Range on 1970's multimeter (Metrawatt)

[stonerbox]

Was about to finish up a semi-new project as my chepo-meter gave up on me. I however managed to clean up and repair a broken Metrawatt UNIGOR A40 (made in Austria!) that I luckily found years ago, dumped in a pile of trash. The problem is that it (seemingly) only give me readings between 0-5000 Ohm. Could this meter really be limited up to 5k? I doubt it.

Educate me, I need range!

[click for full sized image]

[Rob Strand]

The problem is that it (seemingly) only give me readings between 0-5000 Ohm. Could this meter really be limited up to 5k? I doubt it.

Educate me, I need range!

Here's the manual for the A43 (there's a few other manuals on-line as well),
https://www.haw-hamburg.de/fileadmin/user_upload/TI-IE/Daten/Labore/Elektrotechnik/Bedienungsanleitungen___User_Manuels/UnigorA43.pdf

At first I thought no way it would limit the ohms range like that.  But when I look at the face of your unit and compare it to the face of the A43 it is very clear the A43 has a heap more ohms ranges.  Yours only shows x1 and x10  for ohms.    So now I'm thinking that unit was purpose-built for a certain field and the ohms range is indeed limited.

I found this article in a magazine,
https://www.e-periodica.ch/cntmng;jsessionid=B46E8AE553C6A4A5E0C5DDAE296EA05C?pid=zbk-003:1979:26::553

If you take the text and translate it.  It mentions the meter is for "Heavy current Engineering" and " it offers uses throughout the area of general electrical engineering".   I saw some high current ranges mentioned but not the ohms range.   Actually,   there's something not clear in the translation but it supposed to say "resistors from 1 ohm to 50 K ohm are measured."

When I zoom in on your pic I see the basic range is 5k. So that means the x10 range will be 50k.   You have to do the x10 in your head.

[antonis]

I shouldn't trust the end of that log band for decision of an uncoloured resistor value between 3k3 & 4k7.. 

[anotherjim]

I wonder if the text "BBC" on that A40 is significant. The BBC had a lot of kit made to their standards.

[stonerbox]

The problem is that it (seemingly) only give me readings between 0-5000 Ohm. Could this meter really be limited up to 5k? I doubt it.

Educate me, I need range!

Here's the manual for the A43 (there's a few other manuals on-line as well),
https://www.haw-hamburg.de/fileadmin/user_upload/TI-IE/Daten/Labore/Elektrotechnik/Bedienungsanleitungen___User_Manuels/UnigorA43.pdf

At first I thought no way it would limit the ohms range like that.  But when I look at the face of your unit and compare it to the face of the A43 it is very clear the A43 has a heap more ohms ranges.  Yours only shows x1 and x10  for ohms.    So now I'm thinking that unit was purpose-built for a certain field and the ohms range is indeed limited.

I found this article in a magazine,
https://www.e-periodica.ch/cntmng;jsessionid=B46E8AE553C6A4A5E0C5DDAE296EA05C?pid=zbk-003:1979:26::553

If you take the text and translate it.  It mentions the meter is for "Heavy current Engineering" and " it offers uses throughout the area of general electrical engineering".   I saw some high current ranges mentioned but not the ohms range.   Actually,   there's something not clear in the translation but it supposed to say "resistors from 1 ohm to 50 K ohm are measured."

When I zoom in on your pic I see the basic range is 5k. So that means the x10 range will be 50k.   You have to do the x10 in your head.

Great finds Rob, thanks for helping me out here! Now if I only got my hands on a A43.. but they are not cheap. Been looking around for the A40 too but not found anyone up for sale or what they are worth.

After the initial post I quickly realized that the meter goes up to 50k when dialed in to x10 and luckily this meter will do for now. I think it is kind of a beauty and does not have a totally crappy board inside either. EEVblog did a teardown on the A43 and it had a much lower quality board than this one, strange since mine is the older one.

[Rob Strand]

EEVblog did a teardown on the A43 and it had a much lower quality board than this one, strange since mine is the older one.

Interesting thing about that A43 is it's an analog meter with a capacitance range.

[PRR]

> manual for the A43

Lot of strange in these models.

