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Date: Thu 9 Feb 89 12:02:25-PST
From: Brad Horak 
Subject: Icom IC-900 mods

Enclosed is a list of frequency range extension modifications for the IC-900.
They are courtesy of KB1MZ and KA1OKQ. Some mods may enable out-of-band
transmission. Transmitting there is ILLEGAL ! These radios are not type
accepted so therefore it is ILLEGAL to use this radio out of the ham bands
even if you have a license to do so. (ie volunteer F.D. etc). All mods
are done at your own risk. This is in NO way an invatation to break the law.
The mods are provided for information only..........................73


#Subject: Icom IC900 mods

Frequency Expansion Modifications for the ICOM IC-900

All changes are made in the control head.
Diodes are arranged in a 8x4 array: A-H, 1-4.

As shipped from the factory, the diodes are arranged thusly:

X = diode in place

                |   | X | X |   |   |   |   | X | 4
                |   | X |   | X |   |   |   |   | 3
                |   | X |   |   |   | X |   | X | 2
                |   |   |   |   |   |   |   | X | 1
                | A | B | C | D | E | F | G | H |

To add recieve coverage on 6 meter and 70 cm bands only:

C=cut diode, or remove

                |   | C | X |   |   |   |   | X | 4
                |   | X |   | C |   |   |   |   | 3
                |   | X |   |   |   | X |   | X | 2
                |   |   |   |   |   |   |   | X | 1
                | A | B | C | D | E | F | G | H |

440 Band - to expand coverage from 420 - 470 MHz:
Remove diode D25 (3,D) for RX
Install diode in (3,C) for TX
VCO in 440 module covers a 20 MHz range - re adjust as needed.

220 Band - to expand TX coverage from 216 - 236 MHz:
* This mod hasn't been tried and verified. *
Install diode (1,C) for TX
Remove diode (2,D) - ?

144 MHz Band - to expand TX coverage from 138 - 174 MHz:
Install diode in (1,C)

50 MHz Band - to expand coverage from 40 - 54 MHz:
Remove diode D32 (4,B) for RX
Install diode in (4,A) for TX

28 MHz Band - to expand coverage from 28 - 40 MHz:
Remove diode D32 (3,B) for RX
Install diode in (3,A) for TX


	ICOM IC-900 Memory Expansion Modification (Preliminary Rev II)

	This modification, done to the controller head, will double
	the amout of available memories to two banks of ten memories
	per band. 

	Remove the screw from the back of the control head, and 
	carefully open the unit. Remove the five screws holding the
	"Display B" unit buard to the rear cover, and unplug the 
	white three pin connector that goes to the "beeper".

	Locate IC2, the 24 pin RAM chip on the back of the board.
	Carefully cut the small etch between pins 19 and 20. Solder
	a #30 wire about 6 inches long to pin 19, (address bit 10).
	Solder a 22K 1/4 resistor to the cathode of D5, (1N4002).
	This is a good "pullup" voltage. Locate this resistor next to
	D5, on the top side of the board. Connect the wire from pin 19
	of IC2, and another 3 inch long #30 wire to the other end of 
	the 22K resistor. (Trim the wires as needed).

	The "DIM" switch is connected to two fine etches that go between
	diode D5 (1N4002)and  the rectangular cutout on the board.
	Carefully cut these two etches, removing at least a 1/16 inch
	section of each etch. Carefully scrape about 1/16 inch of the 
	green varnish on the "upstream" side of these two etches (one of 
	them is connected directly to the cathode of D5. Bridge these two
	etches together. (The display will now be only "bright", but this
	is no real loss, since the display isn't all that bright anyway).

	On the bottom side of the board, there are three pads for the 
	"DIM" switch, with two of them connected together. There will be 
	a glass zener diode (D2) across this switch. Remove this diode.
	Solder a short piece of wire from these two pads to a convenient 
	ground point on the board. Solder the 3 inch piece of wire from 
	the 22 K resistor to the third pad of the DIM switch. When the DIM 
	switch is in the "bright" position (down) the memory will contain 
	the original data. If the switch is in the "dim" position (up), 
	then the second bank of memories can be loaded and used. 
	Combined with the RX and TX expansion mods, this is a good way to 
	have the first bank loaded with ham band freqs, and the second bank 
	filled with police/fire/whatever channels. 

	The new bank of memories will read 999.9995 MHz when first called 
	up, and will have to be loaded with the desired frequencies, offsets
	and PL tones. (This is not a way to open up the RX to receive the 
	800 - 950 MHz band, unfortunately).

	Ed Brown     KB1MZ		
Copied from the QRZ! Windows Ham Radio CDROM


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