Very old Multimeter repair / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Very old Multimeter repair

1 2 next

Move Thread

LAN_403

Nik

Mon Oct 06 2008, 09:38PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I picked up a "Superior Instruments Co." Multimeter "model 710" for 10$ at the shop down the street. Unfortunately it does not read voltages (AC or DC) correctly. DC voltages read much higher then the actual voltage and AC voltages read slightly lower then their actual voltage. I am fairly sure that the galvanometer's full scale deflection has changed because I used my digital meter to check voltage drops across the galvo and the current limiting resistors and all the readings made sense.

Heres where I need your help to tell me if I am going about this the right way. I am going to find the current required to make a full scale deflection then calculate the resistance required for each of the voltage ranges in order to make make the max voltage of each range pass the full scale deflection current though the galvo. If any one sees a problem with this method or knows an easier one let me know.

All the components inside the meter are resistors and switches (other then 1 mystery part that is not apart of the DC voltage reading circuit) which makes me think this is a doable project.

Bored Chemist

Tue Oct 07 2008, 05:51AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I would statr by checking that it's repeatable and linear.
It's never going to be as good as it once was but the idea you have seems OK.

Bamacoiler

Tue Oct 07 2008, 11:52AM

Registered Member #1628 Joined: Wed Aug 06 2008, 08:48PM
Location: Huntsville, AL USA
Posts: 95

I agree fully. That's about the best way I can think of as well.

Nik

Tue Oct 07 2008, 09:39PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

Thanks for your input, I like to check before I start work.

I measured the voltage required for full deflection to be 62.5mV, with an internal resistance of 150ohms that gives me a full deflection current ot 1/2400A (0.00041666A).

Crunching the numbers ive worked out the resistances required for each of the voltage ranges in order to get full deflection as follows (including the resistance of the galvanometer)

15v scale = 36kohm
60v scale = 144kohm
150v scale = 360kohm
300v scale = 720kohm
600v scale = 1.44Mohm
1500v scale = 3.6Mohm

Once I get the resistors I'll start the math on the Ohmeter circuit and Ameter circuit.

1223415595 53 FT55227 Multimeterinternal

EDIT, Made a typo its 62.5mV not mA for full deflection.

Steve Conner

Wed Oct 08 2008, 08:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The FSD of the meter should be equal to the FSD of the unit's most sensitive DC current range, since that's what that is: the meter is just connected straight to the terminals with no other components in the way. It should be a nice round number like 1mA, 500uA, or if you're really lucky, 50uA, which is what the old Avo Model 8s had.

Sensitivity was also often quoted in ohms per volt. So the Avo Model 8 had 20,000 ohms per volt. (50 microsomethings = 1/20,000) You multiplied that by the voltage range you were set to, to get the actual input resistance, which was important in those days, since it was low enough to load tube circuits down.

The resistors have probably drifted with age, and the "mystery component" (probably a copper oxide rectifier for the AC ranges) might well be leaking too.

Nik

Thu Oct 09 2008, 02:02AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I started to work out the resistance needed for the ameter and ohmeter sections of my meter and I think I may have hit a problem. The scale printed on the galvo was made for a galvo that was ~1mA for full scale deflection. But since the spring has become weak and it only takes 416uA to full deflection I don't think the scale will be usable or anywhere near accurate even if I replace all the resistors and set my own supply voltage.

Bored Chemist

Thu Oct 09 2008, 05:50AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

That makes it easy. You just need to shunt the galvo with a resistance of a bit less than 150 Ohms.

Bamacoiler

Thu Oct 09 2008, 12:50PM

Registered Member #1628 Joined: Wed Aug 06 2008, 08:48PM
Location: Huntsville, AL USA
Posts: 95

As a calibration technician, I'm sorry but if the full scale changed from 1ma to 416 micro that meter will never work right again unless you replace the meter. Even then I doubt still. I think you are probably wasting your time trying to get that working.

How are you determining what full scale deflection is? Are you using a calibrated current source? Good luck and keep us updated.

Nik

Thu Oct 09 2008, 06:09PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

Even if I only get the Volt meter working close to accurate I will be happy, Really I just want it for show. I determined the FSD by putting a voltage across it until the needle deflected all the way, then i measured the voltage 62.5mV and calculated the current using the meter's internal resistance of 150ohms.

Would a shunt be able to correct for old age in all metering modes or would i need a different one for each mode? (Off to do some more math)

Bamacoiler

Thu Oct 09 2008, 08:23PM

Registered Member #1628 Joined: Wed Aug 06 2008, 08:48PM
Location: Huntsville, AL USA
Posts: 95

Even if I only get the Volt meter working close to accurate I will be happy, Really I just want it for show. I determined the FSD by putting a voltage across it until the needle deflected all the way, then i measured the voltage 62.5mV and calculated the current using the meter's internal resistance of 150ohms.

Would a shunt be able to correct for old age in all metering modes or would i need a different one for each mode? (Off to do some more math)

Well, I didn't think of that. But I tell you what though you guys are impressive. You learn to do a lot with just a little. That method should work pretty accurately in all voltage ranges but you will need to shunt each range individually. Current should be simple, just shunt it with some type resistor, measure the voltage drop and use ohm's law for the correct current to drive the meter. Resistance not too sure on probably some wheatstone bridge type method. How do the current and resistance ranges work? Or do they work at all?

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob