Author Topic: Fluke 8840A stuck in measurement  (Read 5841 times)

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Offline bdunham7

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Re: Fluke 8840A stuck in measurement
« Reply #25 on: January 18, 2025, 02:21:56 am »
So I retrieved my test unit.  It's an 8842A but similar enough for our purposes.  I don't have a TP306 that I can see, but for GND 2 you can use the INPUT LOW terminal on the front (provided the F/R switch is out) or the rear lead of CR308 or the front lead of CR310.  For GND 1 the L-shaped metal shield between U202 and U101 seems to be the recommended spot.  I measured basically a dead short between GND 1 and GND 2 with the unit powered off.  With it on, I measured zero voltage except that I was able to see a few hundred microvolts when the 2V range was overloaded.  I also showed the exact same voltage at TP103 regardless of which reference point was used.  So can you specify exactly where you are connecting to GND 1 and GND 2 to make those measurements of TP103?  If you truly have a difference between them, then that needs to be explored.  If not, as in there was some error in selecting the test point, then you can proceed onwards into ADC troubleshooting. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline ulixTopic starter

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Re: Fluke 8840A stuck in measurement
« Reply #26 on: January 18, 2025, 09:57:33 pm »
At the time, I measured on Pin22&Pin21 of U101.
=> Checked, and I'm getting now: GND1 & GND2: Short(off) and 0V(on);
=> at TP103 (between Pin 22 of U101 and Low Input/Pin21) 0.015V to 0.018V (Input shorted).

Had a quick look at some of the inputs/outputs of U101:
- PIN23: 1.23V (Ref GND2)
- PIN20: 1.138V (Ref GND2)
- PIN19: 1.275V to 1.825V (Ref GND2)
- PIN18: 0.027V to 0.031V (Ref GND2)

What do you mean by "proceed onwards into ADC troubleshooting"? Do I understand it correctly that U101 is the ADC? Can this even be troubleshot further?
- Does it make sense to remove U101 (it is seated in a socket)?
- Should I remove R318?

BTW thanks a lot for your effort, much appreciated  :-+


« Last Edit: January 18, 2025, 10:05:37 pm by ulix »
 

Offline bdunham7

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Re: Fluke 8840A stuck in measurement
« Reply #27 on: January 18, 2025, 11:33:55 pm »
What do you mean by "proceed onwards into ADC troubleshooting"? Do I understand it correctly that U101 is the ADC? Can this even be troubleshot further?
- Does it make sense to remove U101 (it is seated in a socket)?
- Should I remove R318?

I would not remove any parts yet.  For further measurements in the reference/ADC area, I'd recommend using the L-shaped shield between U101 and U202 as your ground reference point so that our readings match if we compare.  Yes, U101 is the main ADC controller and switch, but there are important external parts adjacent to it that might be the issue.  We should be able to determine what the issue is eventually.  It is very possible that the issue is one or more of the analog switches internal to U101, but it's worth investigating.

The measurement points you mentioned are mostly not static and would have either be looked at with a scope or with the ADC stopped.  You can stop the ADC by pressing the EXT TRIG button on the front, the display will show no reading as it is waiting for that external trigger signal that will never come.  I have not delved deeply into ADC troubleshooting on these because I haven't had any with those problems, so I'll be exploring this with you for the first time.  I'd recommend you read the ADC troubleshooting section in the manual as it is pretty detailed and does explain what to look for.

Perhaps you could go through that part of the manual and post readings of all the pins of U102, U103 with the ADC stopped and whatever else you test.  You can also have a go at using your scope to look at TP101, etc as suggested in the manual.  It will be a few days before I can do extensive comparisons because I have a large Bang & Olufsen turntable strewn all over my bench and it is needing more and more as I go along.   
« Last Edit: January 18, 2025, 11:39:29 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 


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