We have 2 sts-2 readout box - pink and blue. Pink outputs 12 DVC - this a problem of amplifier. This box has a rectifier (the box works from AC power) and an amplifier for velocity channels. Mass positions, calibration channels are connected by a wire from input to output. The amplifier for velocity channels does not work properly, so I connected velocity channels directly to the output - the signal from sts-2 is large enough even without amplification. When I plugged sts-2 to pink readout board, on the velocity output I saw ~4 VDC. Sts-2 was needed to be recentered. I pressed AUTOZERO command, but this did not work out. Before I had checked that this readout box indeed gives an autozero logical signal - 5VDC for ~2 sec. I think it does not provides sts-2 with enough current, seismometer needs 0.1 A in autozero regime.
Blue readout box after switching it to 1 sec regime and zeroing sts-2 started to output reasonable signal for gains = 10. I tried gains = 100, X velocity channel started to output noise. Now the gain is 10 and the response is 120 sec. But at least this box works. Still performance is not clear as well as noise level. To determine this I've put sts-2 to isolation box.
After I've put Guralps in the isolation and waited for a couple of days, Guralp noise has been improved a little more.