Kai, you may simulate a signal generator for a special test. Use some meter isolated wire as an antenna. Take care the wire doesn't connect anything electrical. Dial your QMX to a FT8 frequency e.g. 14.074 MHz and set it to digital mode. Listen to the audio and connect the wire to the connection between T401 and IC403 pin 9 (the wire from T401 next to the edge of the PCB and a screw hole).
It would be fine to check first by using a different radio if loud FT8 signals are present at the time. If not use a different band or an other time.
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If you hear a difference between wire connected and not connected and some FT8 sound when wire is connected, the signal path from here up to IC407 is working in some way. If not maybe no FT8 signal is in the air or the signal path from the testpoint up to IC407 is broken.
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Sorry it's not so specific but a simple way to gain some more information without special equipment.
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73 Ludwig
