Not so good news today, after characterizing all kinds of noise sources, the 8970A stopped working. Can’t get it to calibrate properly, or to show any reasonable noise power values. A quick check revealed that the detector output (voltage proportional to the noise power measured) is good. But no proper display when activating the 8970A-internal volt meter (special functions 80, 81).
Checking various traces and signals – the issue seems to reside with the A7 assembly, voltmeter.
Red – input voltage section; green – reference voltage section (about 1.2 V); blue – auto-zero section.
How it works, quite well-established dual-slope integration with autozero – a capacitor, initially at zero volts, is charged first from the input voltage, then from a (negative) reference voltage, until zero is reached again. The time it takes to do this directly relates to the input voltage.
See here, working example (sorry a bit dim- see the triangular shape in the lower left hand corner of the scope screen).
Here, non-working condition – integrator not working.
After checking various FETs, and timing signals – the TL072 integrator opamp appears to be the faulty device. It is a strange, intermittent fault – not triggered by vibration, but appears to be intermittent with no direct external cause – maybe a defective output stage of the opamp? Removed it from the circuit; unfortunately, all spare back at the main workshop in Germany, but fair enough will get some TL072s in soon.
…. once repair is done, noise source project will continue asap!