Fluke 8000A Help Needed

[Goatropercu]

Hello,

I recently inherited my dad's old fluke 8000A multimeter. It seems to work well enough measuring ohms and DC volts, but when I measure AC voltage it wanders fairly erratically. For example, when measuring mains voltage (115vac at my house) the reading jumps around from 110vac to 140vac, and anywhere in between. When connected to a known 6vac voltage source it bounces between 0vac and 5vac. With the leads shorted it bounces up to as much as 9vac.

This meter does have the mA/sec function that I believe was an option. The service manual/schema that I downloaded does not include the additional board that I believe to be associated with this function.

I have cleaned the controls (twice).

I replaced the following:
C6 or C10 hard to say which (.47uf/35v tantalum)
C20 and C9 (10uf/20v tantalums)
C17, C18, C25, and C31 (.47uf/20v tantalums)
C14 (.22uf/50v electrolytic)
I also replaced two 4.7uf/25v tantalums on the additional board that I assume is associated with the mA/sec functionality.

R45 was visibly cracked and spewed apart upon removal. It measured 60ohms both before/in circuit and after removal. I am no expert, but I assume this resistor may be more involved in current measurement rather than the AC volts function. It does look like the range switches are directly tied to these, but since AC volts is off in both the 200vac and 20vac range I do not think R45 is the root cause. Either way a replacement has been ordered.

Is there any way I can tell if the op-amp U1 is bad? It is stamped Motorola 352930 7098.

Any help is greatly appreciated. Thank you.

[wn1fju]

I would first check the output of the buffer (Q1 source) in the AC Converter section and make sure it follows the applied ACV input reliably.  If not, I would suspect the current limiter, CL1.  I had a similar Fluke 8600A DMM with erratic ACV readings and that was the problem.

As far as checking the op amp, U1, when operating properly the two inputs, pins 2 and 3, to the op amp should be very close to equal and the output, pin 6, should generally not be near the +15 or -15 supply voltages.  This goes for almost every op amp.

[Goatropercu]

Pardon my ignorance, but can you explain what you mean by the Q1 source should "follow the applied ACV input reliably"? I assume you mean that the voltage at Q1 source lead should "move" with respect to different ACV's being measured/applied to input, but specifically what am I comparing Q1 source voltage to and where?

Thank you.

[Dave Wise]

I know you said other ranges were working.  For what it's worth, the only thing that's ever failed in my 8000A is the power supply reservoir caps.  When approaching any test instrument for troubleshooting, the first thing to do is confirm that the power supply rails are good, because if they are not, the instrument can malfunction in every way imaginable and then some more.

[Goatropercu]

I am getting +19vdc and -19vdc from power supply, apologies for not mentioning this in my initial post.

Thank you.

[wn1fju]

Q1 acts as a "source follower" and so the output of the transistor should be a "copy" of the AC waveform you are supplying to the front panel jacks.  The amplitudes may be different because there are attenuators in the signal path, depending on the selected range, before reaching Q1.  If you have an oscilloscope, you would be better served using that.  You get a more complete picture of what's going on that by using a voltmeter.  Usually when probing a DMM, your "ground" reference is the negative input terminal. 

I only suggested Q1 because when you are experiencing erratic behavior, you want to logically start at the input and work your way through the signal flow until the observed signal suddenly becomes erratic.  Q1 is a logical place to start because it is before the active rectifier circuitry.  Sometimes this can be difficult if there is feedback in the circuitry you are analyzing - then the question is who is at fault.  At that point, you usually have to take out the soldering iron and lift a component or two to break the feedback before further debugging.

But, as the previous poster suggested, this all assumes the power supplies are working.  Again, a scope is more useful than a voltmeter because you can spot simple things like 120 Hz ripple indicative of a failed electrolytic capacitor or 60 Hz ripple indicative of a failed rectifier. 

[Goatropercu]

This is incredibly helpful. I’m out of my element here, but I do have a scope. Let me see what I can find. Thank you very much for your time and assistance.

[Goatropercu]

Still trying to put together some useful information. I replaced R45 and U1, but the issue persists. More to come.

[Goatropercu]

Since I seem to be able to trace a sine wave "further" in the low voltage range settings I input a 1.5vac signal into the test lead jacks and moved through the AC converter section with the range set to 2.

