Goerz Unigor 1 - 1960s Analog Multimeter Restoration

[ZeroCubic]

Hi  Guys, I have a Goerz Unigor model 1 multimeter that I'm trying to restore to working order.

Background:
- The multimeter belongs to my girlfriends dad, and I'm wanting to fix it as a gift for him. Plus I think this thing just looks really cool, and I want to see how it compares to my modern DMMs.
- The only info I can find about the meter is from this page: https://www.radiomuseum.org/r/goerz_unigor_1_type_226201.html
- The meter comes with a very cute manual booklet meant to look like the device itself.
- The company that made it, Goerz Electro, was Austrian, founded in 1907 and originally made optical assemblies, then later electronic test equipment. The company shuttered in 1991.

Where I'm at:
- Currently only the DC voltage range is working, and even then not very accuratly, reading about 10% below true. The Amps and Ohms ranges don't seem to work at all. I haven't tried AC yet for fear for frying it.
- Theres some corrosion from a dead battery that needs cleaning, but beyond that I'm not sure where to start on diagnosing this. I can't see any components that are obviously damaged, though many are discoloured from age. I've tested only a few dozen of the components, the ones I can easily access, and they all seem to be working and reading correctly.
- According to the schematic, theres at least a couple diodes and caps, but I haven't been able to find them on the device yet.
- I'm hesitant to disasemble the boards from each other, since it requires desoldering a number of components. But of course I will if theres no other way to repair it.

Advice I like:
- I'm not so crash hot with analog electronics like this so feel free to pretend I'm an idiot when it comes to this.
- DC volts range: What am I looking for to determine why its reading low? it seems to read low by about the same amount on all the DC volts ranges. Would it be just a resistor thats moved out of spec?
- DC amps: none of the ranges seem to work and I have no idea why. broken shunt connection somewhere?
- ohms: again, nothing working here.

[ZeroCubic]

Here's a couple more photos:

[Messtechniker]

Step 1 would be: Disconnect the meter movement
to ensure that the meter on its own is indicating the
right values from Zero to full scale. Within its spec of 5 %.
Be careful though, because the meter on its own
is quite sensitive. So connect a suitable dropping resistor
for this linearity test. If OK then continue troubleshooting.

[ZeroCubic]

Thanks Messtechniker, I'll give that a go.
How do I know what the full range input voltage into the meter movement should be?

it says 0.06 to 1200v as the range on the movement display, but that's surely just for input into the meters probe terminals, not into the movement itself?

[david77]

At the top near the banana sockets it says +12mV. That could be a direct connection to the meter movement, so it would mean the meter shows FS when 12mV are aplied. Your manual should explain this.

Correction: It's explained on the bottom of the meter. This is the input for a thermocouple and yes, I'd guess it goes directly to the movement. I'd choose a resistor for much less than 12mV first, just to be on the safe side. Maybe 2-5mV or so?