As you may already know, the IC-765 has IF-shift but no passband tuning. There is, however, a simple mod originally designed by Gerd Henjes, W2ISB, which allows the IF-shift button to tune the passband while the IF-shit button is in OFF position. This mod is described in the VE3HUR IC-765 notes and reproduced here with some illustrative pictures, as well as a video demonstrating the difference between PBT and IF-shift
What’s the difference between IF-shift and PBT?
Traditionally, IF-shift shifts the local oscillators, which effectively moves the pass band of the IF filter in frequency. The pass band width, however, does not change. On the other hand, PBT shifts the passband of two different IF filters (typically 2nd and 3rd IF) so that the amount the passbands overlap changes. The net effect is that the passband does not move but narrows down. Depending on how the shift is adjusted, the passband narrows from the high end or from the low end.
I have tried to demonstrate the difference between IF-shift and PBT by recording the audio spectrum:
You can also watch the video in high definition on my YouTube channel.
In order to do the PBT mod you need an additional 1N914 or 1N4148 small signal Si diode, and you have to access the main PCB in the bottom of the IC-765. Remove the top and bottom covers and turn the IC-765 up-side down. Then proceed as follows:
- Cut (or lift) the cathode end of D63. It is located approximately in the middle of the board.
- Solder the anode end of the new 1N914 to the anode of D64 as shown on the drawing below.
- Cut or de-solder W31 (brown wire) and solder it to the cathode end of D63 and cathode end of the new diode.
You can use the pictures below if you have difficulties in locating the parts. Also note, that if you want to de-solder the cathode of D63 and W31, you will have to lift the board and make some arrangement to hold it. I do not recommend to completely remove the board, since that would require removing a lots of connectors. If you don’t feel quite comfortable by doing this you can cut instead of soldering, although it can be difficult to access D63 with a cutter.
Cut (or lift) the cathode end of D63. It is located approximately in the middle of the board. Solder the anode end of the new 1N914 to the anode of D64. Cut or desolder W31 (brown wire) and solder it to the cathode end of D63 and cathode end of the new diode.
The PBT mod is now finished 🙂