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RE: Measure Quad 989 trigger level?

When the sniffer circuit engages it internally shorts the speaker causing the output to drop, 10-15dB, and the output it distorted. It sounds like the circuit is working properly. ESL-63 up to serial number 29000 had an additional protection circuit a soft clipping amp and once a certain voltage threshold is reached the soft clipping amp introduces some distortion into the signal and there will be no drop in output. No 98 series will have the soft clipping amp.

The sniffer circuit rarely fails and when it does it typically fails in two ways. It fails completely and never triggers or it fails and it is clamping all the time. I have never seen the circuit fail and clamp at lower than ideal output levels.

Sparking is occurring somewhere either inside the speaker or close to the speaker. I have seen very low humidity levels cause the sniffer circuit to trip since there will be some sparking occurring around the speaker, i.e. walking across the carpet, etc., which creates some static electricity.

The best way to determine what is going on is to disconnect the sniffer circuit antenna wire that goes through the delay line assembly. This lowers the sniffer circuit sensitivity. Darken the room so that you can see if any sparking is occurring. Apply signal to the level where you have been having shut down issues and see if you can find any sparking in the panels. If the panels appear OK than lay the speaker on its face apply signal and see if you can find any sparking in the electronics. Be very careful when looking at the electronics there are lethal voltages present when signal is applied to the speaker.






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