Needed schematic for pH meter Elwro N5170

[bborisov567]

Hello, i am repairing this old ph meter. Unfortunately when powered up only the minus sign lights up. All power supply voltages look good. But i need a schematic or service manual to further continue repairing. Even a user manual still would be helpful. Thank you in advance!

[Kleinstein]

A PH meter is a realtively simple instrument. Not really that much inside: a very low bias input amplifier and than scaling (can be temperature dependent - thus the pot and maybe exter PT100 input) for a mV meter.
The line of - signs could be indicating overflow for the ADC part.

Even without schematics one could likely check the input amplifer / buffer part.
It may be normal that the meter reads an overflow with an open input. For a test one should try with a short across the input and than get 0 mV and ~ PH7.

[doktor pyta]

Hi,
not much but still better than nothing.
It is a part of a manual of a similar model.

[bborisov567]

I have tried shorting the input and supplying 200-300 mv but there is no change. Thank you very much for the pictures! This is the model with the nixie tubes, right? I believe the schematic could be very similar. Could you scan the whole manual (you can use one of the smartphone scanning apps like Camscanner, no need for high quality), especially if there is a schematic. Also does somebody recognize the multi pin connector on the back? It looks like something similar to GPIB but it is definitelly not.

[bborisov567]

It turned out that the schematic regarding the digital circuitry inside this Elwro is the same as Meratronic V640 .
Regarding the problem - the display reading came after i replace C502 (68p) which is responsible for the clock of the D triggegrs.
Also I had to change all 4 7-segment displays because of dead segments.