FT847 ALC modification
You know this situation from the VHF Contest for sure - on the opposite hill is the station with commercial equipment and solid state PA. His signal has splatters over whole band and the result is, that our competirors already have within few hours of contest tes QSO more. When we visit that station, the only response to our complaints is uncomprehending look of operator, because he is using "professional equipment" and so all should be perfect.
And because FT847 transceiver is quite popular we are pleased to bring you recommendation how to modify ALC circuits for transmitter splatters suppression.
ALC modification is focused on the circuit changes on AF-CNTL board. Original ALC circuit is in the modification changed to peak detector. On the board is removed original C1070 1µF/50V. Into ALC loop is added new tantal capacitor C1070 as well on value 1µF/35V, which together with Q1023, R1082 and R1086 creates quasipeak detector with regulation time abt. 0,5 msec and deregulation abt. 1sec. This modification is very simple, in the transceiver is not needed any readjustment and all functions (such a SWR protestion) stay the same as before. Transceiver modification can be reversed to original status easy.
Modifications schematic drawing:
And what is the result? Check audiorecords (bad quality, but still clear) before and after modification here and here. Both transceivers (tranmitter ned receiver) were interconnected on test stand to have on receiver signal S9+ 20dB and detuned by 20kHz. In the second audio sample you can recognise, that splatters are gone and only audio cross talk and a bit higher noise due to phase noise of both local oscillators can be observed.
How to do it:
Remove the bottom cover, remove original C1070 and add new tantalum C1070. Solder them carefully by grounded micro soldering iron! Assembly your transceiver again and check all functions. No changes should be observed except suppression of splatters.
Look at original AF CNTL board Detail and modification drawings of original C1070 removal and addition of the new one Look at modified board
BTW - when you have your FT847 already open - add to frequency synthesis principal crystal oscillator the cap from foam polyuretan (the white "cube" on picture below). It improves fcrequency stability of transceiver a lot - it is important particularly for EME on WSJT. Recommended!