7
Figure 5
I.F. Filter Passband Improvements:
I believe the crystals that Heath supplied for the carrier oscillator were of fairly wide
tolerance, thus the frequencies of the LSB/USB/CW carrier injection may not be properly
positioned on the slope of the I.F. filter. This can affect both the receive and transmit
audio response to a great degree. Telltale signs of this are not having the same audio
response on USB and LSB, and reduced CW power output when using the CW filter.
It is relatively easy to tell if the USB/LSB carrier insertion points aren’t placed
equidistant from the center of the I.F. filter passband. After the rig reaches a stable
operating temperature (1/2 hour) disconnect any antenna and peak the preselector for
maximum receiver gain. Next turn up the volume control to a slightly higher than normal
level and listen closely to the hiss coming from the speaker. Then switch to the opposite
sideband. The pitch of the receiver background noise should be the same if the USB &
LSB carriers are both placed equidistant from the filter center frequency.
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