The IC-765 has an IF shift feature that allows one to move the passband around for the best SSB audio quality or to reduce interference. The same type of IF shift is offered in the IC-706 series and the R9000 receiver. This should not be confused with the variable bandwidth modification often performed on the 765, which allows one to reduce the IF bandwidth by offsetting the 9 MHz and 455 kHz filters.
When the IF shift button is pushed “IN”, the 9 MHz crystal filter is completely removed from the circuit, to allow the 455 kHz filter to shift its passband position. Unfortunately the elimination of all selectivity at 9 MHz degrades the performance of the radio significantly, as noted in the QST review of December 1990. In that review the ARRL reported the 5-kHz dynamic range is reduced from 85 dB to 73 dB. The 2-kHz dynamic range is degraded even more.
This problem can be completely eliminated by adding the Sherwood distributed roofing filter. Instead of removing the SSB filter at 9 MHz, we insert a 3.3 kHz crystal filter that allows the IF shift to function without degrading the radio’s dynamic range. This allows the FL-96 2.3 kHz filter (or optional FL-80) to move around inside the Sherwood 3.3 kHz filter, yet still keeping strong adjacent signals out of the 9 MHz IF chain.
The only compromise with this upgrade is the IC-765 will no longer function on 10 meter FM. The 3.3 kHz filter that is in the circuit on FM is too narrow for that mode. Addition of this filter does not degrade blanker performance. This kit is compatible with the Inrad 69 MHz roofing filter modification. |
