ICOM IC260 - 9600 BIT/SEC MODIFICATIONS

                        (PRELIMINARY RELEASE)


Rel. 1 Nov.1994


DOCUMENT'S PURPOSE


 Following document describes the hardware modifications and configurations,
 done on the ICOM IC260E s/n1151,to allow satisfactory operations with
 the G3RUH modem.

 Extensive reliability test have not yet performed but the unit shows good
 performances.


RECEIVER SECTION

 The only modification required consists in the optimization of the link
 between the output of the receiver detector stage and the pin#4 of the
 ACCESSORY SOCKET. (Refer to page 12)

 As shown in the schematic diagram (ref.A-0239), the output of IC4
 (MC3357P-FM demodulator) pin #9 is connected through the RC filter R51-C53
 (22K-0.001uF).

 The only modification consists in removing the capacitor C53; thus the B.F.
 filtering action is done exclusively by the input filter of the modem.



TRANSMITTER SECTION


 There is no direct connection between the accessory socket and the
modulator;
 the only connection between the transmitter section and this socket is the
 pin # 5 (Output from the MIC amplifier stage).


 The modification consists in using this pin as an input to the modulator.


 The modification has been done without cutting any part of the PC board; the
 ferrite bead (unnamed) on the path between the pin #5 and the C115 has been
 removed and a small piece of wire has been placed between the input trace
and
 the upper side of the trimmer R115.

 The value of R114 has been changed to 6.8k.


CONCLUSIONS

 The suggested modifications seems to work satisfactory.

 Any further modification shall be placed on the net.

 Put yourself in much confidence with the soldering iron....



                                (UPDATE)


 After extensive checks, a better modification to the transmitter side of
the IC260 has been developed.

 In  the  preliminary  release,  the local audio amplifier was not decoupled
from the external data; this results in a bad error rate.

 In the final release of the modification reristor R114 has been removed.

 Please  take  care  that  in  the  components  positioning on the PC board,
capacitor C103 is connected to the central pin of the trimmer R115 and R114
is connected to the diode.

 After the R114 and C103 have been removed, a capacitor of value between 22nF
(0.022u) and 47nF(0.047u) must be placed in the position of R114.

 Input  connection  from  accessory socket is connected directly to this new
capacitor.

 In  order  to  check  the  results, an analog link has been done, using the
transmitter on a dummy load and receiving the PRBS pattern with a different
receiver.

 The Rf level was lower than the normar level received from a good BBS.
In this condition the modulation level has been adjusted for the lower BER.

 Several minutes with no errors have been obtained.

 I hope this modification will be satisfactory.

 The units can sill be used in SSB and, eventually, it should be possible to
use  an  another  pin  of  the  accessory  socket  to be used as jumper for
operations in FM voice.

 Hope to receive acks for this message.

Best 73 Stefano IV3URK
/ex

