Gossen Metrahit 25s

[Giorg]

I'm grappling with the challenge of restoring this ancient treasure that I stumbled upon.

Green boxed  components have been replaced due to damage.

The main issue at the moment is the power draw:
Around 9 mA when turned on,
About 11 mA when turned off. No logging is active.

When I touch G4 with a probe or a resistor connected to ground, I observe 0.15 mA in the OFF state. I believe this is a good value, considering that a part of the circuit is always on.

Everything is functioning well except for the function generator. While I am receiving pulses on the gate (G4), both the drain and source remain at 0. It's possible that certain components or vias are missing (yellow box?) or damaged on the drain/source side.

Would anyone have advice or a schematic diagram pertaining to this meter? It would bring me great joy to fully restore it and acquire new knowledge. Thank you!

[coromonadalix]

sadly no schematics are know to have leaked on the web,   unless you have some of them based on known dmm chipsets, lower series

remove the ic's stickers  and check the writings on them

you'll have to figure out  by yourself

i think there is a tear down of one of them here or the web ..

[Uunoctium]

In the yellow square there are two MELF components mounted. Orientated in opposite direction. Marking is '7 - 7'
Uf ~0,5V. Reversed 0,49V
Maybe antiparallel zeners for protection?

[coromonadalix]

i think they could be tvs mov  or gas discharge tube  ?

glass tube casing      could be  signal diode, zenner, diode, gas discharge tube ?

[giosif]

The main issue at the moment is the power draw:
Around 9 mA when turned on,
About 11 mA when turned off. No logging is active.

I suggest checking the 5.6V Zener I mention in a previous thread here:
https://www.eevblog.com/forum/testgear/gossen-metrawatt-metrahit-29s-(m229a)-issues/msg3145154/#msg3145154

In the yellow square there are two MELF components mounted. Orientated in opposite direction. Marking is '7 - 7'
Uf ~0,5V. Reversed 0,49V
Maybe antiparallel zeners for protection?

They are Philips/NXP BYD77D and, yes, they are installed antiparallel for protection.

[Uunoctium]

btw
who replaced the main IC/processor? There is a lot of flux left on it and the green capacitors above + the two white below are molten at edges by touching them during soldering

[giosif]

btw
who replaced the main IC/processor? There is a lot of flux left on it and the green capacitors above + the two white below are molten at edges by touching them during soldering

Slight correction: that square IC is not the processor, but the "multimeter" IC / ADC.
The processor is the next-biggest rectangle IC.

[coromonadalix]

some previous teardown

https://www.eevblog.com/forum/testgear/metrahit-25s-teardown-component-analysis/

and yes the gmcad30 is a custom asic dmm ic,  no datasheet exist about it