Fluke 73 III repair  - Page 1
Forum section: electronics / repair

author: bborisov567
author_rank: Regular Contributor
timestamp: July 27, 2023, 06:52:20 pm
content: I got the unit with leaked battery. Fortunately there was little damage to the board. I cleaned the board very well, even removed the analog chip and cleaned under it. Now the unit is in somewhat better condition but still there is a problem with the ohm meter function. In open circuit it reads about 8-10 megaohms and when shorted it read about 40 ohms and the value is gradually going to zero, it's like there is a charging capacitor. All other modes are working accuratelly. When measuring voltage it needs about 3-4 seconds to settle. I tried removing the MOVs at the input but there is no change. Also when i short the "OHMS" pin to ground it reads around -40 ohms, note the minus. The 33pF cap between OHMS pins and ground is changed. The refference voltage and the supply voltages are also stable. I don't have the exact schematic but i've been using the schematic for the 73 ii, which is quite similar. I have checked Q1,2,3 with a transistor tester and they look okay. Can somebody suggest what might be wrong?

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 27, 2023, 07:24:10 pm
content: What did you clean it with? Usually for alkaline battery leaks, cleaning with some vinegar and a flush with clean water works well. For the remaining greasy residue I use a 10% ammonia/water mix with a few drops of dish soap. Scrub with a short haired paint brush, rinse well with water, and blow dry with air.Then dry in a very low temp oven / drier (100C / 212F) for an hour before powering back on. (Take LCD off first)There could also be some contaminants in the yellow sparkgap next to the fuse. You could run a piece of paper soaked in 99% alcohol through the gap.

author: bborisov567
author_rank: Regular Contributor
timestamp: July 27, 2023, 07:37:47 pm
content: I cleaned it with vinegar after that with alcohol and in the end i rinsed the board with soap water and let it dry. I tried cleaning the spark gap with paper soaked in alcohol. I am wondering if the acid could be in the pcb material. But still that would explain the 10 meg reading but not the 40 ohm.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 27, 2023, 08:11:47 pm
content: Remember that an ohm meter is just a volt meter and a current source. So if there's any voltage leakage into the ADC, then that'll look like a resistance. Does the meter read zero on the 300mV DC scale?Here's alink to the Fluke 77/73 manuals.Diagram is from the 77 manual, but the 73 will be of similar in design.

author: Paceguy
author_rank: Regular Contributor
timestamp: July 27, 2023, 08:16:17 pm
content: I could be wrong,but other than some residue from cleaning it could be an open fusible resistor. I circled the two resistors that could be suspect. Also bad contacts on the rotary switch can cause this kind of problem.

author: iJoseph2
author_rank: Regular Contributor
timestamp: July 27, 2023, 10:06:02 pm
content: Found these threads which talk about cleaning helping their faults ..https://www.eevblog.com/forum/chat/fluke-77-ii-incorrect-readings-on-all-ranges/https://www.eevblog.com/forum/repair/fluke-73-series-iii-slow-to-gt-to-reading/

author: bborisov567
author_rank: Regular Contributor
timestamp: July 28, 2023, 05:41:40 am
content: Well, on DC voltage there is some reading, around 100-200 mv when floating but when i short the inputs it goes to zero, unlike the mysterious 40 ohms. Also i was going to ask about C1. In the manual it says it is a dc blocking capacitor but we are actually measuring dc voltage? And as far as i understand the potential representing the unknown resistance is measured at pin “OHS”, right? So if i short this pin to ground i must get a zero reading? And last question - there might be flux left under the chip. What would be the best to try cleaning it?EDIT: Yesterday i left the meter on my desk with the battery removed. This morning i decided to give it another try. Now it it reading 28 megaohms and 13-14 ohms when shorted. And it hasn’t been touched at all.

author: mentholflash
author_rank: Contributor
timestamp: July 28, 2023, 10:47:39 am
content: Could be this icjust watched itRegards

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 28, 2023, 05:22:35 pm
content: Quote from: bborisov567 on July 28, 2023, 05:41:40 ams1 bypasses C1 & R2 when the meter is in the Ohms, VDC, or MVDC modes.QuoteThat does appear to be correct. OHS is defined as "Ohms Sense" in the manual.QuoteMaybe a bit of a soak in alcohol 1st, and then flushing it repeatedly with fresh alcohol.QuoteDefinitely sounds like it's drying out. So maybe a bit of time in a low humidity heated environment will dry it enough that the ghost readings go away.

author: Paceguy
author_rank: Regular Contributor
timestamp: July 29, 2023, 12:27:46 pm
content: EDIT: Yesterday i left the meter on my desk with the battery removed. This morning i decided to give it another try. Now it it reading 28 megaohms and 13-14 ohms when shorted. And it hasn’t been touched at all.[/quoteSounds like it's still drying out to me too. Try lightly running a hair dryer over the back of the board.

author: bborisov567
author_rank: Regular Contributor
timestamp: July 31, 2023, 07:09:54 am
content: I dis some cleaning and let it dry for a few days. Now with the input floating i get OL reading but with the input shorted it get around -9 ohms. What does that minus mean? Does it mean there is a higher voltage on the OHS pin than expected?

author: Paceguy
author_rank: Regular Contributor
timestamp: July 31, 2023, 11:44:25 am
content: Maybe during the cleaning process, the calibration pot got turned. I would check to see if the DC calibration is spot on first. There is just one cal pot on these.

author: bborisov567
author_rank: Regular Contributor
timestamp: July 31, 2023, 12:38:43 pm
content: I already recalibrated it an works fine and accurate on DC with the exception of some small voltage reading (0.2-0.3v) when the input is floating. But when it is shorted i get a solid 0.00 v reading.

author: marcovanyken
author_rank: Newbie
timestamp: November 11, 2024, 10:03:31 am
content: Quote from: bborisov567 on July 27, 2023, 06:52:20 pmHi Bborisov,I see this is an old thread but hopefully you receive my message.Can you perhaps tell me what Transistor is the one marked in my attached pic. Its the pic of your board but I have the exact same board.Thank you

author: mqsaharan
author_rank: Regular Contributor
timestamp: November 12, 2024, 03:52:39 am
content: Quote from: marcovanyken on November 11, 2024, 10:03:31 amI don't have the service manual or schematic for this particular meter. But my guess is the encircled transistor will be 2N3904, exactly the same as the one next to it. Together these are used as a high speed clamp.Please be careful while taking my suggestion. I could be completely wrong.

author: marcovanyken
author_rank: Newbie
timestamp: November 12, 2024, 04:02:29 am
content: Quote from: mqsaharan on November 12, 2024, 03:52:39 amThank you for the reply, I also thought it would be a 2N3904 but wanted someone else's input as well.

author: bborisov567
author_rank: Regular Contributor
timestamp: November 13, 2024, 08:21:43 am
content: I can check in a few days if still needed but as far as i remember both transistors were the same.

author: marcovanyken
author_rank: Newbie
timestamp: November 14, 2024, 06:26:32 am
content: Quote from: bborisov567 on November 13, 2024, 08:21:43 amIt was indeed a 2N3904 as well. Fluke like new again.