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

[TiN]

Got a new toy..
Since mostly these units require some tweaking and modifications, I think this should belong to here, rather than just repair section..



Really clean and nice condition outisde:



Let's see inside:



Guarded area:



Woohoo 

I was expected to see infamous neon tubes, light guides and photoresistor chopper amplifier, but was greeted insteal with alien-looking green PCB with micropower regulator Linear LT1120CN8 and few HP 2731 optocouplers(?) !
LT1120 dates with 1993 week 52. Is it that optically-isolated Bilateral Analog chopper, which mentioned in Fluke 845AB manual's note in 7/1993 print?

Repairs

* Meter does not read anything in BAT OPR mode. It's just dead on zero, like no power is supplied, even though BAT CHK shows good battery
* In LINE OPR mode meter is unable to zero on ranges below 30µV

First issue was resolved by fixing blown track on power PCB.

As usual with equipment with age over 15 years I replace all electrolytic capacitors to avoid electrolyte leaks and further desctruction.



There is plenty space inside, so I just used usual good quality Chemicon radial capacitors instead of axial. I used 470uF 63V instead of original 470uF 25V and 6.3V caps.



Green PCB with chopper assembly have blue multiturn trimmer pot, accessible from the side. That pot adjustment allowed to shift zero, so now meter can be zeroed on lowest 1 µV range.

Test



Settings: Fluke 845AB Range: 1 µV, OPR switch = ZERO, 3458A DCV auto, 10 NPLC, AZERO ON

I will post YouTube video later.

Perhaps worth to reverse that green PCB schematics?

Future plans:

* Add backlight for meter
* Remove input resistance to have high impedance input
* Study chopper operation ,perhaps replace with something more stable?
* Ultimate mod - replace analog meter with digital PCB module with battery-powered Sigma-Delta ADC to show readings in digital?

+ As usual article with bit more details on my site.

[manganin]

Is it that optically-isolated Bilateral Analog chopper, which mentioned in Fluke 845AB manual's note in 7/1993 print?

Yes. The manual supplied with the modified meters was not updated and therefore didn't match the instrument.

Perhaps worth to reverse that green PCB schematics?

That would be very interesting.

No improvement in specifications, but the new version solved the well known neon lamp issue. The oxidation of the new style binding posts caused some problems because they were plain copper and not gold plated like the old ones.

[Dr. Frank]

Hi TiN,

Great find of this new version, and a reverse engineering is greatly appreciated!

You already have repaired this instrument, as you measure its output?

Then, I also would be interested in the leakage / bias current of this 845AR..
This important parameter is lacking in all specifications, only a high / infinite resistance is proclaimed, due to zero volt input at bridge zero.

I am really skeptical about the bias current, maybe modern chopper amps or low bias bench DMMs like the 3458A , 2002 and 8598A really are on par, at least.

Frank

[Kleinstein]

I won't expect very good performance from the circuit (from xdevs website): there is a rather high series resistance (300 K total) at the input, which provides quite some noise. Also the LDRs won't be very high impedance.

I can understand they did an update to the old circuit. So the impedance might be higher with the modified chopper part.

When it comes to input bias, the low chopper frequency helps. Also as the input range for the amplifier is so small input protection can use just two back to back diode and this way cause less leakage. So the bias / leakage can be really low.

If it really comes to mods - for better performance / stability, this might end up as a build from scratch.

[Jay_Diddy_B]

Hi group,

I had a look at the manual on the ko4bb website. The are changes in the manual to reduce the input resistance on the 1uV thru 1mV from 10M Ohms to 1M Ohm by reducing the value of R104 from 10M to 1M.



There is also reference in the manual to increase the input resistance by removing R104 completely.


I will have to have a look at my 845A.

Regards,

Jay_Diddy_B

[quarks]

Nice score, congratulations. In my my 845 I only replaced the batteries so far. Looking forward to see your video.
Thanks for sharing.

[Jay_Diddy_B]

Hi group,
I opened mine. the unit I have was made in 1974. It has neon bulbs and plastic rods, light pipes.

Here are few pictures of working:





 
[Very Sexy] 

I suspect that the green board in TiNs unit is an oscillator and driver for Vactrol, Led/LDR. I don't think the op-amp is part of the precision circuitry.

Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

I looked at TiN's photograph. I think the green board gets timing information from where the neon lamps used to be connected. The board probably has Vactrols H11F1 Photo FET Opto-couplers where I have indicated. Excuse the scribble writing, I am not very neat using the mouse.



Regards,

Jay_Diddy_B

[TimFox]

I thought I had a bad 845, but found that the input transfer switch needed a mechanical tweak.  The rotate/push mechanism is a simple switch without contacts, that drives the real transfer switch through an insulating rod.  I needed to bend the leaf holding the contact from the front panel connector so that the moving leaf would contact it appropriately.  Before that, I could zero the unit but it did not respond to external voltage inputs.

[Squantor]

Hi there,

Fluke 845A owner but it is still under repair. I completely revised all caps/resistors, replaced the neons by leds and also changed the zero circuit to using two 1.25V references. But still having problem with what I think an unstable zero. I am also noticing when I put the zero completely left or right that the +15V and -15V vary by a volt or so. Anybody else experiencing this? This was before and after the reference modification. Does anybody see a lot of variation in the +15 and -15V in general?

@TiN:
Holy crap, a Fluke 845 with the fabled electronic chopper! Please reverse engineer so I can upgrade my 845 too! It is unusable as it is now... Seems like a more complicated circuit then I expected, lots of components there. I wonder why, if it only needs to drive the optoFET leds... Maybe it has to do with the dutycycle and timing?

PS: I revised all the caps, because they where the old philips variants and they all had the bulge on the underside that makes them dry out. All the resistors where replaced as almost all of them where 20% or more high. A normal problem with carbon composition resistors.

[doktor pyta]

Only for reference here is  measurement of my Fluke 845AR on 1uV range, input shorted. Sampled every 10s, using K2001 2VDC 1NPLC Filter=AVER10, total record time ca.11h

[TiN]

doktor pyta
Your unit seem to be much more stable. Look like I was too quick saying it's fixed! Doh 
I'll have to check more in detail. I can see needle sway slowly from -4 to 4 reading on 1uV range, with OPR switch set to ZERO.

[picburner]

This is a part of the new chopper circuit diagram derived from the photos.
Unfortunately there are things that I can't see and, although I tried to turn inside the pictures like in the Blade Runner movie, I'm not really able to do it....
Someone wants to continue correcting errors and filling in the missing values?
TiN, I lend you my endoscopic camera if you want....

[TiN]

Wow, somebody did my work. I'm impressed, really. I will be out of the country this week, but will definately remove covers and post more pics next week, no worries.

[enut11]

Hi All. Just purchased an 845AB from the States and awaiting delivery. Mine has black knobs and looks older so probably has the neon tubes.
This is an interesting post and am looking forward to further discussions. Will post if I find something of interest in mine when it arrives.

Questions
1) Here do I buy Chemicon caps in Australia?
2) Can I replace the carbon resistors with 1% metal film?
3) The Fluke manual says not to clean the rotary switches. Any thoughts?

enut11

[Vgkid]

1) Here do I buy Chemicon caps in Australia?
2) Can I replace the carbon resistors with 1% metal film?
3) The Fluke manual says not to clean the rotary switches. Any thoughts?

1) Mouser, Digikey, RS Australia, Element 14. Any high end capacitors should work.
2) Looks like sqauntor, has done it. No problems there.
3) Not sure, maybe if it causes issues.

[enut11]

2) Can I replace the carbon resistors with 1% metal film?

2) Looks like sqauntor, has done it. No problems there.

Thanks vgkid. I missed that in the posting. I need to work out the best way to preserve the PCB when I do it.
enut11

[Squantor]

2) Can I replace the carbon resistors with 1% metal film?

2) Looks like sqauntor, has done it. No problems there.

