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

[enut11]

Hi All
Just purchased a Fluke 845AB High Impedance Voltmeter-Null Detector on eBay. The 845AB usually sells for >$1000US so was happy to get one delivered for under $275AU. Of course, with any good deal, you may not get a working instrument and this was the case for me.

Externally, the unit was in good condition for its age (circa 1971?) with no major scratches or dents.

The 845AB was powered by a Nicad battery pack and these are notorious for leaking. Getting to the pack required a fair bit of disassembly. The cells are held against the back of the power supply PCB by a plastic moulding. Even before I had removed the battery holder I could see evidence of battery electrolyte corrosion. On removing the holder, all the cells fell out! What was strange was that someone had replaced one of the Sub C cells with a C cell and did a very bad job of securing the holder back onto the PCB.

One of the pictures that I have posted shows the extent of the corrosion on the tinned copper tracks. This board also holds the transformer and other power supply components. Any advice on how to clean this board would be appreciated.

Thankfully, the main PCB with the voltmeter components is housed in a separate aluminium shield and did not show evidence of corrosion.

This looks to be a long time restoration. Will post more info as I progress further into the disassembly and clean/repair phase.

enut11

[builder]

Hi Enut11,

I have just done a complete rebuild of an 895A DC differential voltmeter, on which the voltmeter circuit (called TVM in the 895A manual) of your 845A will, if made in 1971, is very likely to be based.

I replaced the neon/LDR chopper isolated with ILED/FER isolators ( later 845 models used these).  Worked brilliantly. Can send more detail if required.

Basher

[enut11]

Hi basher. As the unit is currently dead I am not able to verify if the chopper works or not. Am interested in your mod. Currently researching best way to clean corrosion off PCB. Enut11

[Squantor]

I replaced the neon/LDR chopper isolated with ILED/FER isolators ( later 845 models used these).  Worked brilliantly. Can send more detail if required.

Basher

Hi Basher,

Would be nicer to share it with all of us with ailing 845's

[TiN]

I'd be interested to see as well, it's never bad to know more details and tricks about these old but still useable tools.

[enut11]

Well, I cleaned the corroded power supply PCB of nicad residue as best I could, replaced the two electros but no luck in getting any meaningful response from my F845AB.
No warm glow from the neons. Random meter movement - at least a good sign that the movement is working. No response from the zero control. No response from input voltage. Even the battery test (8.4V) reads low at 6v.. Lots to be done to get this unit operational.

As this unit is difficult to get apart for service, I decided to add connectors from the power supply to the main PCB.
enut11

[Squantor]

Well, I cleaned the corroded power supply PCB of nicad residue as best I could, replaced the two electros but no luck in getting any meaningful response from my F845AB.
No warm glow from the neons. Random meter movement - at least a good sign that the movement is working. No response from the zero control. No response from input voltage. Even the battery test (8.4V) reads low at 6v.. Lots to be done to get this unit operational.

As this unit is difficult to get apart for service, I decided to add connectors from the power supply to the main PCB.
enut11

First check the +15 -15 supplies, I am suspecting the 83Hz supply oscillator if the neons are not burning. Good that the movement is working, I am always scared with shipping equipment with mechanical indicators due to shock sensitivity.

When you get the +15 -15 supplies, the neons should glow unless they are completely shot, but usually they do not fail that abruptly.

Unfortunatly, cant help you with the battery issues. I have the 845A, so no batteries there.

[enut11]

Power supply is way out, 13v and -8.5v. Still looking into it.

Does anyone have experience repairing nicad damaged boards. As reported above, I scrubbed the PCB first with diluted vinegar to neutralise the electrolyte, then hot soapy water followed by a pure water rinse then drying in a low temp oven. There is still some residual corrosion around the component leads solder pads. See previous photo.

I am trying to decide whether I have to remove every component from the PS PCB, clean the bare board and resolder the parts back on.
enut11

[Squantor]

Power supply is way out, 13v and -8.5v. Still looking into it.

Does anyone have experience repairing nicad damaged boards. As reported above, I scrubbed the PCB first with diluted vinegar to neutralise the electrolyte, then hot soapy water followed by a pure water rinse then drying in a low temp oven. There is still some residual corrosion around the component leads solder pads. See previous photo.

