The Russian ZEVS station is probably transmitting on 82Hz. More information about the station here:
http://www.vlf.it/zevs/zevs.htm
https://en.wikipedia.org/wiki/ZEVS_%28transmitter%29
The station uses two 60km ground dipoles as the transmitting antena.
On 28 may 2017 at 16:27 UTC there was a weak signal at 82Hz (received at SQ5BPF, Warsaw/Poland KO02md):
The 75Hz signal is probably local, although i haven’t seen it previously.
Later the signal shifted to 88Hz. This is the spectrogram from 28 may 2017 at 19:18 UTC:
Next day it was at 90Hz. Thisis the spectrogram from 29 may 2017 at 6:37 UTC:
Few hours later at 08:05 UTC the signal went to 92Hz and disappeared a few minutes later at 09:11 UTC:
The signal is stable in frequency. This can be a local signal, intermod at my antenna/receiver, or maybe this is antenna work at ZEVS and an attempt to test it outside the normally used 82Hz frequency. According to VLF.IT the transmitter can work at 20-250Hz.
EDIT: signal analysis from the whole day shows a stable carrier at 82Hz, and various signals which are not that stable, probably resulting from intermodulation:
