IC275

    29-Mar-90 01:44 EST
Sb: IC275 Mods for FO20
Fm: Jack Mathias - W9FMW [70275,1336]
 
PSK Transmit Audio Modification for ICOM IC275 & IC271 - Garry VK5ZK
From AMSAT-Australia Newsletter #59 (Graham Ratcliff, VK5AGR)
 
After reading a FUJI-OSCAR-20-PRBBS message from James, G3RUH,
regarding the modifications he made to his Yaesu FT736 to improve the
PSK transmit audio, I decided to investigate the situation on my ICOM
IC275H to see if the same problem existed. I was certainly less than
satisfied with the results I was getting in the first few days with
FO-20 Mode JD. I was feeding the TX audio in at the front microphone
socket and results on the uplink were quite poor but my downlink copy
was 100%.
 
The first change I made was to move the Tx audio to the rear
connector, ACC1. Results improved but I was still not satisfied. Some
scoping around the Varicap diode, D40, revealed considerable distortion
of the PSK audio. To prove if this was the cause of my problem I
studied the IC275 circuit to see how I could feed the TX audio in as
close to the Varicap modulator diode, D40, as possible.
 
In the IC275H there are several Audio feeds to D40, including 3 via
Q33, and in my unit one of these, the feed from the Tone Squelch is
unused. P46 normally plugs into the Tone Sq card but as this option
is not installed it is left unterminated inside the rug. I also found
that Pin 1 on ACC1 was unused so a shielded wire from P46, pins TSTN
and GND to ACC1, Pins 1 and 2 would give easy access from outside the
rig to D40 and NOT alter any of the existing functions of the IC275H.
 
The installation of the wiring is not as easy as it sounds however:
 
(1) You have to find a way to connect P46. I stripped a small amount
of insulation from the wires just behind the plug housing and soldered
the extension wire there.
CONTINUED IN IC275.1