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

[enut11]

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.

Thanks. I have a feeling that the 845AB  neons need certain colour characteristics. Anyway, for the moment, I will try an LED mod and see if that works.
enut11

[enut11]

I have removed the neons, light rods and posts. Changed several components as outlined above from Dallas Smith. Added LEDs to directly activate the LDRs (in the yellow box).

Note that CR106 and CR107 diodes have been reversed, capacitor C119 replaced by a link, LED resistor R154 now 6K, all driven from the low voltage winding on T202.

I am using off-white LEDs that I already had and spent some time finding a matched pair for current and forward voltage.

Fingers crossed.
enut11

[SeanB]

I would not use white leds because of the slow turn off and variability of the phosphor in them. Use a blue LED instead, which will do the same with the fast turn on and turn off, plus it will activate any LDR as fast as possible, but here any LED from blue to green, orange or red will work just as well, as the LDR resistance will work with the red neon lighting of the original.

[Dr. Frank]

LDRs usually are most sensitive in the IR range.

Therefore I would avoid blue, green and especially LEDs with phosphor.

Go for red or IR LEDs instead

Frank

[tronde]

The original neons will most likely have a kind of orange light colour. I don't think they have a selected colour, but the circuit will not work when they become unstable.

[Kleinstein]

There are different types of LDRs. Some are IR sensitive, but the normal ones are not IR sensitive at all, they may not even react much to red light. So an orange or green LED would be the best choice - yellow might be the most effective for the LDR, but yellow LEDs are often less efficient. As the frequency is not that high the phosphor in the white LED is not a problem. The color of the LED should not make a big difference. In theory red light will penetrate deeper and thus give more volume effect, while blue light is more a surface effect. It might be more important to get an even light distribution on the LDRs - gradients in the light intensity could cause some minor offset.

The color of neon lamps is fixed by the neon gas. But this does not mean that this color is best. The problem is more that neon lamps get darker and might start to flicker if old.

[enut11]

Thanks gents. My LED mod did not work but it may not be the LEDs at fault. I am now finding that the output of the 84Hz oscillator is weak so am looking at replacing the germanium with silicon transistors. Then I will try the various coloured LEDs suggested.
enut11

[enut11]

I have had some success with changing the neons (which were dead) for LEDs in the 845AB. I have now gotten a response from the LEDs. I measured around +-12v on the rails (a little low) and I was able to zero the meter on the lowest ranges. The meter responded to a DC input on the 3v range (only one tested). This is a big step forward so am happy.

Only problem encountered is trying to set the 84Hz oscillator - I get a fluctuating frequency that I need to look at. It is not a nice clean square wave and seems to have lower and higher frequencies mixed in. I am not yet in a position of save oscilloscope screens but working on it.

Meanwhile, any suggestions on stabilising the circuit below would be appreciated.
enut11

[Kleinstein]

The LEDs might be a rather high load for the oscillator. This can also make the frequency less stable and explain the lower voltage. So it might be a good idea to reduce the current to the LEDs a little and thus high efficiency LEDs may be needed.
One could also use the supply half of the transformer  - the LEDs don't need 100 V.

[enut11]

Hi. I posted the wrong circuit. The LEDs are powered from the low voltage winding. Will look at reducing the LED current further. Thanks.
enut11

[enut11]

Finally got an oscilloscope screen capture program working. Had to revert to a PC running Win XP and with an RS232 port. I have a TDS220 scope and am using OpenChoice Desktop from Tek. Below is the scope trace for the Fluke 845AB oscillator taken from pin1 of the transformer (TP10). It is supposed to be an 84Hz square wave! As you can see it hardly resembles a square wave nor is it running at 84Hz.

The trace was unstable and I was not able to adjust it to 84 Hz using the 2000 ohm trimmer pot.

Power supplies are +-11.8v (nominal 15v) and the meter is reading input on all ranges, although out of calibration.

I would like to know why the square wave is missing/distorted and how I can get the frequency right.
enut11

[Kleinstein]

The exact frequency is not critical - so the 65 Hz would not be a problem.

But the waveform is really bad - so bad it might damage some parts.

