes positive, DS101 lights, turning on V101.  (When it goes negative DS102 lights turning on V102.)
But in an upgraded instrument with Fluke's H11F1 board, when terminal 9 goes positive, I calculate that U5, the H11F1 replacing V101, turns OFF instead of on.
How can this be?  The meter would read downscale for positive input and vice versa.  Did they modify other places?  Why not just make the H11F1 board do the right thing?

It makes my head spin.

Here is my writeup, attached and inline.  It refers to posts by TiN, Jay_Diddy_B, and picburner, especially post #7 and #14, which for me show up on page 1 of this thread.
For easy reference, I also attach pictures and schematics from the above posts.

I'm looking forward to your analysis and comments.
Thanks,
Dave Wise

FLUKE 845A CHOPPER REPLACEMENT BOARD
David Wise
September 2019

The Fluke 845A, 845AB, and 845AR Null Detector was designed with
a photocell chopper.  But see the NOTE in the Change/Errata pages
of 845A_AB_imeng0000.pdf .  (This is pdf page 3.)  Transcribed below.
See https://https://xdevs.com/fix/f845ab/
See https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)
Photos at https://doc.xdevs.com/doc/Fluke/845AB/img/f845_chopt.jpg and
https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/
reply #7 March 3 2016 by Jay_Diddy_B, attachment "green board.jpg" .

NOTE

A major circuit modification has been made to the Fluke
Model 845AB.  This change entails the complete redesign
of the Photo Modulator, previously based on
photo-conductive resistors.  The new circuit now employs
optically-isolated Bilateral Analog FETs, and is
fabricated on an additional circuit board.

Updates to the relevant sections of this manual are now
in process and will be available for shipment
approximately december 1993. This manual is completely
accurate with regard to specifications and operating
instructions. The circuit modifications are totally
internal and are completely transparent to the user.

End quote.

Nobody has the new manual.
One guy (TiN at xdevs and eevblog) has an updated instrument.
Another guy ("picburner" at eevblog) drew up a partial schematic,
https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/
reply #14 March 7 2016 by picburner, attachment "845A_324_RevB.pdf" .

He got it almost right.

ERRATA

1. U2 pins 3 and 4 are K2 and A2, respectively.  Therefore,
section 1 closes on positive and section 2 closes on negative.

2. For Q1 and Q2, the base resistor is R10 and R11 (1K); emitter
resistor is R12 and R13; pulldown resistor is R16 and R17 (10K).

3. The two H11F1's are U4 and U5.  U5 pin 6 goes to CN8 BLUE.
U4 pin 4 goes to R15 (220 ohms) to CN7 ORANGE to R14 (20K)
to U5 pin 4.

4. U4 pin 6 goes to R7 (1Meg variable) to R6 (562K).
This is offset trim.

5. U3 is misdrawn.  The input (marked pin 1) is actually pin 3.
The output (marked pin 3) is actually pin 1.  R4 goes from pin 1
to ground, not pin 1 to pin 3.  The junction of R5 and R6 goes
to pin 3.

6. I believe U3 is an LT1009 or LM336-2.5, wired as a negative
shunt reference.
Pin 3 is minus, pin 2 is plus, and pin 1 is trim.
It's not an LM385 whose pin 1 is a no-connect.
It's not a TL431 whose pin 2 is minus.

7. R3 is 2K not 1.6K .

PRINCIPLES OF OPERATION (speculative reconstruction)

DS101 and DS102 are removed.
R154 is changed from 33K to 6.5K.
C119 is changed from 0.22uF to 1uF.

Positive pulses from transformer T202 light transistor-output
HCPL2731 optoisolator U2 section one (A1/K1/C1) via R154, C119,
CR106, and ORANGE wire CN1 where DS101 used to go.  Negative
pulses light section two via R154, C119, CR107, and YELLOW wire
CN2 where DS102 used to go.  Where C119 used to create dead time
between DS101 and DS102 lit periods, now it ensures equal timing
even though T202's negative and positive peak voltage may be
different.  The increased load on T202 reduces its peak voltage
to less than U1's max rating of 36V.

Positive pulses also power U1 (an LT1120A regulator)
via RED wire CN5, rectifier CR1, and reservoir/bypass C1 and C7.

