Problem:
ICOM has chosen to remove the PBT in the later models, and replace it with a "data level adjustment". This change removes the possibility to profit from the PBT in case of QRM or to use the PBT as a cheap CW-filter.
Furthermore the two extra IF-mixings is done with only a minimum of filtering, making the receiver very susceptible to spurious IF breakthrough. This is often heard as high-pitch tones or as a high IF noise level.
Solution:
The solution is to re-wire the "data level" potentiometer so that it adjusts the PBT VXO-oscillator, and to re-install FI3. FI3 is a 455kHz ceramic SSB filter and can be bought under the type number: MURATA CFJ 455 K6.
Implementation:
- remove the capacitor (C61) fitted in place of FI3
- solder in FI3
- readjust R72 for proper IF gain
- cut wire PBTV (pin 4, P1/J1, MAIN, yellow) about 5cm from the plug (P1)
- cut wire AMO2 (pin 3, P2/J18, MAIN, red) close to the plug (P2)
- cut wire AMO3 (pin 4, P2/J18, MAIN, metal baird) close to the plug (P2)
- connect PBTV wire (the end not connected to the plug) with AMO2 wire (the end not connected to the plug)
- connect AMO3 (the end not connected to the plug) to +8V e.g. by tapping (pin 1, J4, MAIN, brown).
- connect the wire stubs on the plug (AMO2 & AMO3) together is you wish to use external modulation input for PACKET etc.
- adjust the coil next to X2 (PBT VXO) for correct middle PBT setting
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