On the original Atmegatron (V 2.4 I believe) the filter LFO tends to malfunction when the parameters are really pushed. If the resonance is high, the LFO rate is above 12 or so, and the cutoff is low (or high depending on the filter) the sound becomes distorted for a moment (relatively speaking lol) and then cuts out completely. Sometimes it can be brought back if the parameters are dialed down but sometimes it just will just stay silent unless you either restart the unit or load a preset.