Good 1.5v sine wave at U1 input pins 2 and 3. Same applies at the junction of R6 and C7.

At U1 output pin 6 the wave is no longer a smooth sine wave; vertically oblong is the best way to describe it. It measures about 3.5v rms.

I replaced U1, and hope/assume (neither of which i rely on) the part installed was good. Lesson learned on establishing a proper baseline for comparison, again. I will see if replacing C8 has any effect.

[bdunham7]

I replaced U1, and hope/assume (neither of which i rely on) the part installed was good. Lesson learned on establishing a proper baseline for comparison, again. I will see if replacing C8 has any effect.

I have an 8000A with a different problem (ADC?) but AFAIK the AC converter is OK.  Here is what I get at the output of U1 and at C10.  C10 is the large-ish capacitor rightmost and frontmost under the shield and the point to measure is on the right.  The schematic is confusing because the 'C10' designator is on the first half and the actual capacitor is on the second half.

The idea is that the output of U1 has to deal with the diodes in the feedback loop.  The stuff connected to the compensation pin appears to be feedback, but the actual feedback is through those diodes and that is why the output has those offsets at the crossover points.  That's normal.  What you should see is a fairly but not perfectly smooth output at C10 that more or less tracks the AC input.  I used 1.5V as you did, I got what you see.  As I change my AC input from 0 to 2V, that signal at C10 smoothly follows it.  That's what you want--that signal at C10 is what the meter sends to the ADC to be integrated.  Unless that is erratic, your problem isn't in the AC converter.

[Goatropercu]

Again, this is incredibly helpful. Thank you. I am learning quite a bit. My dad actually just loaned me his functioning 8000A for comparison.

With an 8vac input and range set to 20 the waveform at the output U1 (pin 6) is the same on both machines, except mine is jumping up and down on the screen where his is stable. More to come.

This is my first rodeo in this fora, and I am impressed. I expected crickets. Thank you very much for your time.

[bdunham7]

With an 8vac input and range set to 20 the waveform at the output U1 (pin 6) is the same on both machines, except mine is jumping up and down on the screen where his is stable. More to come.

1.5VAC on the 2V range is probably the most useful signal to use because there is no scaling or extra gain involved--it just goes (relatively) straight through.  So next you need to see what is on the ouput at C10.  You mentioned it jumps around even with no input, how about shorted?

[Goatropercu]

Yes it jumps around w inputs shorted.

[Goatropercu]

I am suspecting CL1 may be the culprit. It was mentioned earlier in this thread. At the + side it is much jumpier in comparison to the working meter with an 8v input (w/ a 1.5v input the jitteriness is much more subtle). Per the sm CL1 is a 1mA, fluke p/n 348482, mfg p/n 89536, and looks to be a TO-92 package (i could be wrong there). Would this be a suitable substitute? If not, any recommendations?

https://www.mouser.com/ProductDetail/Semitec/E-102?qs=wgO0AD0o1vuW%252BfX3FIcJ1g%3D%3D

I may get bold and swap in the one from the working meter.

Thank you.

[bdunham7]

What do/did you see at C10?  What is your reasoning for suspecting CL1? 

[Goatropercu]

I suspected CL1 because that is where the scope image starts swinging around. I swapped CL1's and the issue was not resolved...moving on.

At C10 i don't get a waveform on either machine, but just a jumbled up mess of a line that looks like noise. The image is swinging wildly at the C10 pin closest to S1. If you are referring to the "point to the right" being from an arial view of the component side, that would be this pin. I put my voltmeter on this point and it is all over the place (in mV).

Where does the negative lead of C10 go? I see a triangle with what appears to be a "1" inside of it.

Another observation is that with no inputs connected the incandescent lamp (DS5) in the problematic machine fluctuates in brightness like something is pulling down the current perhaps. The lamp in the working device stays well lit (and for the most part brighter) with no fluctuations in intensity.

[bdunham7]

I suspected CL1 because that is where the scope image starts swinging around. I swapped CL1's and the issue was not resolved...moving on.

This is where the voice in your head should tell you "Sir, put the soldering iron down and back away from the instrument!"   

