Fluke 85 Repair (Strange 10x voltage) - Page 1
Forum section: electronics / repair

author: Spekham2013
author_rank: Newbie
timestamp: August 17, 2020, 09:06:08 pm
content: Hi everyone,I recently acquired a old broken fluke 85 with the service manual. All the ranges work perfectly fine except for the voltage ranges (mV works normal). When reading a voltage I quickly get a overflow and it takes a long time to go to zero. It is as if a capacitor is really slowly discharging. When I read a small voltage (0.3 volt) the multimeter shows a voltage of exactly 10x higher. When going up a range this becomes 100x. I know the multimeter IC will probably not be fault because the mV range still works like normal just like all the other ranges. Has anyone seen a problem like this before and if so can you point me in the right direction?

author: dacman
author_rank: Frequent Contributor
timestamp: August 17, 2020, 09:29:47 pm
content: First, try cleaning the switch and use a lubricant on it.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 17, 2020, 09:44:29 pm
content: 1. Clear focused pictures of the inside PCB helps us help you.2. If you measure a 9V battery, what does the lcd do and show?3. When you short the probes, what does it show on DCV?4. Do you have another working meter to take measurements?

author: mzacharias
author_rank: Frequent Contributor
timestamp: August 17, 2020, 11:31:13 pm
content: Also check the input impedance (resistance) using another multimeter.These sometimes get bad input jack assemblies.

author: Spekham2013
author_rank: Newbie
timestamp: August 18, 2020, 06:50:39 am
content: 1. Added pictures2. The multimeter goes into Overload3. Goes down to 0v not instant though it does go fast4. Yes I have two more working multimeters. (one Prema 5000 and one crappy multimeter)I have also tried to measure input impedance. Only for the mV range could I get a reasonable reading (10.8 MOhm). For the voltage ac/dc range the meter world overflow. I cleaned the input jacks with same kontakt cleaner.

author: Pete66
author_rank: Regular Contributor
timestamp: August 18, 2020, 07:24:09 am
content: Its probably your rotary switch.Remove it as described here and clean board underneath with IPA and toothbrushhttp://mrmodemhead.com/blog/fluke-83-dmm-repair/

author: Spekham2013
author_rank: Newbie
timestamp: August 18, 2020, 09:38:23 am
content: Quote from: Pete66 on August 18, 2020, 07:24:09 amI tried this but it doesn't change anything.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 18, 2020, 09:57:26 am
content: hows the dial pins are, are they physically okay or they are worn outI have 83 (first series) at my job who does pretty funny stuff, they even beep when off loll, a good cleaen up and i did add more tension on the dial contacts by bending them a little more to add some pressure ??The 83 are heavily used at my job, and their life are about to end when were gonna buy new ones, im waiting for the eevblog 150$ meter ...

author: Spekham2013
author_rank: Newbie
timestamp: August 18, 2020, 12:04:14 pm
content: Quote from: coromonadalix on August 18, 2020, 09:57:26 amTook the dial out and cleaned it. Looks fine little bit of wear and tear. The problem still persists. Any more idea's?

author: mzacharias
author_rank: Frequent Contributor
timestamp: August 18, 2020, 01:39:01 pm
content: This pretty much proves your input jack assembly is bad. Take the main board out, plug a known good set of test probes into the main AC/DC input jacks and simply use another meter to measure from each probe tip to the appropriate points on the underside of each input jack. You'll find the bad one pretty quick.Replacements are available on eBay and other sources as well; they run in the 20-30 US dollar range."I have also tried to measure input impedance. Only for the mV range could I get a reasonable reading (10.8 MOhm). For the voltage ac/dc range the meter world overflow. I cleaned the input jacks with same kontakt cleaner."

author: coromonadalix
author_rank: Super Contributor
timestamp: August 18, 2020, 02:22:11 pm
content: Do they have a good pressure ?

author: retiredcaps
author_rank: Super Contributor
timestamp: August 18, 2020, 10:02:19 pm
content: What does measuring a 1k ohm resistor show? What does the meter show when you short the probes when it's on ohms setting?

author: Lifeborne
author_rank: Contributor
timestamp: August 31, 2020, 01:10:09 am
content: Check Z1 values, if they are good then it may be the internal MCU switches that are faulty. Check a voltage in all the manual ranges.