We can't read the printing on R1, so we don't know Vcc for sure,
but it ought to be less than 18V, HCPL2731 optoisolator U2's max
rating.  It's probably just 15V, being a separate supply to
keep current pulses off the original +15V unregulated rail.
(It was okay to use the old -15V rail to power U3, because
it's a constant DC load.  +15V means that R1 is 1Meg.

Reducing R154 increases the current driving the LEDs inside U2.
The new R * C product is the same as before, but this is misleading;
with the low start voltage of the LEDs, each LED is lit for the
entire half-cycle.  All the dead time comes from C4 and C5,
which also give a soft turn-on.  (I don't know if that's
important.)

When T202 goes positive, U2 section one is lit, shorting
C1 (pin 7) to ground, discharging C4 and extinguishing U5.
Meanwhile, C2 (pin 6) opens, allowing C5 to charge and Q2
to light U4.  The same action occurs on the opposite
components on the other half cycle.

Note that U5 is turned off (and U4 turned on) when DS101
used to illuminate V101, so the modulator and demodulator
have the opposite phase relationship to original construction.
This would reverse the meter polarity, so I am obviously
missing something.

C4 * R9 = 0.93ms .
With 15V, at phase begin, C4 or C5 charge at 16V/ms or
about 15mA/ms, but that doesn't continue.  After 1ms we're
up to about 10V or 7mA which is still way past U4/U5 turnon.
Factoring in transistor and LED diode drop, we probably get
an actual dead time on the order of 100us, which is plenty
for the H11F1.

U3, an LT1009 or LM336-2.5 -2.5V shunt reference, provides
offset trim to U4 and U5.  Minus pin 3 is powered with 6mA
from the -15V rail via R3.  Plus pin 2 is ground.  Trim
pin 1 is set by voltage divider R4/R5 which is wired across
Minus and Plus.  I presume this zeroes out U3's temperature
coefficient, since they are two different resistor types.

U4 and U5 act as an SPDT switch with dead time in the
middle.  U5 pin 6 or CN8 BLUE connects to the input
voltage on C103 in place of V101; U4 pin 6 or CN6 RED
connects to return/feedback in place of V102; their
junction, CN7 ORANGE, goes to the AC amp at C104.

When lit, U4 and U5 produce on the order of 100uV of
offset voltage, with source negative and drain positive.
Asymmetrical resistors R14 and R15 forming the common
point between U4 and U5 transmit the respective offsets
unequally, resulting in a net positive, which current
from U3 via R6 and R7 trims back to zero.  The trim
current is adjustable from 1.6uA to 4.4uA, developing
16 to 44uV across R114.

[Kleinstein]

Swapping the switches at the chopper would do more than just change the polarity of the reading. This part is inside a feedback look - so a reverserd polarity would cause wrong polarity feedback and thus not work at all. Chances are the H11F1 and neons would use a different transformer as they also need a much lower voltage.

[Dave Wise]

Oops, you're quite right, it would turn negative feedback into positive feedback.  That never ends well!

This board was intended to be an in-place retrofit.  I suppose they could have changed things elsewhere - after all, they changed R154 and C119.
I don't know if this was only applied to new production, but similar things were done as an upgrade performed on instruments brought in for service.  My 887A is an example of the latter, 1960's date codes everywhere, except one board from the 1990's.  (Different board but the same idea, ditch the old chopper.)

I don't think they changed T202.  The daughterboard has design elements that suggest it was working around characteristics of existing parts.  The only thing I know they did - and as far as I know, the only things they had to do - was
(a) Fit the board instead of DS101/DS102/V101/V102, and
(b) Change R154 and C119.
If you gloss over the phasing issue, that's all they would have had to do.

It would have been trivial to lay out the board with opposite phasing, so why do it wrong and then correct it elsewhere?  I just want to know if I've lost it.  Can you trace through and check me?
Here's a trace on the original circuit.

Let's say the input is positive.
V101 passes the positive when DS101 is lit, so the square wave phase is positive when DS101 is lit, negative when DS102 is lit.
The AC amp inverts, so it presents a square wave to Q106 that's negative when DS101 is lit and positive when DS102 is lit.
When T202 is positive at terminal 9, lighting DS101, it's negative at terminal 7, grounding Q106.
Therefore, negative peaks are grounded and positive peaks are passed.
Therefore, positive input to the 845A means positive input to the DC amp.
The DC amp is noninverting, so the meter reads upscale.