It is well possible that old Ge transistors (Q201,Q202) with to much gain are a problem. Thus range of the 2 K pot is not sufficient anymore to control the oscillator gain. The missing capacitive load of C119 with the neons might be a factor - though it should tend to increase the frequency. It might be a good idea to have the neons connected, even if not used for the chopper. They also act to reduce the peak voltage a little. They also provide some capacitive loading - C119 might be important to set the frequency. Alternative just C119 a series resistor and two zeners / suppressor diode might work.

Also to much capacitance for C201 could be a problem. But I am not so shure of the exact operation of the oscillator.

[enut11]

Hi All. Thanks for your input. Your suggestions have all been worthwhile. I am happy to report that my Fluke 845AB is now working after much time spent on cleaning, troubleshooting, mods and repairs.

This turned out to be my most challenging project as this device has a very complicated physical structure and must be dismantled to get to the main board. I inserted a couple of connectors into the wiring looms to facilitate disassembly and testing.

For the main oscillator, I substituted a couple of switching Si transistors for the Ge. The LEDs for neons mod suggested by Dallas Smith also seems to work well.

Power wise, in the end I decided to run mine on 2S LiPo battery pack as the total meter current draw is under 20mA. There is provision for 230vAC operation later on if I need it.
enut11

[Squantor]

This turned out to be my most challenging project as this device has a very complicated physical structure and must be dismantled to get to the main board. I inserted a couple of connectors into the wiring looms to facilitate disassembly and testing.

For the main oscillator, I substituted a couple of switching Si transistors for the Ge. The LEDs for neons mod suggested by Dallas Smith also seems to work well.

What transistor substitution did you do? Just generic 2n3904/2n3906?

I have troubles with fluctuating voltage rails on my fluke, I still need to revisit my Fluke one of these days. Maybe I am experiencing the same problem, your information/waveforms are going to be a big help. I have already done the LED mod, replaced all the resistors and caps (except for the tantalums and films). This only leaves the transistors.

I wish I had a list of the transistors used in TiN's 845 so I could start substitutions .

[enut11]

Hi squantor. For Q201/Q202 I tried MPS404a pnp [ref picburner] and they worked. I also tried common BC350 pnp and they worked. I suspect the transistors are not critical as long as they are matched. The MPS404a had higher gain (>150) compared to BC350 (<100). 

I could not make sense from some of the waveforms shown in the 845AR manual. For example, TP10 on pin 1 of the transformer T202 should show a square wave. In fact pin 7 on the the transformer secondary shows a beautiful 12vRMS square wave at 84Hz.

My voltage rails are just under 12vDC compared to 15v nominal but since these are unregulated I am not worried as it is probably related to the lower battery voltage (8v LiPo) that I am using.

With the LEDs mod, I elected to mount mine up against the LDRs. Dallas mounted his LEDs where the neons were so he used the original optical rods. I am using efficient LEDs and the current is ~2mA (12v/6K). I am thinking of reducing this to 1mA to see if it has an effect on pointer stability (currently very slowly moving +-3 minor divisions around zero on the 1uV range with a shorted input).
enut11

[Squantor]

Hi enut11, hope your Fluke845 is still open. Could you try something out for me? Can you put it on 3uV mode, short the input with the zero switch and then turn the zero pot fully clockwise and counter clockwise and measure the voltage rails at both settings. Do they differ a lot from when you have Nulled the pointer? Mine fluctuates a full volt and a half on the negative rail. I want to know if that is normal.

Thanks for the transistor tips. Maybe I am suffering from the same problem but not so severe as yours.

Do you get a big difference between line powered and battery powered?

[enut11]

Hi squantor. It is all together now as I wanted to measure performance with the internal shields in place. However I am already thinking of some changes so it wil be apart again soon. I do not recall any bias in the 15v supplies as a result of moving the null control. I will measure this next time around.

I have not powered it with AC mains yet because it requires a different power socket for 230vAC.

[enut11]

I measured the 'total noise' of my fully assembled Fluke 845AB running off a 2S LiPo pack (8.2vDC). I set the output pot to deliver 1v while injecting 10.0v on the 10v range. Once set, the full scale output is 1v on all other ranges. I then shorted the input by selecting OPR-Zero, selected the 1uV range and centered the pointer to zero. The chart below shows a gradual deviation with a small negative slope. Not sure what is causing this but it seems to be linear with time. Temperature over the ~30 min test was 21C, humidity low (winter).

