Electronics > Repair
Fluke 289 not quite sorted.
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nixxon:

--- Quote from: retiredcaps on October 14, 2018, 04:13:00 am ---
--- Quote from: nixxon on October 13, 2018, 10:15:36 pm ---After turning the dial into resistance mode, the display shows 25 M Ohms while probes are open circuit.

When the probes are separated again, the display doesn't return to 25 M Ohms, but shows ~3.7 M Ohms (while probes are open circuit)

--- End quote ---
The non display of 0L might indicate that one or more of your input protection circuits has been damaged.

You can measure all in circuit and see if they are good.  If not, desolder them to verify.

A MOV should measure infinite resistance.  The PTC should be around 1.1k ohm.  The fusible resistor should be around 1k ohm.

--- End quote ---

I will have to figure out a way to identify the components that are involved in measuring resistance. I guess watching some of David L. Jones' old DMM teardown videos will be helpful.

In the meantime I used another DMM to measure the voltage output of the 289 terminals (COM and V) while the 289 was in resistance mode. Initially (while displaying 25 M Ohms) the voltage is 1.92 volts. After the terminals have been shorted momentarily the voltage increases to 2.76 volts (while displaying 3.7 M Ohms).

Does anyone have any idea what is going on, and why the voltage increase after the probes have been shorted?

I measured the blue Metal Oxide Varistors (MOV) and they did not show infinite resistance:
RV1: 17.5 M Ohm
RV2: 24.0 M Ohm (not stable)
RV3: 23.5 M Ohm (not stable)

Can they all be bad? I have read that "When MOVs degrade they become more conductive after they have been stressed by either continuous current or surge current." Source: https://iaeimagazine.org/magazine/2004/03/16/metal-oxide-varistor-degradation/

The black Positive Temperature Coefficient (PTC) thermistors were measured ok:
RT1: 1.1 k Ohm
RT2: 1.2 k Ohm


The fusible resistor R4 has a good resistance of 1.01 k Ohm

I also measured the main fuses
XAF1 ("DMM 44/100"): 0.8 Ohm
XAF2 ("DMM-11A"): 0.1 Ohm.

Is XAF1 supposed to have such a high resistance? It seems wrong.

Now I will read the Service Manual for Fluke 76 Chapter 2 Theory of operation and hopefully learn quite a few things about how this and other DMM's work...
I found it here: http://assets.fluke.com/manuals/76______smeng0000.pdf
retiredcaps:

--- Quote from: nixxon on October 17, 2018, 04:06:25 pm ---I measured the blue Metal Oxide Varistors (MOV) and they did not show infinite resistance:
RV1: 17.5 M Ohm
RV2: 24.0 M Ohm (not stable)
RV3: 23.5 M Ohm (not stable)

Can they all be bad?
--- End quote ---

Desolder all the MOVs and re-measure them out of circuit.  With the MOVs removed, see if the meter works on resistance mode.

Verify that ACV and DCV works with the MOVs on low energy circuits (i.e. no mains, etc).


--- Quote ---I also measured the main fuses
XAF1 ("DMM 44/100"): 0.8 Ohm
XAF2 ("DMM-11A"): 0.1 Ohm.

Is XAF1 supposed to have such a high resistance? It seems wrong.

--- End quote ---
The fuses only affect current measurement.
retiredcaps:

--- Quote from: nixxon on October 17, 2018, 04:06:25 pm ---In the meantime I used another DMM to measure the voltage output of the 289 terminals (COM and V) while the 289 was in resistance mode. Initially (while displaying 25 M Ohms) the voltage is 1.92 volts. After the terminals have been shorted momentarily the voltage increases to 2.76 volts (while displaying 3.7 M Ohms).

--- End quote ---
When the non working 289 is set to DCV, its input impedance should be around 11.11M ohm.
nixxon:

--- Quote from: retiredcaps on October 17, 2018, 10:37:03 pm ---
--- Quote from: nixxon on October 17, 2018, 04:06:25 pm ---In the meantime I used another DMM to measure the voltage output of the 289 terminals (COM and V) while the 289 was in resistance mode. Initially (while displaying 25 M Ohms) the voltage is 1.92 volts. After the terminals have been shorted momentarily the voltage increases to 2.76 volts (while displaying 3.7 M Ohms).

--- End quote ---
When the non working 289 is set to DCV, its input impedance should be around 11.11M ohm.

--- End quote ---

When the non working 289 is set to DCV, its input impedance is shown alternating between 11.004 or 11.005 M Ohm on my good 287 and alternating between 11.00 and 11.01 M Ohms on my good 83V. So the impedance of the non working 289 is ~0.105 M Ohm or ~105 k Ohm lower than what you suggested.

A comparison:
When my good 287 is set to DCV, its input impedance is shown as (10.982 M Ohm on the non working 289 and) alternating between 10.99 and 11.00 M Ohm on my good 83V. (and steady 11.00 M Ohm on my 87 (version 1)).

After shorting the probes on the non working 289 for a moment (open probes display 3.7 M Ohm), the reading on the 289 is lowered from 10.982 to 3.6542 M Ohms while the DUT 287 shows 2.7498 VDC. Lots of figures. Go figure...

Are you shure the imput impedance of a 287/289 in VDC mode is supposed to be 11.11 M Ohms?  (rather than 11.00 M Ohms)

The input impedance of my 83V and 87 (version1) in VDC mode are both ~11.11 M Ohms, though.
nixxon:
The faulty 289 (while in resistance measurement mode) goes from 25 M Ohms open circuit to 3.7 M Ohm open circuit after a resistance below 3.7 M Omhs is registered. Check enclosed log graph where you se me touching both the probes firmer and firmer until measured resistance goes below 3.7 M Ohm.
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