Thanks,
Dave Wise

[Dave Wise]

Here's my marked-up schematic, goes with my writeup and reflects the errata there.

[picburner]

Thanks for the corrections, I post the updated pdf.

R5 is connected right.

From an pix posted by TiN, even if blurred, I can read for U3 "LM" but I can't see the numbers.
Only a few unknown values remain that only those physically own the board can say what they are.

[Dave Wise]

Thanks for the updated schematic, picburner.  The only thing missing is annotation on the input/power leads.

ORANGE replaces DS101
YELLOW replaces DS102
GREEN is ground (at DS102)
BLUE is -15V
RED is T202 terminal 9 i.e. red wire in cable

I looked at every voltage reference Mouser knows about that begins with LM.  Rejecting parts where pin 3 is NC or pin 2 is minus (i.e. LM329, LM285/385, LM431, LM4040/4041), we are left with LM336 only.  It has 2.5V and 5V versions.

I still think the phase has gotten reversed, please review and help me figure out where I've gone wrong.

Dave

[picburner]

It could be that, instead of connecting the "HV" side of T202 intended for neons, they connected the other side of the secondary of T202, the one at low voltage, since the optocouplers require a lower voltage.
Someone on "volt-nuts" made a similar change.
This would explain the phase reversal but I can't see if it is so only look at the pics.

I have a doubt about the R3 value, maybe it's too low, it would load the -15V line very much.
Looking at the pic better the colors would suggest that it could be 20K or 200K.

[Dave Wise]

Pictures clearly show the orange and yellow wires soldered in place of DS101 and DS102.

The LM336 wants between 600uA and 10mA.  20K is not enough when you factor in the load current of R6.  200K no way.  2K draws a comfortable 5mA or 7.5mA, what's wrong with that?

[TiN]

I can take few more pics of chopper board, as I found 845AB guts while cleaning up the place.

[Fred_47]

The LM336 wants between 600uA and 10mA.  20K is not enough when you factor in the load current of R6.  200K no way.  2K draws a comfortable 5mA or 7.5mA, what's wrong with that?

I greatly appreciate the work you and picburner have done to document this 'secret' Fluke mod.

The 845 ckt is practically identical to the null detector used in the 335A/D and suffers the same problem with aged neon lamps in the input chopper.

I've been working all summer on a replacement for the neons or the entire photochopper in my 335D.

Initially I looked at MOSFETs similar to the Keithley 155 but I couldn't get rid of the switching spikes. Then replacing the neons with LEDs. I actually made a new block out of Al to replace the neon blocks and designed a constant current ckt to drive the LEDs. That sort of worked but there was an offset beyond the null zero capacity. I blamed that on the difference in the photo resistor on/off resistances. V1 had a much larger swing than V2.
 
Right now I have H11F3 optocouplers with the diodes driven thru a constant current circuit powered by terminal 7 of T1 (T202 in the 845). The offset is beyond the range of the zero null control in the positive direction on ranges 1mV and higher and in the negative direction for the 2 lowest ranges on the 335D, 100uV & 10uV.

I've attached my latest schematic.

Could Fluke have hand selected LM336's that work at low currents, or just ignored the minimum current?

As part of exploring alternate chopper lamps (LEDs) I measured the ±15v supplies at different loads. Winding 7-8 of T202 (T1 on the 335A/D null det PS board) has a high DC resistance, ≈600 and terrible regulation. Several posts upthread comment on the low voltage. Winding 8-9, which powers the neons, is about 4k .

My testing indicated a voltage droop of 1-2V per mA of load on each of the 15V supplies.

Other testing indicated that the meter ckt takes a about 1.25mA for full scale deflection. The meter in the 845 is different from the one in the 335A/D so I don't know if this carries over.
 
Since these transformers (and the meter) are the ultimate unobtainium, I've been very leery of overloading them.

Can others measure the DC resistance of their transformer to confirm, or deny, my measurement?

edit to correct T202 was T201.

[dietert1]

When i experimented with optofets i made the mistake of giving the LEDs currents of 5 to 10 mA, like other opto couplers. It works much better at 1 or 2 mA. Probably you also want to have separate adjustment of the two LED currents in order to avoid selecting optofets.

