[volt-nuts] Solartron 7081 noise

[Михаил]

Joe,
It's а very interesting result! I think the only internal reason of such slow
sine-wave fluctuations is a interference of the mains frequency. During the
power-on Floating Logic CPU IC801 is autodetect the mains frequency (50, 60, 400
Hz) and setup appropriate output lines (400Hz, /60Hz) and ADC clock divider for
proper PLL phase synchronization. It may take more than 5-10 minutes, during
which the filter's capacitor C806 voltage and V-T (ADC) clock changes slowly. After all,
clock oscillator outputs 5.24 MHz which must varies with the mains frequency.
If phase synchronization is failed due to some reason, then effective Common
Mode Rejection is largely reduced.
First of all you can try to adjust initial PLL frequency in accordance with the
Solartron 7081 Maintenance Manual, 9.8 (page 9.5). It's very simple.
The next steps, I think, are isolation of the input leads and Input Amplifier
infuence. To do this, I recommend: 1) disabling probes and short the input with a
copper wire (photo). 2) repeat testing after the execution of a command "MODE,
TEST 0". This command disables the input amplifier and ADC shorts.

With best regards,
Mickle T.
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