Are you saying that every 750ms you get a registered count (i.e. LED lights)? That isn't normal. However, there is some noise from the oscillator on the tube, but the Vmax through the TTL circuit is no more than 1V, so it shouldn't register an event. During my testing, I couldn't correlate the periodic noise to an event and the geiger counter I have connected to the feed, seems to work just fine.
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