Fluke 27 multimeter part needed - Page 1
Forum section: electronics / testgear

author: pinpassion
author_rank: Contributor
timestamp: April 04, 2025, 08:54:16 pm
content: Hi gang,I just picked up a Fluke 27/FM on Ebay sold as parts or repair. Of course, I want to repair it.I found a component on the board that is cracked straight through. In the manual for the 27 it is listed as RT1. I think it is associated with the input protection circuitry. It is located just above the MOV's on the main board. There are two of them, the other being located on the opposite side of the board. In the manual, the description of the part is:Thermistor, Rect., POS., 1K, +/- 40%, 25c Manufacturers Part # 911P84E102YU13 On the part it is labeled:TDK 8049I searched Digikey and got no hit. The other one on the board looks perfect. The rest of the board looks like new.Can anyone suggest a decent substitute? I am attaching a picture. This is the cracked component.Any help is appreciated.Mike

author: robert.rozee
author_rank: Frequent Contributor
timestamp: April 05, 2025, 02:24:54 pm
content: for the purposes of testing, you could replace (or bridge) it with a small 1k resistor. this will enable you to verify that the rest of the meter's circuitry functions, but until replaced with thecorrectthermistor severely compromises the meter's safety. ie, test that everything works, then REMOVE the 1k resistor and source the correct replacement part.a possible source may be using an otherwise non-functional 70 series meter as a donor. they have a similar thermistor.cheers,rob :-)

author: bdunham7
author_rank: Super Contributor
timestamp: April 05, 2025, 04:05:27 pm
content: Which service manual are you looking at? I have not been able to find one for the 27/FM, all the ones I have are for the newer IEC 60101 version.I think earlier versions used the 1k fusible resistor and later ones use a 1.1k PTC. The later service manual lists an RT1 and RT2 (later changed to just RT1 as they eliminated one PTC) with a p/n of 602995. It has the same description as yours and in the later models it is actually a GE/Amphenol YS4020. The TDK part may have just been an early PTC as they were a bit new at this time--Fluke adopted them to replace that fusible resistor pretty much as soon as they were available. The YS4020 is ~ 1.1k cold and I refer to it as the "Godzilla of PTCs" because its specs are pretty impressive. They're readily available and I would just replace both of your PTCs with YS4020. You'll need to use some care positioning them because they are physically a bit different and the leads are exposed.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2025, 04:37:47 pm
content: Please avoid double posting, it leads to wasted time...https://www.eevblog.com/forum/testgear/fluke-27-an-manual-needed/msg5871391/#msg5871391

author: pinpassion
author_rank: Contributor
timestamp: April 05, 2025, 08:42:13 pm
content: Robert and Bdunham7, thank you for the response. Much appreciated.I was going to bridge the circuit with a 1 k resistor based on the manual part callout. I didn't add the resistor for two reasons. I found a seller on Ebay that had NOS thermocouple's that were OEM and were a perfect spec match. Only difference is they were green instead of gray. I placed that order and am waiting for it to come. Also, the 27 is a pain to take apart and I did not want to join the display board until I was relatively confident I would not have to redo it. I worry about breaking or bending the pins. I have a feeling the replacement will work. Fingers crossed. The Ebay seller mentioned the 70 series multimeter compatibility in the auction.It is NOT the manual for the 27/FM I had referenced, just the one I found online commonly available. I could not find one specific to the 27/FM either.The voltage measurements were perfect when I first tested the meter. First thing I tried out of the box. Spot on! However, the resistance check was a different story. In diode / buzzer mode, the buzzer sounds immediately and continuously. In ohms mode it just kept wandering in auto range. That's when I cut power and disassembled. So, I know the processor and other elements of the meter are working. I would have opted to use the YS4020 replacement if I had known before the Ebay find. With a little luck, I will have a fully working meter when the part arrives. We shall see.What I wonder about is the failure mode of these thermocouples. Are they supposed to split open like the current part did? I know if that is what they are supposed to do when the internal heat rises to the threshold point, that can't be considered a failure, but I don't know much about them. I would have thought the internal resistance would have just gone so high the circuit would be starved for current. This one acted like a fuse and just went completely open. It looks like a robust component. Unlike the internals of a fuse.Gyro, I did not mean to double post. I'm sorry about that. I was having some problems at the time of posting. It was totally unintentional. Thanks for bringing that to my attention.Thanks guys. If the replacement works, I will let you know. Otherwise, the saga continues.Mike

