K1 Builder Alert #3:
Inability to hear transmit signal when making the TX Offset Adjustment
26 Feb. 2001
Impact: All K1's
When making the 'Transmit Offset Adjustment' (K1 Rev D manual, page 47) on some K1s no TX signal tone is heard. (This adjustment has you match the K1 sidetone frequency with the TX offset.) The reason this happens is because on some rigs we have two mute circuits that work almost too well. Q10 is a shunt mute FET, and Q11 is a series mute FET. Jumpering OFS (as noted on page 47) prevents Q10 from muting, but it does not prevent Q11 from muting. Some J309s mute so well there is not enough AF signal leakage from U3 to U4 to be heard in the speaker. TX offset can still be set using an external receiver as a reference, but the following procedure is suggested if you want to use just the K1.
Note: Version 1.09e (and later) firmware disables the sidetone in Tune so the rig must be keyed to hear the TX-generated sidetone for the TX Offset adjustment. Use the Menu-->InP-->Hnd mode to make this easy.
Technical details: The K1's Tune sidetone had to be disabled once the KAT1 atu was added. The sidetone cannot be generated when either micro has control of the auxbus.
Modification to TX Offset Adjustment instructions. (Please read the TX Offset instructions on page 47 first.):
Set the RX BFO properly first (K1 Rev. D manual page 40), by adjusting C20 until maximum amplitude is found using an AC voltmeter on the AF output, or using Spectrogram to peak the AF signal on noise.
Then turn off the sidetone completely (StL 0).
Set the power level for 2-3 watts.
Try adjusting TX BFO trimmer C13 to see if you can hear the beat note. If you still cannot hear the TX-generated tone, temporarily place a resistor of between 1k to 10k across the Drain and Source leads of Q11 to allow more audio to leak into the AF amp at U4. Now you should be able to hear the TX tone well.
You can then increase StL to 1-5 or so to match the sidetone and TX signal levels. You may need to adjust the StL and Power level slightly so both are at about the same audio level for hearing the beat note.
When the TX offset adjustment is complete, remove the OFS jumper and the 1k to 10k resistor from Q11.