In the first 10 minutes the EM84 bulb moves correctly in the playback mode and in recording mode.
After this it starts gradually to lose sensitivity . This can be more clear in the playback mode because there it stops completely to move.
In the recording mode continue to move but only with loud sounds in microphone.
I didn't notice any big chance in the sound quality after 10 minutes in the playback mode, with older recordings.
But when is in playback mode after 10 minutes the recording quality gets really bad.
It reminds me the sound of some old transistor recording tapes When the battery was very low.
Sorry for my English
