Fluke 8050A fault - reading "OL" on all ranges - diyAudio
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Old 30th December 2012, 08:27 PM   #1
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Default Fluke 8050A fault - reading "OL" on all ranges

Can anyone help me with this fault - I have a Fluke 8050A bench DMM, it worked fine for years, then suddenly one day, it displayed OL (overload) with no input when set to any AC or DC voltage/current range.
When set to DC, fault condition shows as a positive overload (+ sign displayed at RHS). The manual suggests a problem with Q17/C8 power-on reset, these check OK.
One important clue (I hope), is that at initial power on, the display will zero for a moment before reading overload, leading me to think there is a problem with the ADC, or something in that area.
Any assistance would be much appreciated, it is still a useful tool (when working!).
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Old 31st December 2012, 04:33 PM   #2
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Location: North Carolina
You've probably already done this, but don't forget to check the supply voltages in case it's just some electrolytic filter cap gone bad or somesuch. And check the voltage reference VR1, too.

You're probably right about the ADC, but the good news is this meter implements its ADC in discreet components, under the direction of the 8251 micro-controller. LM358 op-amps, CD4016 CMOS switches and so forth. It can probably be fixed if you can locate the fault.

Use TP6 as a convenient place to separate the input circuitry (voltage divider, switches) from the ADC and make sure it's not the front end. You never know. In 200mV and 2V ranges, the voltage divider is in 1:1 mode, so TP6 should reflect the voltage at the input jacks. Don't forget that many points in this circuit are high impedance and can be affected by poking around with a 10Meg DMM.

I think some of the CD4016s are socketed. You could switch them around and see if things change. Do make note of their original locations because I see the parts list says they're 'selected'.

I'm no expert on this thing, but I do have a working one here and if you spot something fishy and need a comparison measurement, I can probably provide it.
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Old 1st January 2013, 08:09 PM   #3
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Many thanks for the pointers, I'll look into it further, good hint re TP6. I've already block changed caps in power supply, that was the first thing I wanted to eliminate. Will post findings, and thankyou for offer of measurements from working unit too.
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Old 4th January 2013, 02:59 AM   #4
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Success! Measurement suggested everything normal up to TP7 (output of U7A), no signs of activity around U7B, TP8, and U9, replaced U7, no change, replaced U9 (comparator), machine now working.
1.9V DC cal needed a nudge, and I had to adjust U33 jumper block slightly, then it seemed to set up fine - without access to a calibrator, I matched readings to a low mileage unmolested Fluke 45 (machine used as 2nd bench DMM, so not mission critical).
Thanks again mrmodemhead, got me pointed in right direction!
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Old 4th January 2013, 03:26 AM   #5
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Location: Canandaigua, NY USA
I had to fix mine a while back. The electrolytics near the center of the main board were bad and the contamination caused issues- had to clean carefully before installing new ones. The ohms readings weren't as accurate as they should have been at the high end. There's a metal can FET not far from the center that's also subject to PCB contamination. Everything looked clean but when I cleaned the area with a Q-Tip and alcohol, including under the leads by stretching the cotton and pulling through with tweezers, the ohms readings came right back where they should be. It's a great meter when working right!
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Old 4th January 2013, 12:24 PM   #6
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Great job, glad you got it sorted! You sound like a pro at trouble-shooting. I enjoy using my 8050A as well, so much that I converted it from LCD to LED display to keep it going.

And yes, it's amazing how many problems you can fix with Q-tips and alcohol.
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Old 8th January 2013, 08:12 PM   #7
ilardi is offline ilardi  United States
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I like my 8050A enough that when my first one died I tossed it and bought a new one on ebay (they are pretty cheap if you are diligent). My first one died because the display failed-which I later learned can be a relatively easy fix. I came across this website describing a way to convert an 8050A to LEDS http://lous.home.xs4all.nl/fluke/Fluke8050Asite.html. Could you share the way you did it?
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Old 8th January 2013, 09:33 PM   #8
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Quote:
Originally Posted by ilardi View Post
... I came across this website describing a way to convert an 8050A to LEDS http://lous.home.xs4all.nl/fluke/Fluke8050Asite.html. Could you share the way you did it?
That website inspired me to do mine, but he didn't reveal much about what was going on behind the display. I didn't do a full write-up but I do have a fairly self-expanatory picture gallery with captions at my website.

The LED displays I used are not optimum in terms of efficiency, I had to push the current limits on the CMOS drivers, but the 8050A has been running 24/7 for 15 months with no problems.
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Old 9th January 2013, 03:24 PM   #9
ilardi is offline ilardi  United States
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Your picture gallery is actually very clear. Looks like you got a very nice result.
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