Fluke 8050A calibration

[CDaniel]

Half range should be fine , and if for some reason it's not perfectly linear here is a good point for compromise

[maginnovision]

If it weren't for COVID I'd offer to let you use my calibrator... Perhaps later. Assuming 91343 is your zip we're practically neighbors.

[bob91343]

Yes that is my zip.  Where are you?

[maginnovision]

Near the mission police station.

[JimS]

I was working on a 8050 about 2 weeks ago.
It had been on a shelf unused for around the last 3 years if not more.
The ohm's with a shorting bar was off on every other ohms range.
A good spray cleaning of the switches and it was ok after that.

[AudioGuy]

I have searched the diagram and there is no pot for the 20V range.  The calibration procedure skips that range.

Not that it's terribly far off, maybe around 0.2% at most, but I'd like to trim it.

Does anyone know about this?

You need to calibrate it on the the 200V range, the 20V is derived from that with precision resistors.

Feed it a voltage while it's on the 20V range and twiddle R5. The reading on screen should change.


For me the 20V range is the most useful so I'd "Lego" two 9V batteries together and use the 18V output to match it against my best meter while the 8050A is on the 20V range... 

I have the exact same issue with my 8050A, so I resurrected this thread.  Everything stated in this thread about the 200V range and the 20V range (DC only) are off on my meter.  I went a step farther and looked at the voltage divider network for the DC path.  Everything is spot on using my [calibrated] meter.  I swear there is something after the A/D process.  I'll explain why and look for some feedback.

I use a 19.00 Volt DC source and select the meter to 20V DC range.  The 8050A reads 19.00.  When I select the 200V range, the meter shifts the decimal point, but reads 18.60.  If I select the 2000 range, the meter shifts the decimal and reads 19V.  I look at TP6, which is the output of the divider circuit.  On the 20V range and the 200V range, the divider is spot on and the reading is .190V.  This is with the meter powered on or off.  No supply offset.  So this tells me the issue is with the 'digital' side of the meter.  But for the life of me, I can't see the difference on the digital side for 20V or 200V.

If I calibrate the meter for the 200V scale, the 20V scale is off.  This is linear through the 20V scale (and if I calibrate for the 20V scale, the 200V is off linear in its range). 

For what it is worth, the 2000V scale seems fine.  I can only go to 500V DC, but it is just about spot on.  AC and Ohms measurements are fine.

What am I not seeing here with the 20 / 200 ranges.  I have the original manual and copied the schematic and color-coded paths, but don't see anything different. 

Thanks

[bdunham7]

But for the life of me, I can't see the difference on the digital side for 20V or 200V.

Read the theory of operation part in chapter 3 of the manual.  The ADC itself has two ranges, 200mV and 2V.  The 20V and 200V ranges both use the same 100x divider and then the lower ADC range for 20V, the higher for 200V.  You need to do the DC range calibrations in order as specified--2V, 200mV, 200V, 1000V.  Then it all should work.

[AudioGuy]

Read the theory of operation part in chapter 3 of the manual.  The ADC itself has two ranges, 200mV and 2V.  The 20V and 200V ranges both use the same 100x divider and then the lower ADC range for 20V, the higher for 200V.  You need to do the DC range calibrations in order as specified--2V, 200mV, 200V, 1000V.  Then it all should work.

My [DC] 200mV, 2V, 20V and 2000V are spot on.  I read the theory of operation.  I also understand the diagram at the top of page 3-7 showing the function and range switches.  200mV & 2V are together, 20V & 200V are divide by 100 and 2000 is divide by 1000. 

If the DC divided voltage value at TP6 doesn't change between the mechanical selection of 20V or 200V (Switch 7 & Switch , then it is not a resistive divider problem.  The only difference from the switches at this point is changing the microprocessor via the S7 or S8 to change the state of RNG B or RNG A (pins 3 & pins 4). So then I assume this means the microprocessor selects AZ+INT+LO or AZ+INT+__LO.  So my issue is the path from U19C or U19D?  CMOS switch going bad or C8 or C33 flaky?

[bdunham7]

My [DC] 200mV, 2V, 20V and 2000V are spot on.

Can you try measuring a 190mV signal on the 2V range?

[AudioGuy]

My [DC] 200mV, 2V, 20V and 2000V are spot on.

Can you try measuring a 190mV signal on the 2V range?

@bdunham7 That was it!  There was just a small difference in with my 190mV calibration.  190mV was 188.5 on the 200mV scale, but 190.0 on the 2V scale.  I went through and recalibrated 190mV, 2V scale was still fine and then 190V needed a small adjustment.  That brought the 20V scale perfectly spot on when I went back and checked it.  So now, 19.00 reads on the 20V and 200V scales.

Did I bump the 190mV pot?  I don't know.  But a second look and input from someone else helps.  I appreciate the support.

I hope this thread helps others in the future.