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Icom IC275/271 mods
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IC275.2
Jack - the notes about mods to the FT736R were specific to that
machine, and won't be relevant to the IC735.
1. 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.
2. 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.
3. 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.
4. 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.)
5. 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.
6. 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.
All is fairly logical. Hope useful. Let me know the results.
73 de James G3RUH @ GB7SPV 1990 Mar 28
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