Author Topic: Fluke-85 VDC error  (Read 10796 times)

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Offline halexaTopic starter

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Fluke-85 VDC error
« on: August 27, 2014, 09:10:15 pm »
Hi,

I have bought an used Fluke 85 DMM. However there is an error on the VDC range, it gives an offset of 14.2V and the Auto range doesn't lock into the 40 range to display these 14.2V. All other functions works fine like the Ohm range which basically uses the same input curcuit.
If I do a REL operation and zero away the offset it works fine. 
The only area in the schematics as I have understand it that is unique to the VDC range is the voltage divider around Z1 (page 80 in the service manual). But I could be completely wrong.
I have checked the resistances of Z1 and they seems all fine. I'm not sure how i shall continue my search for the error. So if anyone have some suggestion how I shall proceed please help me out.

Service manual:
http://assets.fluke.com/manuals/83_85_87smeng0500.pdf
 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #1 on: August 28, 2014, 02:00:50 pm »
When the 85 is set to VDC, what is the input impedance?  It should be around 11.11Mohm.
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #2 on: August 28, 2014, 04:32:38 pm »
Thx for feedback,

By simply hook up the input of my Fluke to another DMM and mesaure the resistance of the Fluke input in VDC mode without the battery I get 14.2M Ohm.
I dont know if this is the correct way to measure input impedence of an DMM though.
 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #3 on: August 28, 2014, 05:57:05 pm »
Here is how I got 11.11M.

Turn 85 on and set to VDC.  Then take another multimeter and measure the resistance between the COM and V input jacks.

Just to confirm, when you short out the probes, does the LCD display 14.2V?

In addition, are there any signs of physical damage inside the unit on the pcb?  Scorch/burn marks, dirt/contamination?
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #4 on: August 28, 2014, 06:03:38 pm »
Ok, I measured with no battery, with battery and in VDC mode I get 6.8 M ohm.

Yes, I get 14.2V when COM and V inputs are shorted.

Nope, no physical damage. I have cleaned the PCB with Isopropanol (even though there were no visible dirt on it) and gone through it visually and everything looks fine.
 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #5 on: August 28, 2014, 06:16:58 pm »
with battery and in VDC mode I get 6.8 M ohm.
6.8M is low.

Your input protection components might be bad?

Check the surge protector (E1),  PTC (RT1), MOV (RV1, RV2)s and fusible resistor (R1).

From the 85 service manual, a good PTC should measure around 1.5kohm.  A good MOV should measure 0L (inifinte) ohm.  A good fusible resistor should measure around 1kohm.

This thread might help?

https://www.eevblog.com/forum/testgear/fluke-87-v-mystery-low-impedance-input/
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #6 on: August 28, 2014, 07:41:52 pm »
Hi,

E1 = OL
RV1 = OL
RV2 = OL
RT1 = 1.3K (Whitin the 30% rating)
R1 = 1k

So everything seems OK. Which is not good in this case.

 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #7 on: August 28, 2014, 08:01:07 pm »
If you put the 85 into manual VDC range, what input impedance to get for each range?

That is,

4V = ???
40V = ???
400V = ???
1000V = ???
 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #8 on: August 28, 2014, 08:09:19 pm »
Do you get the 14.2V DC offset when the 85 is in manual range for the following?  That is, what the does LCD show for each of the following manual ranges?

4V = ???
40V = ???
400V = ???
1000V = ???

Any offset on the mV range?
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #9 on: August 28, 2014, 09:16:35 pm »
Strange now i don't get that 14.2V offset, but some other strange readings. This is after disassemble the unit for checking the input protection and the reassemble the unit again.

4V = -3.8V,  6.1M Ohm
40V = -30.1V, 6.9M Ohm
400V = 4.0V, 10.050M Ohm
1000V = 4V, 10.009M Ohm

In the 4V and 40V the readings is not stable it jumps up and down a few 10mV same with the input resistance. The voltage reading is done with shorten V and COM input.
The mV functions shows 000.0mV with shorten V and COM input and an input resistance of 10.9M  Ohm
 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #10 on: August 28, 2014, 09:30:22 pm »
I have to leave for a few hours and might look at it later tonight or tomorrow.

Just curious after the re-assembly if your other functions work okay now?
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #11 on: August 28, 2014, 09:38:23 pm »
It's late her in Sweden. So I will contiune testing tomorrow evning.
 

Offline ModemHead

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Re: Fluke-85 VDC error
« Reply #12 on: August 28, 2014, 10:27:59 pm »
It's time to clean the PCB in IPA.

Remove the shields and LCD assembly, be very careful not to break any of the fragile plastic bits.  You may find it helpful to look at the disassembly steps here.

Immerse the PCB in about 1cm of the strongest isopropyl alcohol you can find.  Use a brush to work around the components.  Pay particular attention to the main chip, make sure there is no dirt, oil, or other contaminant between the leads.


Allow the board to dry thoroughly before re-assembling.  A fan speeds the process.
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #13 on: August 29, 2014, 01:05:49 pm »
It's time to clean the PCB in IPA.

Went to the store today and picked up 1000ml of 99.6% Isopropanol. I will give this approach a try this evening, wait for an update later of how it went.
« Last Edit: August 29, 2014, 01:07:33 pm by halexa »
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #14 on: August 29, 2014, 07:53:29 pm »
No change after IPA cleaning VDC is still broken.