Regards, Dieter

[Fred_47]

When i experimented with optofets i made the mistake of giving the LEDs currents of 5 to 10 mA, like other opto couplers. It works much better at 1 or 2 mA. Probably you also want to have separate adjustment of the two LED currents in order to avoid selecting optofets.

Regards, Dieter

At this time I'm controlling the LED current with a jFET in a bridge rectifier, similar to the preregulator ckt used in the 335/332/A/D. I've set it to 2mA. Being able to individually control the LEDs may be a necessary change.

I've been resisting the use of brute force, i.e., using a nulling ckt to overwhelm the offset, in favor of fixing the source of the offset (if I can find it). Apparently Fluke didn't think that is the way to go. Maybe there are so many different ways that the ckt could be off that they decided to use a one fix fixes all ills approach.

Looking at Fluke's optoFET ckt., I'm wondering what is the purpose of R14 & R15 between the FETs?

[Dave Wise]

R14 and R15 intentionally generate an offset, so that the trim circuit can be unipolar and still cover a range from left of zero to right of zero.

[notfaded1]

So the neon bulbs arrived today. Here is a direct comparison of the new AC3/NE2U (left), the originals bulbs that were installed (middle) and the Barthelme 82305 (right). However, the Barthelme 82305 do work for me. Enough spare parts for the future though

-branadic-

banadic-

Did you ever try the new longer NE2U spec'd in the manual in the 845AR?  I'm curious because I have the same exact box of 10 tubes coming and I would think, from my experience with neon, that they would produce more light.  BTW with neon they're called tubes not bulbs FYI.  That's a querk for neon lovers and common error for neon newbs.  https://en.wikipedia.org/wiki/Neon_lighting

Thanks for all your work on this... it saves me a LOT of time reinventing the wheel.  Also I appreciate you EE's putting up with me.  I don't have the EE experience you all have but... I love this stuff and it takes my mind off the computer science, networks, and security I have to deal with during the day at my real job.  I know I've irritated Dr. Frank a couple times.  I hope you guys can forgive me.  I love all the graphs, pictures, and videos people post but most especially the measurement data and tips on how to fix stuff. 

Bill

[Dr. Frank]

I'm not aware that you irritated me, so don't worry...

Maybe you can check these NE2U on your own, first if the dimensions are ok, and second, whether they show that Dark Effect. This test is easy to do, like I have described.

If they show no Dark Effect, just plug them in.

Frank

[branadic]

notfaded1

I bought AC3/NE2U, but haven't tested them in circuit, as the Barthelme 82305 already worked for me. See also this post https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/msg2597721/#msg2597721

-branadic-

[notfaded1]

Thanks I figured that's what you did... well I'm going to try them when they come so we'll see what happens.  Also a maybe dumb question.  I think I know the answer.  Does (on the low ranges) the needle jump up if you get you hand near the inputs while in OP mode?  I printed out the full manual... it seems to really really help on this device.  I knew exactly what you were talking about testing with shorted inputs and the full deflection... it's all in the manual.

Best Regards,

Bill

[branadic]

It does, though it depends on how you put your hands on the inputs. If you put it symmetrical to both input connectors (high and low), the needle stays almost where it was.

-branadic-

[guenthert]

Does (on the low ranges) the needle jump up if you get you hand near the inputs while in OP mode? 

  That would be either due to thermal EMF (if nothing is connected to the inputs that shouldn't happen, as the junctions which could then contribute would be inside the case, protected from the air movement) or electrcal fields.  The Fluke is shipped in a configuration with 1MOhm input impedance, but can be reconfigured to high impedance (>100MOhm iirc).  The manual probably has a performance evaluation section, which might have a subsection determining the input impedance.  If the instrument is new to you, it might be worthwhile going through that in any case.  There should also be a section with grounding/shielding recommendations.

  I always wondered why Null meters have their inputs next to the operational elements ...   I think Keithley got it eventually right with a 'remote' Nanovoltmeter, separating pre-amplifier and user interface.

[notfaded1]

Here's the datasheet for the various NE-2 type neon tubes.  The F845AR manual calls for NE2U neon tubes.  The stats vary on these tubes.  An interesting thing with these, which makes sense, if you feed them DC then one of the metal bars inside (acting as the cathode) lights up.  If you feed it AC both sides light up because the bars take turns being the cathode lighting on the alternating current.

Bill