author: bdunham7
author_rank: Super Contributor
timestamp: April 05, 2025, 09:50:14 pm
content: Quote from: pinpassion on April 05, 2025, 08:42:13 pmTheoperationmode of the PTC thermistor is that its resistance should quickly rise when more than its trip current is present. I believe RT1 is protecting the ohms source and would be activated if you did something like measure your service panel voltage when the meter was in the ohms range. The original Fluke 27 is only rated for 500V protection in the ohms and mV ranges so I although I don't have a spec or datasheet for this part, I'd guess they were rated for 500V at most. Thefailuremode of blowing up is just what happens when you go beyond their limits.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2025, 10:28:48 pm
content: From my own Fluke 25, Fluke seem to have contorted the fairly thick leads of these slim thermistors in order to fit them into the PCB. There is no sign of overheating or arcing. I think that there is the distinct possibility that the package was under mechanical stress over the years and finally broke due to an impact, or maybe a normally harmless thermal cycle.

author: daisizhou
author_rank: Super Contributor
timestamp: April 06, 2025, 02:57:36 am
content: Quote from: pinpassion on April 04, 2025, 08:54:16 pmhttps://www.eevblog.com/forum/testgear/fluke-27fm/You can try searching for AN/PSM-45A

author: pinpassion
author_rank: Contributor
timestamp: April 06, 2025, 04:21:36 pm
content: Well that makes perfect sense Bdunham7. It even reassures me somewhat that the replacement will fix the issue. The ohms range was the issue during testing, and I did not test further. You may have nailed the fault condition. It seems VERY plausible to me that may be just what happened. Hopefully, nothing else was damaged downstream. Time will tell. Thanks for explaining the thermocouple operation mode. I guess they are not used so much in new multimeter designs. This one did its job. I did a resistance check of all the MOV's and they all were open circuit and look perfect physically.In a sense, I guess the mistake made is the only reason I have this meter. It probably would not have been for sale for $34 dollars otherwise. The HRC fuses were missing, and the thermocouple on Ebay set me back $10 bucks. Still a bargain if it works.Thanks again!Mike

author: bdunham7
author_rank: Super Contributor
timestamp: April 06, 2025, 04:40:39 pm
content: Quote from: pinpassion on April 06, 2025, 04:21:36 pmIt is "thermistor" not thermocouple, that is just terminology but it will lead you far astray if you are looking for equivalents. PTC thermistors are actually used almost universally in multimeters these days as the primary protection for the ohms source and overvoltage clamps, any circuit that can't be protected simply by very high input resistance. The newer designs have them earlier in the circuit, pretty much in the former position of the fusible resistor that they no longer use. Open up any modern Fluke multimeter and you'll see at least an RT1 plus an RT2 if it has Low-Z.

author: pinpassion
author_rank: Contributor
timestamp: April 06, 2025, 04:52:13 pm
content: daisizhou, Thanks for the info. I now have two alternate thermistor parts to reference if needed. It's also cool to have the army manual for it. Unfortunately no schematics or parts list, but it's still cool.Mike

author: pinpassion
author_rank: Contributor
timestamp: April 06, 2025, 04:58:59 pm
content: bdunham7, Thank you again!! I stand corrected on the terminology. Also, on its use in modern multimeter designs. I was under the impression that the fusible resistors still prevailed.Much appreciated!Mike

author: daisizhou
author_rank: Super Contributor
timestamp: April 06, 2025, 10:13:41 pm
content: I have 2 27/fm, one is my daily use multimeter，The other one is newer and has high voltage accessories。I have been using 27fm daily，I found that the transparent display window tends to turn yellow，The shell is relatively fragile and easy to crack, which seems to be due to the materialEverything else is perfect

author: pinpassion
author_rank: Contributor
timestamp: April 10, 2025, 08:38:24 pm
content: Hi gang,The thermistor arrived today in the mail. I immediately installed it and went to the resistance range to test. It's perfect! I have a couple of .01% resistors for testing. They were spot on. The replacement part was absolutely on the money in size and specs. It dropped right into the circuit board.Thanks for all the help! Mission accomplished.Mike

author: bdunham7
author_rank: Super Contributor
timestamp: April 11, 2025, 06:52:25 pm
content: Quote from: pinpassion on April 10, 2025, 08:38:24 pmI'd say that supports the theory that the crack was due to physical stress and/or old age rather than electrical abuse.