At C10 i don't get a waveform on either machine, but just a jumbled up mess of a line that looks like noise. The image is swinging wildly at the C10 pin closest to S1. If you are referring to the "point to the right" being from an arial view of the component side, that would be this pin. I put my voltmeter on this point and it is all over the place (in mV).

Where does the negative lead of C10 go? I see a triangle with what appears to be a "1" inside of it.

What you should have at C10 is a relatively smooth, stable DC with a bit of ripple if your source is 60Hz.  If that is going crazy, that might be your issue--but if you see the same thing on the other instrument, perhaps there is an issue with your measurements?

The symbol represents the guarded ground or common.  That should tie in with nearby TP4 and that is where the ground lead of your oscilloscope should go.  Is your AC source ground-referenced?  If so, is the ground side connected to the COMMON input on the DMM?  Is your scope DC-coupled?  Once you are sure all that is good, check these points with your scope and a 1.5V_RMS input.

Q1 inputs pins 2 & 3, you should see a sine wave corresponding to the input but offset somewhat.  These should be quite stable and smooth.  The offset is the result of using CL1 and Q1 as the input buffer.  If it is not steady, try to describe accurately in what way it is unsteady.

Q1 output pin 6, you should see the waveform I posted, again quite stable and smooth.  If not, note how jumpy and irregular it is in terms of millivolts of deviance or instability.

R10/C9/CR5 junction, you can get this on the rear end of R10.  What I see on mine is a pretty good sine wave with a lot of offset.  It has a DC mean and an AC RMS value (set your scope to calculate those if you have that feature) that are both about 1.5V, same as the input. 

[Goatropercu]

This is where the voice in your head should tell you "Sir, put the soldering iron down and back away from the instrument!"   

Noted.

What you should have at C10 is a relatively smooth, stable DC with a bit of ripple if your source is 60Hz.  If that is going crazy, that might be your issue--but if you see the same thing on the other instrument, perhaps there is an issue with your measurements?

It is not the same on both instruments. The DC is incredibly unstable on the C10 pin closest to the switch (S1B), TP4 reference (triangle 1), opposite from R10.

The symbol represents the guarded ground or common.  That should tie in with nearby TP4 and that is where the ground lead of your oscilloscope should go.  Is your AC source ground-referenced?  If so, is the ground side connected to the COMMON input on the DMM?  Is your scope DC-coupled?  Once you are sure all that is good, check these points with your scope and a 1.5V_RMS input.

Understood on the TP4 designation/representation. I have had my scope ground and non-DUT multimeter's common connected to the DUT's common input, rather than TP4.

My AC source is a signal generator that I use for audio stuff, best I could come up with. I have the sig gen output ground going into the DUT's common input.

Q1 inputs pins 2 & 3, you should see a sine wave corresponding to the input but offset somewhat.  These should be quite stable and smooth.  The offset is the result of using CL1 and Q1 as the input buffer.  If it is not steady, try to describe accurately in what way it is unsteady.

With a 1.5vac input the sine wave at U1 input pins 2 and 3 indicates marginal up and down movement. The instrument display indicates anywhere from 1.3vac to 1.6vac. However, as the input voltage is increased the variation/deviation increases as well. At an 8vac input the wave on the display erratically moves up and down and the instrument display deviates further from the input.


Q: "Q1 output pin 6, you should see the waveform I posted, again quite stable and smooth.  If not, note how jumpy and irregular it is in terms of millivolts of deviance or instability."
A: Same waveforms on both machines, same as your post. The instrument with the issue jumps up and down from 3.77v to 3.59v. The other is stable around 3.52v.

[Goatropercu]

Clearly I need guidance on quoting partial posts.

I believe the issue is upstream of U1. That is where the instability begins to occur. Specifically at the anode of CR1. More to come. Thank you for your time.

[bdunham7]

It is not the same on both instruments. The DC is incredibly unstable on the C10 pin closest to the switch (S1B), TP4 reference (triangle 1), opposite from R10.

Understood on the TP4 designation/representation. I have had my scope ground and non-DUT multimeter's common connected to the DUT's common input, rather than TP4.

My AC source is a signal generator that I use for audio stuff, best I could come up with. I have the sig gen output ground going into the DUT's common input.

