Fluke-85 VDC error - Page 1
Forum section: electronics / beginners

author: halexa
author_rank: Regular Contributor
timestamp: August 27, 2014, 09:10:15 pm
content: 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

author: retiredcaps
author_rank: Super Contributor
timestamp: August 28, 2014, 02:00:50 pm
content: When the 85 is set to VDC, what is the input impedance? It should be around 11.11Mohm.

author: halexa
author_rank: Regular Contributor
timestamp: August 28, 2014, 04:32:38 pm
content: 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 28, 2014, 05:57:05 pm
content: 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?

author: halexa
author_rank: Regular Contributor
timestamp: August 28, 2014, 06:03:38 pm
content: 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 28, 2014, 06:16:58 pm
content: Quote from: halexa on August 28, 2014, 06:03:38 pm6.8M 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/

author: halexa
author_rank: Regular Contributor
timestamp: August 28, 2014, 07:41:52 pm
content: Hi,E1 = OLRV1 = OLRV2 = OLRT1 = 1.3K (Whitin the 30% rating)R1 = 1kSo everything seems OK. Which is not good in this case.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 28, 2014, 08:01:07 pm
content: If you put the 85 into manual VDC range, what input impedance to get for each range?That is,4V =40V =400V =1000V =

author: retiredcaps
author_rank: Super Contributor
timestamp: August 28, 2014, 08:09:19 pm
content: 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?

author: halexa
author_rank: Regular Contributor
timestamp: August 28, 2014, 09:16:35 pm
content: 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 Ohm40V = -30.1V, 6.9M Ohm400V = 4.0V, 10.050M Ohm1000V = 4V, 10.009M OhmIn 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

author: retiredcaps
author_rank: Super Contributor
timestamp: August 28, 2014, 09:30:22 pm
content: 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?

author: halexa
author_rank: Regular Contributor
timestamp: August 28, 2014, 09:38:23 pm
content: It's late her in Sweden. So I will contiune testing tomorrow evning.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 28, 2014, 10:27:59 pm
content: 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 stepshere.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.

author: halexa
author_rank: Regular Contributor
timestamp: August 29, 2014, 01:05:49 pm
content: Quote from: ModemHead on August 28, 2014, 10:27:59 pmWent 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.

author: halexa
author_rank: Regular Contributor
timestamp: August 29, 2014, 07:53:29 pm
content: 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.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 30, 2014, 12:50:59 am
content: 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.

author: halexa
author_rank: Regular Contributor
timestamp: August 30, 2014, 08:14:46 am
content: Quote from: ModemHead on August 30, 2014, 12:50:59 amShorten 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.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 30, 2014, 11:35:46 am
content: Quote from: halexa on August 30, 2014, 08:14:46 amOops. Second pin from the other end. It's the downstream side of the precision 10Meg input resistor (the output of the divider, essentially.)

author: halexa
author_rank: Regular Contributor
timestamp: August 30, 2014, 02:11:51 pm
content: OK, then i get:Input shorten = -410mVInput 10Vdc = -380mV

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 30, 2014, 02:27:15 pm
content: 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 30, 2014, 05:40:33 pm
content: Quote from: halexa on August 29, 2014, 07:53:29 pmDid you also clean the range switch? Particularly contacts S1 1,2,3 and 4.

author: halexa
author_rank: Regular Contributor
timestamp: August 30, 2014, 05:54:17 pm
content: Quote from: retiredcaps on August 30, 2014, 05:40:33 pmYepp, and I also double checked that the switch is making all the right connections.

author: halexa
author_rank: Regular Contributor
timestamp: August 31, 2014, 08:02:42 am
content: 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 = 10VZ1 pin2 = 5V (10Meg input impedance @ my DMM)U4 pin99 = -380mVSee attached test setup pic.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 31, 2014, 02:35:40 pm
content: 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 = 0.000/0.001Input 5.000V: Z1P2 = 51.39mV 85 display = 5.00Input 10.000V: Z1P2 = 102.80mV. 85 display = 10.00While 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.

author: halexa
author_rank: Regular Contributor
timestamp: August 31, 2014, 02:57:34 pm
content: 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.

author: halexa
author_rank: Regular Contributor
timestamp: September 01, 2014, 10:07:59 am
content: Latest status,I desolder all caps surrounding Z1 so I could access all traces around Z1, and gave the PCB a new IPA bath with a good scrub partially around Z1 and U4. I examined the PCB under microscope to see if there was anything interfering with the pins of U4 or the traces around it and followed all traces to and from Z1. And everything looks good.But the status of the 85 in the VDC mode is still broken.So as a desperate action i desolder the U4 to see if there was something under the IC that I was missing, but that did not help either.Was next a new CPU? or a new meter?

author: ModemHead
author_rank: Frequent Contributor
timestamp: September 01, 2014, 12:35:06 pm
content: I'm impressed with your determination! Sounds like you've eliminated the possibilities for a simple repair. I'm sorry I can't think of any other ideas now, if the input to the ADC is wrong, the meter is going to read wrong, simple as that.Quote from: halexa on September 01, 2014, 10:07:59 amThis model is too old to find a new CPU. There's the possibility of finding a parts donor, which according to the parts list, can come from an 85 or an 83.

author: halexa
author_rank: Regular Contributor
timestamp: September 01, 2014, 12:56:18 pm
content: Thank you all for superb guidance in my attempt to fix my Fluke85.Guess its time to start scanning the auction sites for an 83 or 85 with a smashed LCD.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 01, 2014, 02:06:24 pm
content: Quote from: halexa on August 27, 2014, 09:10:15 pmJust curious if this 85 you bought was advertised as working or for parts only?

author: halexa
author_rank: Regular Contributor
timestamp: September 01, 2014, 02:26:02 pm
content: It was advertised as " powers up but unknown functionality" so it was a gamble from my side. Only paid 30usd for it.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 02, 2014, 05:16:03 am
content: Quote from: halexa on September 01, 2014, 02:26:02 pmYou can easily recoup that money by selling the Fluke 85 parted out. The holster, fuses (if they were good), front/back case, zebra strips, lcd, input jacks all sold separately could net around $60 to $100.