Dou you know what this is? - Page 1
Forum section: electronics / beginners

author: tomy983
author_rank: Contributor
timestamp: May 03, 2015, 09:20:08 pm
content: Hello,I have this old thing which was my grandfather's.Chances are it is Italian made.I would like to know what it is, and I thought some of you might know...It has an analog indicator going from -10 to 0 to 10, units are not stated. on the right side there is a small symbol with 1,5 written next to it. it is like a resistor symbol wit a line crossing it vertically.On the other side of the display it saysMODEM70N 62255Other interfaces are:One pushbuttonTwo wire connectors, marked with an "X" between themOne pin selector 0.1 1 10 100 1000 It selects different coils , I think.One large wheel selector from 50 to 0.5 I tried to measure difference in resistance, but I think it probably interacts with the wire you can see in the box, in one of the lasts pictures..Here are some pictures

author: electr_peter
author_rank: Unknown
timestamp: May 03, 2015, 10:07:38 pm
content: Hi,My guess it is voltage meter or manual battery internal resistance tester. Few wires are broken off, I cannot reconstruct full circuitry at this moment.Highest range resistor on x1000 is broken off. And just look at that hand made potentiometer!

author: tomy983
author_rank: Contributor
timestamp: May 03, 2015, 10:19:28 pm
content: My very low res multimeter says that the resistance of that pot goes from .1 to 2.5 ohms ~Two are the disconnected wires:the gray one, attached from one side to the center of the potand the black one connected from one side to the pushbutton, at the right of the 20 ohms resistor.The most curious thing to me is the wire still firmly connected hanging in mid air in the box... It stays right under the big pot...

author: jeroen79
author_rank: Frequent Contributor
timestamp: May 04, 2015, 12:36:52 am
content: Looks like some sort ofbridge measurementinstrument.Connect an unknown component between the 'X' terminals, pick the most likely order of magnitude with the pin and then turn the dial until the needle reeds zero.

author: barry14
author_rank: Regular Contributor
timestamp: May 04, 2015, 12:38:14 am
content: It certainly looks like a Wheatstone bridge to measure resistance. The meter is the null detector and thus has a zero centered scale. The multiplier resistors are selected at the bottom and determine the measurement range. The only thing missing is how power (probably from a battery) is connected.

author: tomy983
author_rank: Contributor
timestamp: May 04, 2015, 08:13:55 pm
content: I am not entirely sure, but it might be something related to the ham activity...Would a Wheatstone bridge be useful to an ham?

author: albert22
author_rank: Regular Contributor
timestamp: May 06, 2015, 05:00:07 am
content: Quote+1you need an external supply and press the pushbutton momentarily to test for a null on the meter.Useful to a ham to measure and match resistors. At that time that was the only way to measure with some precision.

author: VK5RC
author_rank: Unknown
timestamp: May 06, 2015, 05:15:09 am
content: ? Is it a noise bridge, a type of Wheatstone bridge to get impedance matching of an antenna, used with a receiver?

author: babysitter
author_rank: Frequent Contributor
timestamp: May 06, 2015, 05:26:01 am
content: Im with barry15.It looks to wire-woundey to be of use for higher frequencies.

author: tomy983
author_rank: Contributor
timestamp: May 06, 2015, 11:56:17 am
content: Last night I had time to have a good look at it.Thank you guys, you pointed me to the right direction.It is something similar to a Wheatstone bridge, although it has a different schematic, which I will post.The strange wire attached to the case I conclude that has to serve to hold a battery.I tried to measure a 100 ohm resistor, connecting it to the "X" connectors.I supplied 1.5 volts.To measure, I started with the pin on the 0,1 position, but could not center the needle. I then tried to move the pin to the 10 position, this time moving the wheel pot I could center the needle at about "10" on the scale.I also tried another value and it works correctly.To calculate the resistance value, all you have to do is multiply the pin selector value (10) times the reading you get on the large pot scale (~10) when the needle is at 0. Also, there is a calibration screw on the front of the gaussian meter, I immagine having a better dmm, I could calibrate it against a known resistor value.I still wonder how accurate this is, since my crap dmm is definitely not accurate. (I am saving up for a Brymen)When I will have a proper working ohmmeter I will report back.Also, I would like to restore the missing resistor. Do any of you have an idea where I could find the proper wire or an already made vintage resistor? (I would not like to trow in a regular resistor...)And for vintage battery connectors/holder? Any Idea?What should the maximum voltage battery I should use? The holder wire looks like it is made for one of those 4.5v flat battery.Thanks again