With a 1.5vac input the sine wave at U1 input pins 2 and 3 indicates marginal up and down movement. The instrument display indicates anywhere from 1.3vac to 1.6vac. However, as the input voltage is increased the variation/deviation increases as well. At an 8vac input the wave on the display erratically moves up and down and the instrument display deviates further from the input.

If you want to add your own comments into the middle of quoted text, you need an 'unquote' ( "/quote" in brackets) before your text and then "quote" in brackets after.

Your setup is correct and your sig-gen should be fine.  TP4 and the COMMON input are directly connected.

So the next step is to see how the signal goes through the buffer.  I'll go one step at a time so I can think about it as we go.  I may disappear for an hour or a day, but I'll be back.

At the back of the region under the shield there is a large resistor (R4) and a diode very near TP4 (CR1).  With your scope test both with a 1.5V input.  You should have a clean ~1.5V signal at the solder turret that is also the junction of R4 and CR1 (the left side) and then on the other side of CR1--right next to TP4--you should have the same clean AC signal but offset quite a bit.  If those aren't perfect, compare both sides of R4--they should be nearly identical in all ways, including amplitude.  The left side may be ~1% lower if you are using a 10X probe, 10% if 1X, but 1X is not so good for this.  100X/100M probes really do have uses.

[bdunham7]

This text will be shown as a quote.

This text will not be shown as a quote

The number of quotes and unquotes must match for the whole post

See the attached picture.  And lets not get started about attachments...

[Goatropercu]

At the back of the region under the shield there is a large resistor (R4) and a diode very near TP4 (CR1).  With your scope test both with a 1.5V input.  You should have a clean ~1.5V signal at the solder turret that is also the junction of R4 and CR1 (the left side) and then on the other side of CR1--right next to TP4--you should have the same clean AC signal but offset quite a bit.  If those aren't perfect, compare both sides of R4--they should be nearly identical in all ways, including amplitude.  The left side may be ~1% lower if you are using a 10X probe, 10% if 1X, but 1X is not so good for this.  100X/100M probes really do have uses.

At the junction of R4 and CR1 cathode I have a stable clean 1.5v sine wave. Probing the anode of CR1 produces a clean sine wave BUT it flutters/jumps up and down on a bit on the screen. This jumpiness is not present at this point on the working instrument. Seems like I’m getting some unwanted dc.

[bdunham7]

At the junction of R4 and CR1 cathode I have a stable clean 1.5v sine wave. Probing the anode of CR1 produces a clean sine wave BUT it flutters/jumps up and down on a bit on the screen. This jumpiness is not present at this point on the working instrument. Seems like I’m getting some unwanted dc.

OK, so I think you've narrowed it down to the input buffer 'stack'.  Try probing points 1-5 in the picture I've attached, both with and without an input signal.  If you can find an instability without a signal that will make things that much easier to figure out.

Points 1,2 and 5 should be fairly steady DC voltages regardless of the signal, although you might see a trace of it at higher amplitudes.  Points 3 and 4 will reflect the signal but they should move together.  What scope/probes do you have?  I could suggest some things with multiple channels and math depending on what you are using.  If you do see a noticeable, repeatable instability without a signal, use a DMM to measure across points 3 and 4--this should remain at a fairly constant ~2.2V (the value may be off 10% or so, but it shouldn't vary with time very much).  I think we're homing in on Q1, but it pays to make sure by eliminating everything else first.  R32 is a less likely but not unheard-of possibility. 

I'll test those points on mine if I get some time later so you can compare.

[Dave Wise]

Looks like should be: point 1 @ -15V, point 2 @ -8V, point 3 slightly > 0V, point 2 = point 3 - 2.2V.  Any leakage in CR1 or CR22 will disturb this.  If they are not painted black, any light falling on them will disturb this on the 0.2V and 2V ranges.

[bdunham7]

So here you go:

Points 1,2 and 5 were as expected at -14.7VDC, -7.5VDC and +15.1VDC (regulation on the power supplies is not fantastic on these...)  with no significant variance with or without the input signal.

Points 3 and 4 are shown here, CH1=point 3 and CH2=point 4, the F1 MATH is the difference, which as you can see reflects the constant ~1.0mA across R32.