Author Topic: Fluke 8846A Ohms repair  (Read 6990 times)

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Offline billclay

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Fluke 8846A Ohms repair
« on: March 14, 2013, 05:53:48 am »
I've got a Fluke 8846A working fine (in spec) on AC & DC volts and amps.
Ohms, continuity, diode, capacitance and temp all give false readings.

Open reading on ohms: 224ohms
Open reading on continuity: 224 ohms
Open reading on diode: 0.224V
Short on capacitance: 28.4uF
Short ohm readings seem to be OK.

With another meter connected on these ranges, all outputs are 0.224VDC. (Specs say 13V).

I don't have schematics for this meter, so I'm at a loss on what to test on the main PCA.
If anyone can provide any tips on what to test, or if you've encountered this issue before, please
send me a message.  Thanks!

 

Offline vtl

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Re: Fluke 8846A Ohms repair
« Reply #1 on: March 14, 2013, 09:26:20 am »
From the sounds of things there's an issue with the Ohms current source. Without the schematic the best thing to do is power up the meter with the chassis open and trace out the signal from the front terminals and see where that 0.224V goes.

A precision current source may include a fancy resistor network and an opamp so you may find some luck there.
 

Offline billclay

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Re: Fluke 8846A Ohms repair
« Reply #2 on: March 14, 2013, 09:54:28 pm »
I was able to trace this output voltage to the following spots.  Since it's a multi-layer board, it's difficult following the path. Attached is a photo.

TP10 connected to CR30 (.224v)
Pin7 of U30 (AD820A Op-Amp) (.224v)

I'm stuck...
 

Offline billclay

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Re: Fluke 8846A Ohms repair
« Reply #3 on: March 14, 2013, 09:56:24 pm »
Also, I wanted to mention that VR9 and VR10 were hand-soldered in by someone afterwards.  There was a lot of flux residue around them which I cleaned.
 

Offline vtl

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Re: Fluke 8846A Ohms repair
« Reply #4 on: March 15, 2013, 07:10:52 am »
According to the datasheet, pin 7 is the supply rail for the opamp. If pin 7 is 0.224 V then the power supply is dead for that opamp.

With a multimeter on continuity you'll want to trace that node and find the regulator that powers it (possibly the VR9 and 10 components you've noted.)
 

Offline billclay

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Re: Fluke 8846A Ohms repair
« Reply #5 on: March 15, 2013, 08:33:28 am »
According to the datasheet, pin 7 is the supply rail for the opamp. If pin 7 is 0.224 V then the power supply is dead for that opamp.

Sorry, I meant pin 6.  Pin 7 does indeed have power.
Furthermore, the opamp U30 seems to connect back to U27 (LM339DG).

If anyone has access to a 8845A/8846A, can they measure the output voltage on Ohms in each of the ranges (manual).

Thanks.
 

Offline mzzj

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Re: Fluke 8846A Ohms repair
« Reply #6 on: March 15, 2013, 02:25:49 pm »
I would check next if U30 opamp inputs and outputs make any sense.
There should be no voltage difference between opamp inputs if the output is not saturated against supply voltages.

You may want to check how ohms current source is done in Agilent 34401A. Schematics and service manual is available from internet to 34401A.
With some luck it may give you some ideas how that thing works and what to look for if Fluke has used somewhat similar circuit.

 

Offline robrenz

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Re: Fluke 8846A Ohms repair
« Reply #7 on: March 15, 2013, 02:37:46 pm »
Are the readings the same in 2 wire and 4 wire resistance modes? 

Offline billclay

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Re: Fluke 8846A Ohms repair
« Reply #8 on: March 16, 2013, 02:54:28 am »
Are the readings the same in 2 wire and 4 wire resistance modes?

Yes, exactly the same on 2x4wire and 4 wire.  Also the same on the rear terminals.
So it's something inside the meter, not the physical terminals.
 

Offline billclay

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Re: Fluke 8846A Ohms repair
« Reply #9 on: March 16, 2013, 03:12:11 am »
I would check next if U30 opamp inputs and outputs make any sense.
There should be no voltage difference between opamp inputs if the output is not saturated against supply voltages.

You may want to check how ohms current source is done in Agilent 34401A. Schematics and service manual is available from internet to 34401A.
With some luck it may give you some ideas how that thing works and what to look for if Fluke has used somewhat similar circuit.

Here is U30 (chip is flipped 180 degrees on board)
« Last Edit: March 16, 2013, 03:16:56 am by billclay »
 

Offline billclay

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Re: Fluke 8846A Ohms repair
« Reply #10 on: March 16, 2013, 03:21:46 am »
Also, here is U27 (LM339DG):

Pin     Voltage (DC unless marked VAC)
---------------------
1 -15.113
2 6.630VAC 9.8Hz
3 14.960
4 1.654
5 1.520VAC 9.6Hz
6 1.653
7 0
8 1.653
9 0
10 1.653
11 3.218
12 -15.131
13 0.214  <--------note
14 -15.113
 

Offline mzzj

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Re: Fluke 8846A Ohms repair
« Reply #11 on: March 17, 2013, 01:11:23 pm »
I would check next if U30 opamp inputs and outputs make any sense.
There should be no voltage difference between opamp inputs if the output is not saturated against supply voltages.

You may want to check how ohms current source is done in Agilent 34401A. Schematics and service manual is available from internet to 34401A.
With some luck it may give you some ideas how that thing works and what to look for if Fluke has used somewhat similar circuit.

Here is U30 (chip is flipped 180 degrees on board)

U30 Looks reasonable.
Without schematic or working unit this goes hardcore level. Keep hoping that someone pops in with solution.
Have you checked if schematics are available for reasonable amount of money?
 


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