I am trying to decide whether I have to remove every component from the PS PCB, clean the bare board and resolder the parts back on.
enut11

I have no experience with cleaning PCB's affected by corrosion from nicads, only from electrolytic capacitors deciding to spray their contents inside a Philips lab powersupply.
Some traces where completely eaten away and component leads where damaged. Re-soldering every joint would be smart. If a trace is not shiny metal scrub away the oxidation until it shines. I used one of those fibreglass cleaning pens (use gloves! those fibres will get everywhere and mess up your day with itching!) until the trace metal was visible or the trace was completely corroded through. In your case the traces look good, no "crust" of gray or reddish residue.

Also check the transistor sockets if some of the electrolyte has wicked up into the socket messing with the transistors. Remove the transistors and check, clean if needed. Did you power it on before you cleaned up the electrolyte? That might have damaged some components.

Have you measured the waveforms of the 83Hz Oscillator?

[enut11]

Thanks squantor. Unfortunately I did power it up when I received it. Currently away for 10 days. Will try your suggestions when I get back. Enut11

[FlyingHacker]

I just recently scored a Fluke 883A locally. It is from ~1966 and has a mechanical chopper you can hear. (Thread here: https://www.eevblog.com/forum/repair/fluke-883a-differential-voltmeter-%28pics%29/ )

I used the same trick with radial caps in most places rather than axials (I had a few axials).

The thing is pretty much spot on with my Keithley 6 1/2 digit DMM.

Is the 845 a newer unit? I see your guys mentioned 1971. The weird optical chopper seems like an older technique rather than a newer one. Anyone up on the history of these?

[Squantor]

I just recently scored a Fluke 883A locally. It is from ~1966 and has a mechanical chopper you can hear. (Thread here: https://www.eevblog.com/forum/repair/fluke-883a-differential-voltmeter-%28pics%29/ )

I used the same trick with radial caps in most places rather than axials (I had a few axials).

The thing is pretty much spot on with my Keithley 6 1/2 digit DMM.

Is the 845 a newer unit? I see your guys mentioned 1971. The weird optical chopper seems like an older technique rather than a newer one. Anyone up on the history of these?

Hi there, I also have a similar unit, the 883AB (with battery pack). The pack was completely shot and needs replacement. It uses a big stack of coincells and a bunch of C sized cells. Bit of a pain to replace but the meter non functional as it is. Still need to look into it as it is a bit of a pain to work on but not as much as the 845.

The 845 is not the same as the 883. The 883 is a differential volt meter that uses a Kelvin varley divider. The 845 is just the null detector circuit with a divider up in front to give it some voltmeter capability. To give the same functionality as the 833 you need to pair it with a kelvin varley divider like the fluke 720 and a voltage reference.

I have experienced that a lot of precision equipment with mechanical or neon choppers usually have problems with the chopper. You are quite lucky that yours is fine :-). I am planning to give the 883 a good look and see if its functional and maybe experiment with replacing the mechanical chopper by something using optofets.

I am reconstructing part of the circuits from the 845 onto vero board to experiment with different chopper circuits to make my own replacement plugin for all the mechanical/neon solutions. Either by using the H11F1 optofets, mosfets (like in the Philips PM2434), or analog switches.

[FlyingHacker]

I just recently scored a Fluke 883A locally. It is from ~1966 and has a mechanical chopper you can hear. (Thread here: https://www.eevblog.com/forum/repair/fluke-883a-differential-voltmeter-%28pics%29/ )

The 845 is not the same as the 883. The 883 is a differential volt meter that uses a Kelvin varley divider. The 845 is just the null detector circuit with a divider up in front to give it some voltmeter capability. To give the same functionality as the 833 you need to pair it with a kelvin varley divider like the fluke 720 and a voltage reference.

I see. So the 845 works like the 883A in TVM mode?

I have experienced that a lot of precision equipment with mechanical or neon choppers usually have problems with the chopper. You are quite lucky that yours is fine :-). I am planning to give the 883 a good look and see if its functional and maybe experiment with replacing the mechanical chopper by something using optofets.

I am reconstructing part of the circuits from the 845 onto vero board to experiment with different chopper circuits to make my own replacement plugin for all the mechanical/neon solutions. Either by using the H11F1 optofets, mosfets (like in the Philips PM2434), or analog switches.

