After reading a message from a friend
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.
(2) To gain access to ACC1 you must remove the PLL PCB that it is
mounted on. With the board removed I soldered the extension wire to
the underside of the board taking care not to short out any
surrounding tracks. The whole exercise took about 30 minutes.
You will probably need more audio level from you PSK modem. You can then
use the pot, F151, in the IC275H to set the TX modulation level.
Results from making this change have been outstanding with many less
retries on the uplink than previously. The waveform being applied to
the Varicap diode is now identical to the output from the modem.
Performance via LUSAT and PACSAT digipeaters have been equally
pleasing. EXTRA hint: Use DWAIT=0 RESPTIME=0 and start TXDELAY=100 ms
and reduce to as low as possible for your radio - I use 30 ms.
A similar change could be performed on the ICOM IC271 by wiring to J12
on the Main Board. J12 is normally unused in Australian IC271's.
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What you need to do is work out a way to belt the varactor
directly. Then inject a square wave (1200 hz will do) and measure the
deviation that results.
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You can do this measurement with either a proper deviation
instrument or more likely with a monitor RX, by looking at the output
from it's discriminator.
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You can calibrate the monitor RX quite easily, Tune in a steady
unmodulated carrier and note the discriminator DC output voltage
Change the RX frequency by 1 Khz (knob on the RX front) and note the
change in DC voltage. That gives you the monitor RX sensitivity in
volts per Khz.
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Now return to the TX, and adjust that driving square wave to give
you approx +/- 3 kHz, you want to see about 6x volts peak to peak
square wave on the monitor. (Somewhere between 4x and 6x will do, no
more.)
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Now return to the modem and juggle about with the TXAudio output
components to achieve this level of drive signal, and look at the
monitor RX too.
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When you have done this, which should take you all of 30 minutes,
you should be in business. If you can't get square waves TX to produce
squarish waves out of the monitor RX, then the varactor dept. must be
suspect, and so on.
Since I am not going to be able to try the mods until I return from a
vacation I won't be able to give you an answer as to whether they work or
not. But it is my opinion they will. What is happening is that between the
mic connector and the varactor the digital signal is being badly distorted
and the bird does not respond to the digital signal. This would not make
much difference on voice but on a digital signal it makes all the difference
in the world.
ICOM has not tried this mod nor have we verified this problem.
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