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I'm trying to modulate a Kenwood TK-880 via the DI input. Have done this on half a dozen radios, but the one currently on my bench won't modulate. MIC input works fine, but I need the DI input. The problem appears to be somewhere in the firmware, as the audio switch controlling DI, IC4, is open. I could fix the problem by jumpering the audio switch, but I've never had to do this before. So I'm wondering what I'm missing in either the codeplug programming or resistor jumper configuration that's causing this DI input switch to remain open. Have been staring at this thing too long (all day today, mostly to get at the buried VCO board to bring the TX in this 450-490 radio down to 420 MHz).

Thanks

Bob NO6B