Check some other functions also.

Ohm mode: Did a comparison against my Fluke 8600A of a range o resistors and the Fluke 87 gave the same reading. So Ohm mode seems to work fine.
Diod test: It gives 3.578V out and I tested a few diodes and it works fine.
Cap: tested a bunch of them and got OK readings.
V-AC: Plugged it in to mains and got 228V/50Hz = OK.
mVDC:Works fine, gets same readings as from my 8600A.



 

Offline ModemHead

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Re: Fluke-85 VDC error
« Reply #15 on: August 30, 2014, 12:50:59 am »
Too bad, you wouldn't believe how often PCB cleaning will get rid of odd readings.

It's fortunate you have an 8600A.  It has a feature that is useful for debugging DMM front ends.  On the 200mV and 2V ranges, it's impedance is over 1 Gigohm.  The usual DMM 10Meg impedance is too low for the downstream side of a DMM voltage divider.

Set the 85 to VDC and short the inputs.  With the 8600A in 200mV range, measure from the COM jack to the second pin up from the bottom of Z1.  It should be very close to zero.

Now apply 10V to the 85 input.  It should auto-range to the 40V range, so the voltage divider will be in 1/100 mode.  You should now measure very close to 100mV on the same pin of Z1.

If your measurements show too high a voltage at that pin, then there is some current leaking somewhere that is pulling the ADC input up, causing a problem.
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #16 on: August 30, 2014, 08:14:46 am »
Set the 85 to VDC and short the inputs.  With the 8600A in 200mV range, measure from the COM jack to the second pin up from the bottom of Z1.  It should be very close to zero.
Now apply 10V to the 85 input.  It should auto-range to the 40V range, so the voltage divider will be in 1/100 mode.  You should now measure very close to 100mV on the same pin of Z1.

Shorten inputs = 00.35mV.
10Vdc input = no stable readings, jumps between 00.32-00.40mV.

Just to be sure i measured the right pin see attached pic.
 

Offline ModemHead

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Re: Fluke-85 VDC error
« Reply #17 on: August 30, 2014, 11:35:46 am »
Just to be sure i measured the right pin see attached pic.
Oops.  Second pin from the other end.  It's the downstream side of the precision 10Meg input resistor (the output of the divider, essentially.)
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #18 on: August 30, 2014, 02:11:51 pm »
OK, then i get:

Input shorten = -410mV
Input 10Vdc = -380mV
 

Offline ModemHead

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Re: Fluke-85 VDC error
« Reply #19 on: August 30, 2014, 02:27:15 pm »
That's totally off, shouldn't be negative with zero or positive input, and shouldn't be that high.  I have to run off and do Saturday-type things, but if you look at the schematic, you may see what I'm asking you to check.  The output of the voltage divider, which is the input to the ADC (U4P99).

The idea is to find out if the offsets you see are a result of leakage currents pulling up the very high impedance ADC input.
 

Offline retiredcaps

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Re: Fluke-85 VDC error
« Reply #20 on: August 30, 2014, 05:40:33 pm »
No change after IPA cleaning VDC is still broken.
Did you also clean the range switch?  Particularly contacts S1 1,2,3 and 4.
« Last Edit: August 30, 2014, 05:42:38 pm by retiredcaps »
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #21 on: August 30, 2014, 05:54:17 pm »
No change after IPA cleaning VDC is still broken.
Did you also clean the range switch?  Particularly contacts S1 1,2,3 and 4.

Yepp, and I also double checked that the switch is making all the right connections.
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #22 on: August 31, 2014, 08:02:42 am »
Don't know if this say anything, but i did a test were I broke the connection between Z1 pin2 and U4 pin99.
I applied 10V to the input and measured the voltages:

Z1 pin1 = 10V
Z1 pin2 = 5V (10Meg input impedance @ my DMM)
U4 pin99 = -380mV

See attached test setup pic.
 

Offline ModemHead

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Re: Fluke-85 VDC error
« Reply #23 on: August 31, 2014, 02:35:40 pm »
The 5V is fine, given a 10Meg input DMM.  The -380mV is not good.  U4P99 should be just floating around if you disconnected it from the voltage divider.

Just to make sure I wasn't leading you astray somehow, I happen to have a Fluke 85 here so I made a few measurements with a Fluke 8600A in 200mV range.  The 85 was powered on and set for VDC.

Inputs shorted: Z1P2 = 0.06mV (unsteady) 85 display = 0.000/0.001
Input 5.000V:   Z1P2 = 51.39mV           85 display = 5.00
Input 10.000V:  Z1P2 = 102.80mV.         85 display = 10.00

While it is possible U4 may be defective, I've found it's beneficial to look really close for contamination sources that may escape an initial inspection or cleaning.  It doesn't take much leakage to influence the very high-impedance ADC input.

Here are some pics of an 83 I cleaned up a few months ago.
 

Offline halexaTopic starter

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Re: Fluke-85 VDC error
« Reply #24 on: August 31, 2014, 02:57:34 pm »
OK, will start up my microscope and closley examine the routing to the adc input. To see if i missed anything that could interfere with the trace and cousing the current flow.
 


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