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2025, 07:13:39 pm
content: Good it's all fixed, some extra life for the multimeterIf you look closely at OP's pic, there are some dents, chips outside of the split line, so it looks like it got hit somehow.I have seen these old rectangular PTC's crack where the leads attach, I believe they are sintered metal oxides compressed into a ceramic pellet.

author: Fungus
author_rank: Super Contributor
timestamp: April 12, 2025, 04:28:31 am
content: These are very good, accurate meters.Basic, oversized, a bit slow... but they work.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2025, 09:53:25 am
content: Fluke 25s seem to be more plentiful and cheaper than the 27, if you can do without the Rel and Min/Max button (literally the only difference is the absence of the conductive rubber button moldings). They still have the Fluke intelligent touch hold, brief good diode bleep and nS range. They often come with the big Mil floating carry cases.

author: bdunham7
author_rank: Super Contributor
timestamp: April 12, 2025, 02:57:44 pm
content: Quote from: Gyro on April 12, 2025, 09:53:25 amPerhaps YMMV by location, but on eBay in the US the 27 is plentiful and cheap--later model versions in good condition can be had as low as $50 or so.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2025, 06:03:00 pm
content: Yes, probably so. Fluke 27 and 27/FM don't come up very often at all here. I think the MOD was quite a big user of the 25 here, hence the number that come with the rugged carry case.PS. I got lucky with mine a few years back, but current listing prices for the 25 here are £50-60.

author: watchmaker
author_rank: Unknown
timestamp: April 13, 2025, 11:53:23 am
content: Quote from: bdunham7 on April 12, 2025, 02:57:44 pmTwo years ago I got one off eBay with the whole kit for $75. Calibration expired just this last January (2025). Great for taking outside. Thought about painting it yellow.

author: floobydust
author_rank: Super Contributor
timestamp: April 13, 2025, 06:44:29 pm
content: Fluke 25, 27 released Spring 1985 and quite expensive, almost double the price of a 75/77. I don't think I've ever seen them in the wild.

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2025, 07:25:26 pm
content: They were certainly built to last, "cases twice as thick as any other DVM we build", 3ft submersion for one hour etc. 0.1% basic accuracy too. Also handy for self defense in an emergency...(sorry the quality is a bit poor, a magazine scan I think)

author: floobydust
author_rank: Super Contributor
timestamp: April 13, 2025, 07:53:27 pm
content: I also note Fluke 27 was on the equipment list for Boeing aircraft maintenance. Considering they are located next door to Fluke in Everett, they might have been part of the new ruggedized feature that came up in handhelds.

author: Gyro
author_rank: Super Contributor
timestamp: April 14, 2025, 09:13:37 am
content: I'd forgotten. Dave did a Fluke 27 teardown and review...https://youtu.be/MKuvmoW0AZQ

author: Dave Wise
author_rank: Regular Contributor
timestamp: April 15, 2025, 12:58:09 am
content: Did the 8025B become the 27?

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2025, 03:04:44 am
content: Quote from: Dave Wise on April 15, 2025, 12:58:09 amNo, it doesn't look like it. There are 8025A, 25 and 27 manuals dating from 1984, the 8025B manual is 1988. My 1996 Fluke catalog still lists the 25 and 27 but there's no 8025B. The 8060A and 8062A are still there. I'm assuming the two different model numbers were just how they were marketed, sort of like the 28 II and 87V-Max today.

author: Dave Wise
author_rank: Regular Contributor
timestamp: April 15, 2025, 03:30:49 pm
content: I never did understand the folks in Marketing.