I am trying to repair Ronan X85 calibrator and I am puzzled with the strange LM312 behavior.
(Judging from date codes, it is from 1981.)
I don't have any manuals on it, but I traced voltage output part to be like the attached schematics.
So with the given values (approx. 15k is measured, I skipped adjustable part), output is correct "100mV", "1V" and "10V" matching the switch position.
The strange thing happens with 100mV range. If I power it up on this range, the output is correct 100mV.
But after I switch to 1V and back, the output becomes ~9V instead of expected 100mV!
"1V" and "10V" are always correct.
I figured out that I can add potentiometer in parallel with R3 to play with different feedback resistor values.
The sweet spot seems to be around 630 Ohm. I.e. swiching to the range with feedback resistor > 630 ohm is stable. When it's less than 630 Ohm - I get 9V on the output.
630 to 500 is flaky region. 500 seems to always yeld 9V.
I verified that OpAmp input (V-) to (V+) is around -8V , i.e. (V-) is 8V greater than (V+), but OpAmp does not regulate.
Replacing LM312H with AD OP27 solved the issue.
But I am puzzled, is it something known?
Also, is OP27 a suitable replacement? (I took it from the analog parts kit that I had.)
On the one hand, on the 100mV range fluctuations are around 10s of microvolts. I.e. 100.0mV is stable with my 34401a, output fluctuates around 100.0XXXmV.
On the other hand, I expect that there should be cheaper alternatives for the OpAmp from 1981.