It looks like the X1X10 machine could have two more resistance ranges, going by the more-range model schematic and the two blank positions on the less-range faceplate.

Yes, the capacitance feature is odd, it has transistors, yet nothing to buffer voltage, the V function is passive.

I don't think this "BBC" has anything to do with Monty Python. There are several other meanings for BBC, some NSFW.

[amptramp]

The problem is that it (seemingly) only give me readings between 0-5000 Ohm. Could this meter really be limited up to 5k? I doubt it.

Educate me, I need range!

Here's the manual for the A43 (there's a few other manuals on-line as well),
https://www.haw-hamburg.de/fileadmin/user_upload/TI-IE/Daten/Labore/Elektrotechnik/Bedienungsanleitungen___User_Manuels/UnigorA43.pdf

At first I thought no way it would limit the ohms range like that.  But when I look at the face of your unit and compare it to the face of the A43 it is very clear the A43 has a heap more ohms ranges.  Yours only shows x1 and x10  for ohms.    So now I'm thinking that unit was purpose-built for a certain field and the ohms range is indeed limited.

I found this article in a magazine,
https://www.e-periodica.ch/cntmng;jsessionid=B46E8AE553C6A4A5E0C5DDAE296EA05C?pid=zbk-003:1979:26::553

If you take the text and translate it.  It mentions the meter is for "Heavy current Engineering" and " it offers uses throughout the area of general electrical engineering".   I saw some high current ranges mentioned but not the ohms range.   Actually,   there's something not clear in the translation but it supposed to say "resistors from 1 ohm to 50 K ohm are measured."

When I zoom in on your pic I see the basic range is 5k. So that means the x10 range will be 50k.   You have to do the x10 in your head.

Great finds Rob, thanks for helping me out here! Now if I only got my hands on a A43.. but they are not cheap. Been looking around for the A40 too but not found anyone up for sale or what they are worth.

After the initial post I quickly realized that the meter goes up to 50k when dialed in to x10 and luckily this meter will do for now. I think it is kind of a beauty and does not have a totally crappy board inside either. EEVblog did a teardown on the A43 and it had a much lower quality board than this one, strange since mine is the older one.

What's so surprising about older equipment being better built than newer equipment?  It does seem to be the natural progression of things.

[stonerbox]

Not really in most cases but the A40 and A43 models are just a few years apart so that is why I thought it be strange for the company to downgrade and mess up so many things. But sure, a company can always chose to cheap out on their productions at any time.

[stonerbox]

It looks like the X1X10 machine could have two more resistance ranges, going by the more-range model schematic and the two blank positions on the less-range faceplate.

Would it be a good idea to add a third step for X100 or would it need a major reconfiguration? I am still looking for the A40 schematic.
A43 schem:

[Rob Strand]

Would it be a good idea to add a third step for X100 or would it need a major reconfiguration? I am still looking for the A40 schematic.

It would make the meter more useful.

You would need to analyze the circuit.   Those meter circuits are a real pain in the butt to go through.  Also you sometimes find little tricks in there that are hard to understand.

Where I see a possible issue is the meter has a 1.5V battery for the supply and the meter part itself is always a current meter.  Both these set a limit on the maximum measurable resistance.  The front panel on the A40 has a marking 3333 ohms per volt.  That usually means the meter part itself is 1/3333 = 300uA.   At 1/10th deflection on the meter the current would be 30uA and the measurement R = 1.5/30uA = 50k.   On the existing 50k *range* at 1/10th the meter deflection the reading (on the scale) is 5k ohm.  So getting a 500k range, with 50k at 1/10th deflection, is *barely* within the capability of the meter.  You might be able to do it but at worst case it might not be possible to calibrate it accurately.

Some circuits will have added resistors for protection which may add a resistance and further reduce the meter's maximum resistance capability.  Not a good idea to remove those either.  There can be other sources of voltage drops as well (eg. the A43 has a current source which prevents you using the full 1.5V). 

You might ask how does the A43 do it?  So firstly from what I can see the A43 has a 100kohm/V movement so that means the meter current is 1/100kohm = 10uA, which is 30 times more sensitive.   The second thing it has is the circuit in the bottom left.  This is some sort of DC to DC converter which boosts the voltage inside of the meter so you can get more current flowing through the larger resistor values.  (FYI, another way meters got more ranges is two batteries, one at 1.5V and one at some higher voltage.)

