Modification Yaesu FT-847 for 70MHz band
v 1.0
Peter SP2DMB and Greg SP3RNZ
www.sp2dmb.cba.pl
The inspiration of this study was the FT847 mod project of Hellar ES1II /8.
The following description applies to modify the transceiver Yaesu FT -847
serving dramatically improved noise performance and sensitivity of the receiver,
as well as TX efficiency, increased power output and spectral purity in 4m band.
Particular emphasis was given on the purity of the signal emitted in the range of
4m and removing unwanted signals and LO mixing products below 4m band which are
present in the TX spectrum after simple unlocking FT847 to gain 70MHz band. In
addition, we believe that this modification protects the 54- 76MHz band pass filter
against L5022 self desoldering and SMD capacitors burn out.
Both RX and TX chain is modified in this case. As in original idea of Hellar
ES1II the solution is to replace the existing factory filters, in range 54- 76MHz – with
band-pass filters made in hybrid technology.
The original filter in the 847 has bandwith of 54-76 MHz, which should give
reasonable RX quality. But it’s not. Receiver sensitivity varies from about 0.3 to 0.5uV
depending on the production model and Year of production. Probably it depends on
minor changes in component values used by Yaesu, and/or destination model.
In addition, broadband response of these filters are also less resistant on
intermodulation products caused by near 4m OIRT stations. Some on air test with two
different 847 on same antenna proved that unmodified radio even on quiet band
receiving more of broadcast harshes even on Meteor Scatter burst, than modified one
- needless to say that unmodified one was less sensitive. This could be another
advantage of limited bandwith of new filter. Of course we cannot expect that with this
dimensions of L, new filter response will be very sharp e.t.c, but limited filter bandwith
(as shown below in RX mod part) is doing good job.
After more than 1 year of using first full modified 847 we can confirm that doing
this mod will give You increased resistance to spurs modulation of OIRT, improved
sensitivity and at last will improve total RX NF.
After this mod use of built-in amplifier is not necessary and even not recommended.
