Gossen Metrahit 25s - Page 1
Forum section: electronics / repair

author: Giorg
author_rank: Contributor
timestamp: August 07, 2023, 11:37:56 am
content: 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!

author: coromonadalix
author_rank: Super Contributor
timestamp: August 07, 2023, 01:16:43 pm
content: sadly no schematics are know to have leaked on the web, unless you have some of them based on known dmm chipsets, lower seriesremove the ic's stickers and check the writings on themyou'll have to figure out by yourselfi think there is a tear down of one of them here or the web ..

author: Uunoctium
author_rank: Frequent Contributor
timestamp: August 07, 2023, 02:26:54 pm
content: In the yellow square there are two MELF components mounted. Orientated in opposite direction. Marking is '7 - 7'Uf ~0,5V. Reversed 0,49VMaybe antiparallel zeners for protection?

author: coromonadalix
author_rank: Super Contributor
timestamp: August 07, 2023, 07:32:17 pm
content: i think they could be tvs mov or gas discharge tube ?glass tube casing could be signal diode, zenner, diode, gas discharge tube ?

author: giosif
author_rank: Frequent Contributor
timestamp: August 07, 2023, 08:14:03 pm
content: Quote from: Giorg on August 07, 2023, 11:37:56 amI 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/#msg3145154Quote from: Uunoctium on August 07, 2023, 02:26:54 pmThey are Philips/NXP BYD77D and, yes, they are installed antiparallel for protection.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: August 07, 2023, 08:19:01 pm
content: btwwho 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

author: giosif
author_rank: Frequent Contributor
timestamp: August 07, 2023, 08:43:25 pm
content: Quote from: Uunoctium on August 07, 2023, 08:19:01 pmSlight correction: that square IC is not the processor, but the "multimeter" IC / ADC.The processor is the next-biggest rectangle IC.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 08, 2023, 12:17:54 am
content: some previous teardownhttps://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