The data from the 845AB was measured with my MS8040 and sent to my XP RS232 port using the Mastech PC program.
enut11

[enut11]

Hi enut11, hope your Fluke845 is still open. Could you try something out for me? Can you put it on 3uV mode, short the input with the zero switch and then turn the zero pot fully clockwise and counter clockwise and measure the voltage rails at both settings. Do they differ a lot from when you have Nulled the pointer? Mine fluctuates a full volt and a half on the negative rail. I want to know if that is normal.

Thanks for the transistor tips. Maybe I am suffering from the same problem but not so severe as yours.

Hi Squantor. Rails tested as requested, 3uV range:
Zero pot centered: +11.28v and -11.14v
Pot fully clockwise: +9.42v and -11.31v
Pot fully anticlockwise: +11.55v and -9.45v

So yes, the zero pot does affect the power rails, the further you turn it the greater the deviation.
enut11

[Squantor]

Hello enut11.

Thanks a lot! Well it saves me from a possible chasing a problem that is not actually a problem. Hopefully in the summer I have time to revisit the 845 and finally fixing it :-).

[TiN]

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.

[2N3055]

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.

I  know you know it but did you check for oscillations? Carbon resistors are noninductive, low parasitics... Horrible noise and tempco too;-)

[enut11]

Hi TiN. Have you checked the 84 Hz oscillator?

[martinr33]

Started looking into my 845AB. It is reasonably stable, but it could do better.

Bottom line: The FET opto isolator, something to tighten up the opto drive, and then removing that

1) This is an AC circuit!
The meter reading comes off of a transformer, and the chopper amplifier circuit shifts the center point of the waveform. The chopper frequency makes it all the way through to  the display; the meter is averaging the output waveform. Very clever stuff that no doubt has its roots in vacuum tube designs. This architecture implies that we can't add a chopper amp in and get anything useful. That said, there is a DC segment, but it is wrapped in that AC technology.

2) That new board is simpler than it looks. It uses very good FET optoswitches to replace the LDRs and neons. It should be more consistent, because there's no dependency on the randomness or deterioration of a neon tube.  You could come off of the transformer, but I think that Fluke wanted the extra control of a regulated voltage supplying the optos. The FET optoswitch is absolutely the way to go.

3) Looks like U3 (and thanks to TiN for the pictures, and Picburner for the schematic) simply provides a DC voltage to offset the zero in the front end chopper circuit. Oddly, all the other front end wiring is carefully isolated with standoffs and air wires. But this little offset circuit is just plain PCB wiring.

4) I'm not sure how important asymmetry is in the optoisolators. Mostly, I see trouble in the switching threshhold, as asymmetric switching will offset the zero. The input shaping that Fluke added should keep that to a minimum.

5) Looking at the input, if the 1M resistor is disconnected, the only DC path is through a dark LDR. Again, the opto FETs should help hereby eliminating the leakage path through the LDRs. Another point for the AC pathway in a DC instrument!

6) If anybody does get into an assist PCB for this, a small microcontroller could do a pretty good job of autozero. It would need a comparator to read the zero, and a way to slightly move the offset voltage in the front end. Could probably use pwm.

[Squantor]

Hello Martin R,

My 845A is lying mostly disassembled in a corner, looking at me, whispering "why am I not repaired yet". I wanted to replace the neon+LDR chopper for a long time but I am probably going with a different route.

I have a bunch of equipment that either uses mechanical or LDR+Neon choppers to create the autozero functionality. I am investigating a replacement for them all by using a small custom wound transformer (using a high Al pot core with shielding) and optofets to recreate the chopping function. The reason I am for a small custom wound transformer is that I want to reuse the neon drive windings but the current is too low on those. The optofets need a substantial current to really go into low resistance mode (30ma according to the datasheet of the H11F1).

Have you reverse engineered the optofet chopper PCB that was posted here? I am interested in the residual zero trimming part. So the 5V regulator is there just to provide a DC bias so the differences between the optofets is canceled?
This might help that you dont need to match the optofets so tightly.