Old Giavazzi/Pantec analog multimeter. Resistive network ...

[saronno]

I have this old analog multimeter from Giavazzi Pantec. This is the schematic.



As you can see, in the middle of the schematics there is a 4-pin resistive network (26 20 025).
This component is clearly burned. The point is I have no idea where and how to find a replacement.

Any idea?

[bob91343]

I think you will have to make your own.  Unsolder it and make some measurements to see which parts are damaged and just replace those.

It will be a lot easier to make up satisfactory resistors than to try to find an original part.  Luckily  you have the values.  I assume 1% tolerance would be satisfactory.

[saronno]

I think you will have to make your own.  Unsolder it and make some measurements to see which parts are damaged and just replace those.

It will be a lot easier to make up satisfactory resistors than to try to find an original part.  Luckily  you have the values.  I assume 1% tolerance would be satisfactory.

I think this resistive net is critical ... if you are off even just a bit, the multimeter get unreliable.
Probably the ratio between the values is most important than the values themselves. This is the reason
they have a single component. It's easier to build resistive net with specific ratio with very good tolerance
if you create a single component.

Anyway, I will give it a try to see how off the multimeter is with a DIY net.

[TimFox]

The resolution of the d'Arsonval movement is on the order of 1%, with similar linearity.  Therefore, 1% metal-film resistors are probably sufficient to replace the four in the network.  You probably need to use two in series to achieve the values required for some of the resistors.  The overall accuracy, including the calibration of the microammeter, can be adjusted with the trimpots for different functions.

[saronno]

The resolution of the d'Arsonval movement is on the order of 1%, with similar linearity.  Therefore, 1% metal-film resistors are probably sufficient to replace the four in the network.  You probably need to use two in series to achieve the values required for some of the resistors.  The overall accuracy, including the calibration of the microammeter, can be adjusted with the trimpots for different functions.

Thanks ... I will give it a try.

How many watts could they be ask to dissipate?

[TimFox]

I believe that that network is only used for ohms.  A quick glance shows 3 V from the battery applied in series with roughly 6 + 8 ohms to the circuit.  The current through that 14 ohms would be 215 mA (max), or 370 mW in the 8 ohm resistor.  You might want to use 2 W wire wound resistors for those small units.  1/2 W metal films should do for the larger units.
My cursory glance at the circuit was handicapped by not knowing the input connections.