Schematic for east german/russian benchtop meter

[Fryguy]

This weekend my oldest benchtop volt/ampmeter died   

I'd love to repair it . . . but without paperwork ?

It is a RFT G-1209.500 - with a lovely nixie display , 2µA full scale at lowest amps range @ 4.5 digits ! , autoranging ! and superb stability and precision .

This thing is incredibly complicated - a quick look inside revealed 23 pcb's , 25 20-turn trimpots , 2 transformers with about 20 taps each , 10 precision resistors (vishay 0,02%) , more than 100 DIL14 bugs with '79 date codes and some parts i can't identify at all . . . most of the components are russian .
19 of the pcb's can be pulled out - 7x analog circuitry , 9x digital logic/control/display drivers and 3x lots of voltage regulators . And nothing is labeled - no voltages , no testpoints . . . 

A nightmare without schematic - and i guess reverse engineering will take too much time and might not be worth it .   

Can somebody help me find the paperwork for this unit ? If that even exists today . . . 

[Hamelec]

frag doch mal bei OSTRON:
https://www.ostron.de/Manuals-Schaltplaene/Manuals-Dokumentationen-und-Schaltunterlagen.html

[Fryguy]

Thanks for the tip Hamelec - unfortunately they only got a 4 page datasheet on this unit .
They got the complete set of manual , schematics and service data only for another model with different functions .

So far i can tell , that the display is gone - the analog section seems to work . A powersupply rail on the digital/display part is most probably dead . . . hmmmmmmm 

I just made a decision - if i don't find the dead part until sunday , i'll turn this thing into 2 piles of christmas presents !
One pile metal for the scrapyard and one pile expensive precision parts for my lab 

I've paid 39 DM for this thing 28 years ago - the vishay resistors in the analog section alone are worth about 10-20 times that money     

[BlownUpCapacitor]

As starting with most things, start with troubleshooting the power rails and PSU. It's just likely a bad filter cap.

[Fryguy]

Easier said than done ! i'll have to turn this thing over a hundred times to find out what goes where - and this thing ist huge and heavy   

That's it - i'll get me a rotating plate (heavy duty turntable bearing + wooden plate or something similar) - that should do the trick 

Want some pictures ? 

[Hamelec]

[Turntable: I rebuilt a height-adjustable bar stool (similar to an office chair, only higher), removed the seat and added a tabletop]

[Fryguy]

That is a really nice idea , Hamelec - but it'll not work for me . My lab is too small since it needed to move to a new location 6 month ago .
I needed a thing to put on the bench . Needed because i just ordered a turnplate - 40 cm wide and rated at 125 Kg for 16€ . That's enough to turn stuff on the bench .
Maybe i'll add a larger plate to put on top to get 60 cm for the larger stuff . This is absolutely perfect for my needs . 

[Kleinstein]

A turnplate on the bench looks interesting and may help with heavy stuff.

Pictures would definitely be interesting.

A repair would likely not get get a good performing meter. Especially when space is lacking a bulky meter is not that interesting. I is more to keep a colectable part alive.

It is a bit odd to find vishay resistors in a RFT meter - that would only for a really high end, small number series. Normally they would go for russian resistors - often also good, just physically a bit larger.

[RoGeorge]

This weekend my oldest benchtop volt/ampmeter died

Died how?

Check the external fuse, then check the power supplies and the voltage regulators.  Usually the power components are the ones who fail.  Inspect visually for broken or smoked parts, for any onboard fuses, resit the boards and the connectors, things like that.

Maybe you can get away with it, without a schematic.

[BlownUpCapacitor]

Is it possible that you can provide some image of the internals? It would make us helping you a lot easier.

Most of the time the power supply board has rectifiers on it.

[Fryguy]

I'm back now from my workshop after 6 hours on the dead meter - i checked all the power rails on the 3 regulator boards and found nothing - they are all powered up but i don't know the correct output values . . .  The fuses are all ok - no luck there . No burnmarks on the pcb's , no onboard fuses found so far - but the precision resistor count is up to 22 !   

Thanks for the input guys , i really appreciate it . Already thought of all this today - my eyes are tired right now .   

I made some photos today but forgot to bring a memorystick - yes , i use a camera and got no dumbphone . 

Electronics are my hobby for about 40 years now - trust me , i know what i'm doing ( BANG - HAMMER ! ) and this thing is very complicated by being russian in the first place .
There are about 300 wires in this thing - 30 of them are white , 50 yellow ones , all nicely bundled together to make it impossible to follow one . . . and absolutely no markings of any kind except board numbers to put all the cards in the right place again .   

I'll upload the pictures tomorrow . And i am 100% positive this thing has got some more or less hidden power rails because i have yet to find the 150V power rails for the nixie tube display . So far i did not find anything on the boards with a 3 digit voltage rating front to back like a 150V zener regulator and cap .
The 40 transformertaps also deliver no clues without a schematic . 

Hmmmpfft 

@ RoGeorge - Died how ? I pushed the powerswitch like always - a millisecond of orange light on the first nixie (AC/DC+/DC- indicator tube on the far left side of the display) and then . . . nothing more - like the 150V rail died at power-up . The rail i have yet to find . . . 

[RoGeorge]

I pushed the powerswitch like always - a millisecond of orange light on the first nixie (AC/DC+/DC- indicator tube on the far left side of the display) and then . . . nothing more - like the 150V rail died at power-up . The rail i have yet to find . . .

So it might be that only one power supply failed, and the rest of the beast is still fine.

Just like you, I'll keep looking for the Nixie supply, starting from the tubes pinout and following wires and PCB traces until the HV source is found.  Would be a pitty to toss the entire instrument.  Good luck with it.

[Fryguy]

Bad news - the HV output of the larger transformer is gone and the digital/display driver section is more or less toast . 

The nixies are all fine (separately tested) , but when i feed HV to the driverboard from an external source , the display shows only mixed garbage and some of the tubes don't ignite at all . I'm not going to troubleshoot this thing any further - without paperwork it is beyond repair .   

A few pictures for you guys . . .

[Fryguy]

Russias finest . . .   Am i crazy or does the last picture in this post look like the engineer who made this has got a very special private hobby ? 

[Fryguy]

The "Reference Cell" - still functioning today after 43 years , and a last couple for our friend Kleinstein !   

[coppercone2]

hahaha you are in east germany without your papers good luck

[Fryguy]

hahaha you are in east germany without your papers good luck

A real *murican reaction    but in this case absolutely true - even today . . .