I do feel quite fortunate that the chopper works, especially for the price I paid for the whole unit. I have dealt with some mechanical choppers before, and they have failed. The thing runs audibly whenever the unit is turned on. I am watching this thread closely for ideas on a replacement chopper.

What are the challenges in the chopper? Looks like it has to both act as an low impedance input switch and control the transistor that pulls the output to common. It has to do that at 84 HZ with a temperature stable 50% duty cycle.

[Kleinstein]

Yes the Fluke 833 in TVM mode works a little like the 845 and similar meters, though likely less sensitive and maybe without the dividers for the higher voltages. So the Fluke 833 is one instrument with
1) a good reference source
2) a Kelvin Varlay divider
3) a reasonable quality null-meter

Each part is not absolutely highest quality, but still reasonably good. Together this is a good quality DVM of it's time, slow but much higher resolution than a simple analog meter.

I am not sure about using a chopper for the null meter in the 833 - the resolution does not look that high to make it an requirement. Just a zero pot and manual reversal would be enough.

There are not that big chalanges with a chopper - no need for exact 50% duty cycle. It's just a slightly more complicated circuit with quite some parts, that made is expensive back than, when every transistor cost's 10s of bucks. The difficulty was having good switching devices, not such a problem today.  Today you get copper stabilized OPs for a few bucks, that may outperform many of the old choppers.

[Squantor]

What are the challenges in the chopper? Looks like it has to both act as an low impedance input switch and control the transistor that pulls the output to common. It has to do that at 84 HZ with a temperature stable 50% duty cycle.

Well it depends, I am quite charmed of a chopper based on the optofets H11F1 but there seem to be challenges with that. See the circuit that TiN found in his 845. I am not done completely reverse engineering it but I think it has a duty cycle control to make sure that it does break before make and some kind of biasing the chopped side with the trimpot. I am first going to try with a mechanical chopper test to make my design is sane and then move to a H11F1 design.

Here is another story[1] of a person replacing the neon choppers in his hp 419a null detector:

If you move on to H11F instead of one of the other analog switches, beware that they have a significant offset voltage, on the order of 100uV, which Fairchild does not test.  It is affected by temperature and LED drive current.  The modulator pair must be matched to within 15uV.  Age them first, by operating them, heating and cooling them, and soldering their pins, over and over until they stop drifting.  Then look for a pair with similar offset, and glue them together for thermal bonding.

[1]: https://groups.yahoo.com/neo/groups/hp_agilent_equipment/conversations/messages/76318

[FlyingHacker]

I wonder if increased modern line voltage has much to do with frequent chopper failure. At least on the 883 the chopper is powered by the unregulated 6V supply. I was measuring just shy of 9V on the output cap of the 6V supply with my line voltage set to 120V. I can't see a voltage rating marked on the chopper. It is possible removing it from the mounting strap would reveal it, but I don't want to risk damage.

I tapped into the 6V supply to add some amber LED lighting to the meter. I'll post pics in the 883 repair thread. I don't feel bad about doing this since it actually brought the voltage down to around 7.5V.

[enut11]

Quote: "I have no experience with cleaning PCB's affected by corrosion from nicads, only from electrolytic capacitors deciding to spray their contents inside a Philips lab powersupply.
Some traces where completely eaten away and component leads where damaged. Re-soldering every joint would be smart. If a trace is not shiny metal scrub away the oxidation until it shines. I used one of those fibreglass cleaning pens (use gloves! those fibres will get everywhere and mess up your day with itching!) until the trace metal was visible or the trace was completely corroded through. In your case the traces look good, no "crust" of gray or reddish residue.

Also check the transistor sockets if some of the electrolyte has wicked up into the socket messing with the transistors. Remove the transistors and check, clean if needed. Did you power it on before you cleaned up the electrolyte? That might have damaged some components.

Have you measured the waveforms of the 83Hz Oscillator?"

Hi Squantor.
Fixed the PS board. Removed most components and resoldered the pads. Had to dismantle the transistor sockets to clean out some green deposits most likely from the nicads. The 84Hz oscillator now works. Also measured 90vAC RMS on the output of transformer T202 which should be enough to drive the neons. However, still no glow from the neons. Now wondering if the neons are shot?? Still getting unbalanced 15v rails. Mine are +10.2v and -12.7v.
enut11

[enut11]

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?

