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Icom IC- 22S mods
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IC22S.2
ICOM IC-22S MODIFICATIONS:
Here are some technical details and modifications for the IC-22S. If you know
some other things about this rig, your messages are welcome.
Before doing any other modification, pins 13 and 16 of IC1 (programmable
divider, TC-5080P, PLL board) should be connected together. This allows the
PLL to accept all the binary codes and then cover all the frequency range
possible with the 22S. This modification is currently done and could be
already done on your rig.
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STEP MODIFICATION (PACKET RADIO OPERATION):
The normal STEP between "channels" is 15 KHz. For example, you can tune
146.610, 146.625, 146.640 MHz, but not 146.620, 146.630, etc...
The step is settled by the crystal X1 (on the PLL board), located near the
VCO (big metal box). The normal frequency of this crystal is 7.680 MHz. This
crystal also sets the offset (normal: 600 KHz) when using duplex mode, since
the offset is made by a logic circuit that adds 40 to the binary code applied
to the programmable divider (PLL). Think about it: 40 x 15 KHz = 600 KHz!!!
The step produced by a given crystal is calculated this way:
STEP = Xtal freq.(Hz) / 512
Example: Xtal 7.680 MHz: STEP = 7 680 000 Hz / 512 = 15 000 Hz = 15 KHz.
The offset is calculated this way:
OFFSET = STEP x 40
or: OFFSET = Xtal freq. / 12.8
Example: 15 KHz x 40 = 600 KHz
7.680 MHz / 12.8 = 600 KHz
If you want to have a 10 KHz STEP, you need to change the crystal X1 for a
5.120 MHz crystal:
STEP = 5.120 MHz / 512 = 10 KHz
Doing so will make the rig operate from 144.390 to 146.950 MHz, in 10 KHz
segments. However, the OFFSET will be 400 KHz instead of 600 KHz. Repeater
operation is therefore impossible, but
this modification is best for PACKET RADIO operation!
Suggestion: You can use a 10.240 MHz Crystal (which is common in CB rigs and
electronic stores) and divide its frequency by 2 with a divider. You can use
the unused pins of IC6 (uPD4013C) as a divider. To do this connect pins 9 and
12 together, enter your 10.240 MHz signal on pin 11 and you get a 5.12 MHz
square wave on pin 13.
If you want a 5 KHz step, You need a 2.560 MHz crystal.
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