STARK AF-2 vintage voltmeter/Ohmeter restoration - broken coil - Page 1
Forum section: electronics / repair

author: carbono.silicio
author_rank: Contributor
timestamp: May 12, 2020, 09:22:47 pm
content: Hello all,Got this old Canadian Volt/Ohmeter from STARK model AF-2.Galvanometer is ok, also resistors, switches and knobs but there's one wood cilinder that looks like to be a coil, whose wire is broken.Is there anyone out there that has an equal or similar one and can tell me the correct inductance?Thank you!

author: themadhippy
author_rank: Super Contributor
timestamp: May 12, 2020, 09:30:28 pm
content: Are you sure its an inductor and not a current shunt for one of the current ranges

author: carbono.silicio
author_rank: Contributor
timestamp: May 13, 2020, 11:31:28 am
content: Quote from: themadhippy on May 12, 2020, 09:30:28 pmNo I'm not sure. I'm fact do not know what an inductor would do here, I've never seen this before. Your suggestion might be possible.

author: Gyro
author_rank: Super Contributor
timestamp: May 13, 2020, 11:51:52 am
content: Yes, it is a wirewound current shunt. Fairly common construction in older meters (and resistance standards).

author: carbono.silicio
author_rank: Contributor
timestamp: May 13, 2020, 12:54:49 pm
content: Quote from: Gyro on May 13, 2020, 11:51:52 amThank you.Took the wire out in order to replace it but i believe is chrome nickel, I suppose I shouldn't replace it with just enameled copper wire!?

author: Gyro
author_rank: Super Contributor
timestamp: May 13, 2020, 06:55:55 pm
content: You would need a rather longer length of thin copper for the same resistance. I'm assuming that this resistor is the current shunt for the 150mA range (you can trace that). You can find resistance wire on ebay. Alternatively, you can probably purchase a low value resistor to use as the shunt.What you need to do is find out the full scale voltage (not current) of the meter movement itself. The easiest way to do that is to use one of the voltage ranges and an adjustable PSU. Adjust the voltage input until the meter reads exactly full scale, then use a DMM to measure the voltage across the meter movement. You will probably find that it is in the 50 - 100mV range.You can then calculate the shunt resistor value which gives the same meter movement full scale deflection voltage when passing 150mA.It depends how authentic you want your restoration to be, both the shunt and other resistors. Looking at all the carbon composition resistors, accuracy isn't going to be great unless you replace them with more modern metal film types.P.S. The other method would be to find the 15mA shunt resistor and make the 150mA shunt resistor to be approximately one-tenth of the value. It will only be approximate because the 15mA shunt will have been adjusted to take account of the meter movement current, whereas on the 150mA range this is insignificant.

author: carbono.silicio
author_rank: Contributor
timestamp: May 13, 2020, 08:45:43 pm
content: Quote from: Gyro on May 13, 2020, 06:55:55 pmYes, good idea! Actually i've done it before by converting a miliamp meter to amp scale, using the same instructions you indicated.It's a restoration and i would like to keep as much original components as possible and also would be nice to keep de shunt coil cilinder, it's made of turned wood for my surprise (i also do woodworking and turning).On the other hand i also like analog meters on my daily elecronics routines, so i guess it would be a possibility to totally replace the resistors, wiring etc and give it a new life on the bench!I like restoration work a lot, also have a few more vintage meters to repair like one Hioki A100D (damaged Galvanometer) and a H artmann & Braun AG - Pontavi WH2 (not really sure whats wrong with it)I'm undecided!!Thank you very much for your kind help.

author: Gyro
author_rank: Super Contributor
timestamp: May 13, 2020, 08:53:44 pm
content: Either way, it would be nice to see it functional again.

author: carbono.silicio
author_rank: Contributor
timestamp: May 14, 2020, 02:25:15 pm
content: Quote from: Gyro on May 13, 2020, 08:53:44 pmYes, I decided to give it a new life. Going to change all resistors and cabling.Not sure how much supply voltage of the battery. I think meters from this Era were common to use 22,5V.