My 845AB also uses germanium transistors. I tested the ones on the power supply board:
Q201 PNP Beta 140, Vf 160mV
Q202 PNP Beta 104, Vf 135mV
Q203 NPN Beta 320, Vf 150mV
Q204 NPN Beta 130, Vf 135mV

Most of the transistors in my unit are the old TO39 metal can type

The 845AB parts list specifies general purpose Si transistors so perhaps any modern transistors will work?
enut11

[Kleinstein]

The circuit should also work with modern Si transistors. Germanium transistors do age, especially if they get hot. Chances are that aged transistors get a higher gain and start to conduct to much, so they get more hot than normal.
In most respects modern Si transistors have better specs. Just the higher U_BE might need some adjustments in some places.

I don't see a problem with Q201 to Q204 in this respect. Q201/Q202  have a pot to adjust the operating point (frequency) already. They also work in switching mode, so trouble with the DC level.

Q203/Q204 might actually profit from the lower CE saturation voltage, but this should not be a problem at the level of accuracy an analog meter provides.

[picburner]

Q106, Q113,Q114,Q203 and Q204 are chopper transistors whith a Vebo>16V.
They can be replaced with silicum type MPSA16/MPSA17 for npn and MPS404A/2N2944A/2N2945A for pnp.
Common transistors don't go well there.
The manual, in part list, say "factory matched" so they have to be selected.

[enut11]

Thanks a lot guys. Gives me something to work with. Looking forward to getting this old test gear up and running.

[enut11]

There is an oddity in the F845AB parts list vs the schematic - they differ in specifying NPN vs PNP for Q201 and Q202. I am taking the schematic to be correct, ie PNP.
enut11

[enut11]

This info came from basher [builder]. It relates to modifying the 845 with LEDs in place of the neons and is something that I will have a look at.

"I got my idea for my 895A (similar chopper circuit to your 845A) from the following,

[volt-nuts] Finally got around to modifying my Fluke 845ab with   LED's
Dallas Smith Tue, 09 Sep 2014 08:57:07 -0700

Finally got around to modify my Fluke 845ab with LED 's for the chopper circuit. Used the 17 volt windings for LED's (Mouser 941-C513AMSNCW0Y0511 Warm White Round LED) instead of the 130 volt, move red wire on transformer pin 9 to pin 7.This winding is 180 degrees out of phase, so I reversed the steering diodes (CR106 & CR107) I left in to help make sure the phase was correct for the LED's when connecting. Change R154 to 6K to set the brightness, selected for good operation of the zero control. Then install jumper to replace C119. Also changed the filter integration response caps C111 to .022uF and C116 to 47uF, this stabilized the jitter to a manageable mode of operation. Meter now works as well or better when the original neon's worked.

As the meter originally had this problem, why is the offset reading different when polarity is reversed at the meter input? About 10uV's.

Lamp Blocks.

However, I used H11F3M  IRED/FET optocouplers and increased R154 as I now had IREDs not neons.  My TVM now has less than 1/2 micro volt jitter and drift, of the TVM, is much less the a microvolt in 20 to 30 minutes.

 I can't recall more as I'm away at present.

Hope this helps, Builder"

[enut11]

Fixed the PS board. Removed most components and resoldered the pads. Had to dismantle the transistor sockets to clean out some green deposits most likely from the nicads. The 84Hz oscillator now works. Also measured 90vAC RMS on the output of transformer T202 which should be enough to drive the neons. However, still no glow from the neons. Now wondering if the neons are shot?? Still getting unbalanced 15v rails. Mine are +10.2v and -12.7v.
enut11

Finally got around to removing main PCB (not easy!). Replaced all electros. Desoldered the neons. They look sick to me. How to test them?

Next step is to try Dallas' LED mod to replace LEDs in chopper circuit.
enut11

[tronde]

Desoldered the neons. They look sick to me. How to test them?

150 kohm series resistor and 100 - 230 VAC should make them glow if basically OK.
I guess yours will be slow to ignite and quite unstable since they have turned dark after many hours of glowing.

New neons are sold on ebay.