Thanks vgkid. I missed that in the posting. I need to work out the best way to preserve the PCB when I do it.
enut11

Hi enut11. Indeed I replaced all the carbon composite resistors in the Fluke, most of them have drifted upwards, especially the higher resistance values. The ones that stay hot like the 150Ohms series resistor was okay, but I replaced it anyways. I dread measurement equipment with carbon composite resistors, I have quite a few HP instruments, that are loaded with them, during repairs I always most of them have drifted upwards. Some of the flakey behaviour is usually to these resistors drifiting upwards making the equipment work maginally. I also have a sizable collection of philips equipment and they really had the rule "eat your own dogfood" with components, they use carbon film and most of them are in spec except for some high resistors. Not a lot repair needed on those, except for some transformers that become leaky.

When replacing them in the Fluke845 take notice of ALL the resistors, a few are underneath the wafer switch and it can be a hassle in removing them, use good strong tweezers. The vias are not plated through so with a good higher power soldering iron and solder sucker you will do fine, at least I did . I used a weller 50Watt iron with a fat chisel tip.

Some of the components are hard to remove due to ground planes and such. Just cut the leads off the old components off and desolder lead by lead.

The shielded part pay attention to that that after soldering, clean the polyethylene grommets with a Q tip and some denatured alcohol.

Unfortunatly the replacement of resistors, the neons by leds and the caps have not solved my problem with a very high drift. It seems that the zero setpoint is drifting a lot and I feel it has to do with the -15 that moves 300mV when the zero is fully clockwise and counterclockwise..

Can anybody measure the powersupply values under the following settings:
Put range in 30uV, set the OPR switch to ZERO.
Now measure the voltages of the supply when the zero is fully clockwise, zeroed and fully counterclockwise.

[enut11]

Hi Squantor
Thanks for the very comprehensive reply. This will be my first time upgrading a piece of antique metrology gear and looking forward to it.
My 845AB is still in transit from the USA and could take up to 4 weeks.

I hope someone answers your question but failing that I will respond as soon as I can. Did you replace the neons because they were faulty or were you chasing the drift problem?

With regard to wattage, modern resistors seem physically smaller for the same rating and I often wonder if this is realistic given that surface area plays a part in heat dissipation. Your thoughts?
enut11

[Squantor]

@enut11:

Indeed, I was chasing the problem and upon first opening I noticed that one of the tubes flickered while the other was a solid orange. So a replacement was in order. I followed the guide mentioned here: http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=Fluke/Fluke_845A_AB

As I can see, most of the resistors have such small loads, especially in the amplifier region that it should not be an issue, the only thing I would pay attention is sufficient wattage for the resistors in the led drive and the powersupply section. The 150Ohm and zener diode get quite toasty.

[Squantor]

Wow, somebody did my work. I'm impressed, really. I will be out of the country this week, but will definately remove covers and post more pics next week, no worries.

Hi TiN, did you find the time to make some more detailed photos of your fluke? Also, mine has a lot of germanium transistors that I suspect of having problems but finding replacements is hard. Can you look at the type numbers of your transistors what type they are?

[TiN]

Sorry everyone, I got busy with other stuff. Just opened this 845AB again and took photos around chopper board.
I had updated article with new pics.



And in process I damaged one of those TO-92 NPNs on base board. Was trying to tilt the thing so can see label on photo and middle pin just snapped. Oh, well, more repairs... 

[Squantor]

@TiN:

Thank you very much, lets start reverse engineering this!
I am sorry to hear about the transistor. I find this Null detector a pain to service, you need to almost completely disassemble it before you can get at the critical parts. It is quite a chore and then you have two boards flopping around. Well, lets find some time to start trouble shooting mine :-).

[zlymex]

It is always a pleasure looking at TiN's thread. Nice and big photo, thorough analysis and repair, and a good test chart at the end.
I had several 845A/AB/AR too but didn't have time/patient to analysis/modify except change line voltage from 110V to 220V.
The chopper amplifier is complex, someone replace it with a chopper stabilized opam such as LTC2054, don't know how it perform against the original.
Also looking forward the reverse engineering.

[enut11]

"Can anybody measure the powersupply values under the following settings:
Put range in 30uV, set the OPR switch to ZERO.
Now measure the voltages of the supply when the zero is fully clockwise, zeroed and fully counterclockwise."
Squantor

Hi Squantor
Received my 845AB today but it was DOA. Lots of corrosion in the power supply area caused by leaking Nicads. It will be some time before I get it going.
enut11