DMM cemetery .. #1 Fluke 87 - Page 1
Forum section: electronics / repair

author: Giorg
author_rank: Contributor
timestamp: January 17, 2019, 10:04:47 am
content: I've received as a 'gift' a lot of brokenmultimeterand I'm trying to get them back from the dead ..The first one is a Fluke 87:Symptoms:mV --> OLV --> around 0.030 V with crossed leads. I can measure correctly a 9v battery--> jumping around 64K. Crossed leads -> still 64kdiode --> OLtest made:dial test (rotary switch test - holding range button while powering )-> OKinput fusible resistor -> around 1kspark gap -> open2 mov -> Openthermistor -> 8.5 K (should be 1,5k but schematic is not 100% matching. swapped with a 1.5 resistor, same behavior)voltage reference zenner -> around 1.2 VmA and A works fine.all power supply lines indicated in service manual are within specs.I cannot spot any damaged components on the board itself.I've looked at the schematics, very well made but I'm not able to spot a component that could trigger this kind of behavior.Is there a way to bypass or force a reading directly to the main uC pins?If anyone as some kind of advice, it would be a great opportunity to learn something new and to put a great instrument into my drawer!

author: Giorg
author_rank: Contributor
timestamp: January 17, 2019, 10:06:26 am
content: more pics

author: retiredcaps
author_rank: Super Contributor
timestamp: January 17, 2019, 07:07:57 pm
content: I suggest you start a new thread for each meter. I'm willing to make suggestions, but I and others will quickly get confused what you are testing and reporting with 3 meters in one thread.I suggest starting with the Fluke as I have several suggestions and there is lots of documented solved cases onhttp://mrmodemhead.com/and full service manual for the Fluke 87 on Fluke's website.

author: Giorg
author_rank: Contributor
timestamp: January 18, 2019, 10:47:56 am
content: You are right, I've modified this thread for the Fluke only.I've looked on your website but I cannot find any information specifically related to my issue

author: ModemHead
author_rank: Frequent Contributor
timestamp: January 18, 2019, 01:40:45 pm
content: Is it possible that some has attempted to replace the rotary switch? It doesn't look quite right. Especially the contact sticking out.

author: Giorg
author_rank: Contributor
timestamp: January 18, 2019, 02:36:30 pm
content: it appears to be just a floating contact, it has no connection with the pcb or anything else. In other pictures found online is just missing. Also solder joints do not look like they have been reworked.pin to pin connection is consistent with schematic.

author: Celectronicamx
author_rank: Newbie
timestamp: January 18, 2019, 04:22:13 pm
content: Maybe this can help?https://elektrotanya.com/fluke_87_service_manual.pdf/download.html

author: ModemHead
author_rank: Frequent Contributor
timestamp: January 18, 2019, 07:28:03 pm
content: Quote from: Giorg on January 18, 2019, 02:36:30 pmOkay, I don't recall ever seeing an actual contact installed on pin 7 on any 80-series meter that I have. There's usually no contact or rivet there. Having that thing flapping in the breeze doesn't seem like a great idea.Okay I'll offer my usual disclaimer, which is I don't know what's wrong with your DMM, but I can throw some stuff out there to ponder on.The thermistor is a problem, but doesn't sound like the main cause of the malfunctions. If it's cold resistance goes too high, it will limit the amount of test current that can be provided in ohms mode, and throw ohms readings off on the low ranges, otherwise not much effect unless completely open.That you can measure a 9V battery correctly bodes well I think. Not zeroing out in DCV mode with the input shorted is usually some leakage current caused by contamination and can often be solved by one (or more) IPA baths and some action with a brush. Resist the urge to turn the calibration pots until all other repairs are finished.The biggest problem seems to be with mV, ohms, and diode-test. These functions share the same voltage sense input on the processor U4P93 (AP1). The route to this input depends on rotary switch pins 11 & 12 being closed, and R2, which is the blue 909Meg resistor beside the fusible resistor. Verify everything along this path. Refer to the manual pages 5-22 and 5-24 to see where I'm getting this.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 18, 2019, 07:52:35 pm
content: Since modemhead spotted that floating contact, a quick easy test is the rotary switch test from the service manual section 3-16 (attached).

author: retiredcaps
author_rank: Super Contributor
timestamp: January 18, 2019, 07:57:18 pm
content: More rotary and code switching and IPA by modemhead ...http://mrmodemhead.com/blog/fluke-83-dmm-repair/

author: Giorg
author_rank: Contributor
timestamp: January 18, 2019, 09:02:02 pm
content: Thank you retiredcaps but I've already done it ( "dial test" in my first post) and results are perfect.I've also already done the general cleaning with IPA..

author: retiredcaps
author_rank: Super Contributor
timestamp: January 18, 2019, 10:28:01 pm
content: Quote from: Giorg on January 18, 2019, 09:02:02 pmOops, sorry didn't see that you edited the first post.

author: Giorg
author_rank: Contributor
timestamp: January 21, 2019, 04:53:02 pm
content: Quote from: ModemHead on January 18, 2019, 07:28:03 pmI've desoldered ant tested all the components on "ohm path " and they are all ok.I've measured -2.8V across COM and V input terminal. I've lifted U4 pin 93 and it's like at -3V from COM.Is it normal ? If it's a sensing input ..is it supposed to be at a negative potential ? I'm starting to worry that U4 might be bad

author: ModemHead
author_rank: Frequent Contributor
timestamp: January 21, 2019, 11:43:34 pm
content: Quote from: Giorg on January 21, 2019, 04:53:02 pmIt would of course be normal to have apositivevoltage across the jacks in Ohms or DT mode, but this test current comes through the fusible resistor/thermistor path.So what you're seeing does not seem normal at all. The ADC input should not source (or sink, in this case) enough current to develop that much voltage. It should be very high impedance. Lifting the pin is exactly what I would have done (as a last resort), and pretty much narrows it down to U4, unfortunately.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 22, 2019, 12:46:52 am
content: I have a 87 with a dead U4 as well. It won't boot at all.Time to start a new thread for your other 2 meters.