From face value getting a higher resistance range is right on the edge of the meter's capability.  I guess you would need to study the meter details and then try it.

[EDIT Forgot to mention, on simple meters the zero pot tweaks the current through the meter to compensate for different battery voltages.  If you mod the meter for a higher resistance range it might work when the battery is new but later you might run out of adjustment on the 500k range even though the battery still has a lot of life in it.]

[Rob Strand]

Here's a reasonably easy to understand analog meter schematic,



Notice:
- two batteries
- slightly different way to do the zeroing
- simple circuit which only uses resistors
   (Not like the A43 with a current source.)

[amptramp]

I have a collection of meters that I use but they are not interchangeable.  Some are analog and some are digital.  One has a voice output so when you touch a terminal, it might say, "Three point four seven volts" or something like that.  It is useful where you have to hold the probes in an awkward position and you can't always see the meter face.  Some have typical resistance scales but one goes up to 20 megohms and another goes up to 30 megohms.  If you are working with tremolo or other low-frequency oscillators, you will run into resistances that go above the typical 2 megohm maximum.  Some have transistor test capability and others don't.  Some have capacitance measurement and some don't.  I have a VTVM that has input DC voltages up to 1500 volts and a matching high-voltage probe that goes up to 20,000 volts.

I would recommend you think of using several meters because one meter that "does it all" is likely more expensive than several meters with coverage of everything you want to measure.

[PRR]

It is an interesting question. However a $10 cheapo-meter will read resistors accurately, and without confusing Antonius on the cramped end of the scale. If you found enough UNIGOR A40s for *everybody*, it would be worth working-out... but I doubt we can all have one.

FWIW, the meter movement is specced "8.8uA". Which makes the ~~300uA sensitivity even odder.

[Rob Strand]

Based on the table on this site,
http://drkfs.net/UNIGOR.htm

We can see the Unigor models 1s, 1p, 1n have 3333 k ohm per V specs, like the A40

I found the PDF manual for the Unigor 1 models here,
https://www.eserviceinfo.com/downloadsm/66776/goerz_unigor-1.html

So maybe a good start would be to see how close the A40 matches up with the Unigor 1.

FWIW, the meter movement is specced "8.8uA". Which makes the ~~300uA sensitivity even odder.

Where was that?  (The posted schem is for the A43, which is about 10uA, and that agrees with 8.8uA)

From that Unigor 1 manual:   It has a 12mV range with 114 ohm input impedance  that implies 105uA input current.  The 12mV range uses a 77 ohm series resistance.  The meter has a 550 ohm parallel resistance.  Put all that together and we get the meter itself is 42 ohms and the full-scale current is 100uA ie. not 300uA.
[I also checked the 300uA current scale and got about 119uA through the meter.   Not sure why I'm off.]

That gives some hope we can add a 500k ohm range.   Also we might be able to simply copy the circuit from the Unigor 1.

[stonerbox]

Thanks for the explanation of all of this Rob! I had zero knowledge of how these things worked but now I'm learning something.  I contacted Metrawatt and asked for the schematic for he 40A but they said they did not have it.. guess I will have to manually compare mine to the available schems. In the mean time I thankfully got a new meter for the precision work, turns out the 40A was very off on most readings.

[Rob Strand]

I had zero knowledge of how these things worked but now I'm learning something.

It's a whole era of knowledge which has disappeared.  Analog meters used to be taught in virtually every basic electronics course.

In the mean time I thankfully got a new meter for the precision work, turns out the 40A was very off on most readings.

It might need calibrating.  Sometimes there's a trimpot inside which calibrates all the ranges (the resistance range may or may not be affected by that calibration).  A meter out of calibration will consistently show low or high readings on all ranges (ohms maybe different or the opposite).

When I use a "new" analog meter I always tap the meter.  They are a notorious for sticking and that causes them to show randomly wrong readings even though the inside electronics is working fine.  Often that can be fixed.  (There was a PDF repairing analog meters on-line somewhere by a guy who built meters in the 1930's(?)).