need help with Tektronix dmm916 repair/calibration

[ErwinsCat]

Hi people,

I received a Tektronix dmm916 recently from my cousin, he said the multimeter was stored in his basement for a couple of years and he received it from his friend and he thought it should be fine and just need a new battery and maybe calibration...
Hi gave it to me and I installed a new battery and test the different functionality. all the ranges look fine except the ohm range and diode test.
in the Ohm test the multimeter showing around 10k ohm without anything connected to the leads... and diode test showing 1.6 Volt open circuit... the open circuit numbers changing slowly in time and I thought it might be a bad capacitor, so I removed the SMD capacitors and test them outside the board, they tested okay.
when I remove the battery and leave the selector in Ohm range, the front end reads around 12K Ohms (when I connect another multimeter into dmm916 Frontend) that made me think maybe some of those protection components gone bad but the Volt and other ranges showing large resistance, so it shouldn't be the frontend.
I like to repair the multimeter if it possible as I really like the feel and it looks and it is the best multimeter I ever owned. also, don't mind to sending it to get repair by professionals(I'm leaving in Sydney, AU) but I doubted if it worth to do that as they sell on eBay under $200.
wondering if anyone here knows if the schematics are available for this model or similar ones. also, if anyone has experience regarding repairing or calibrating this device or similar models. Any advice appreciated. please let me know what you think regarding the whole situation

PS: Please excuse my ignorance regarding technicals and language! as I'm a none-English speaker and beginner hobbyist.

[texaspyro]

The DMM91x series meters are my "go to" meter... I have around 10 of them... I keep one in about every room. 

I've never seen a schematic, but the "service" manual with calibration procedure is available.  Search for:

061-4123-00

[ErwinsCat]

Hi,
Thank you Texaspyro, now you made me 10 times more jealous
unfortunately, the service manual has all the calibrations procedures but the Ohm range! 
I'm guessing multimeter uses other functions to perform ohm measurements, perhaps milliampere.... not sure.
I contacted Tectronix agent in Australia and they said they don't have parts for repair as this model is discontinued long time ago but they can calibrate it for $800 which is a ridiculous price for me.
I think I have to find the damage component and fix the multimeter my self. I'm wondering if anybody can explain how digital multimeters miserere resistance, that way I might be able to narrow my search in the circuit to find the damaged part(s) I'm assuming something damage in the front end and that act as a load for ohm range. does anybody know the typical "ohm meter " topology?
I might better off if I buy another one of the eBay but I can not stand not to be able to fix broken things!!! they sometimes get fixed and more often they get totally destroyed by me!!!

[texaspyro]

Thank you Texaspyro, now you made me 10 times more jealous

Let's see if I can make you 100 times more jealous...  I paid about $20 each for them (two lots of 10 at an equipment auction).  One was a '916, one was a new-in-box '912.  The only issue with them was missing or dead batteries.  One was missing the blue rubber meter condom.

Most meters read resistance by driving the resistor with a stable current source and measuring the voltage produced across the resistor.

[mzacharias]

Look very carefully for circuit board contamination. This would affect the ohms and diode test.

You can pull the battery, turn the selector to DC volts, measure the meter input resistance at the input jacks using another meter. Normally around 10 meg, I'm expecting you'll see something much less.

If no contamination, maybe a blown varistor or the like.

[MosherIV]

Hi, sounds like the input protection, voltage clamp is damaged.

Look at Daves video on dmm input protection


The active clamping components are either MOVs or gass discharge tubes.

Hope this helps.

[ErwinsCat]

Hi,
Thanks for the reply, yes you right the input protection diodes and 1 clamp transistor were shorted.

[ErwinsCat]

Hi,
Thanks everyone!!! yes, you right the input protection diodes and 1 clamp transistor were shorted, also the board contamination had some effect on the reading. but the main problem was the rotary switch contact which I think someone before me moved it into a wrong slot. I couldn't find a photo of the position of the contact on the web(David video just showing the top side ) anyway, I start moving suspected contact into empty slots until the ohm/diode range start working properly.
Now I need to know how to calibrate ranges as best as possible at home or cheaply(texcrtonix asked for $700-800 !!!)
I have one of this voltage references kits from eBay but not sure how accurate they are to start with. I thought if they accurate enough, i might buy some %0.005 resistor to calibrate the ampere and voltage range ... but I prefer to find someone to do that professionally and cheap...

https://www.ebay.com/itm/AD584-4-Channel-2-5V-5V-7-5V-10V-High-Precision-Voltage-Reference-Module-AD-L2/173019953183?epid=1984261241&hash=item2848caf81f:g:Th8AAOSwCcZaKPhD

[Samogon]

$700 Wha? they asked $600 to calibrate and adjust my Keithley 2002
Most important calibration of DCV so you have to have calibrator able to provide .1 to 1000V with at least 5.5 digits accuracy.