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Re: IC-820H Mods
Les,
Some time ago I downloaded this from some FTP and kept it in my HD, I
believe it is still around somewhere on the internet.
This MOD unlike the 821's was not written by myself and I did not even
read it completely, so I cannot comment on it.
I did read the section of X-band repeater enabling mod!, does anybody know
of something similar for the IC-821 ?.
73 de Fernando, CX6DD. (text is below)
ICOM IC-820H
EXTENDED RECEIVE/TRANSMIT MODIFICATION
--------------------------------------
This modification is to open the IC-820H for extended receive
(and transmit) operation. This is a two (four) step modification
which will open the IC-820H to the following frequencies:
VHF Band 136 - 174 MHz
UHF Band 420 - 460 MHz
These frequencies are not guaranteed for accuracy or sensitivity.
1. Remove Diode D25 on logic board for VHF RX Expansion.
2. Remove Diode D26 on logic board for UHF RX Expansion.
3. Remove Diode D27 on logic board for VHF TX Expansion. *
4. Remove Diode D28 on logic board for UHF TX Expansion. *
* RX modification must be done before the TX modification will work. *
======================================================================
ICOM IC-820H
CROSS BAND REPEAT MODIFICATION
------------------------------
1. Add a diode on the blank position between D27 & D28 on the
Logic Board. The part is 1SS353 P/N 1750000390.
AFTER HARDWARE MODIFICATION
1. Set lock mode. Push [LOCK]
2. Turn Power off.
3. Hold [M/S] and turn radio on.
4. Turn repeater off. Push [LOCK].
Note: If you removed Diode D25 or Diode D26 in the first modification
then it can be used in the second modification as it is the
same part.
IC-820H DISPLAY BOARD
---------------------
The following is a partial, not-to-scale, parts placement layout of
the IC-820H DISPLAY UNIT circuit board (bottom view). The DISPLAY UNIT
is the large circuit board that is located directly behind the front
panel. Locating the following four distinctive parts will ensure you
have the right diodes: the main dial encoder, the microphone compression
level variable resistor (R101), J10 a ten pin header and a row of surface
mount resistors immediately above J10.
CAUTION: These diodes are NOT labelled sequential! Ensure you are
certain of which diode you wish to remove.
|
|
|
UHF TX --> -[ []- D28 |
CB RPT --> BLANK |
VHF TX --> -[ []- D27 ----- |
-[ []- D24 [R106] |1o | |
UHF RX --> -[ []- D26 [R107] | o | | B
-[ []- D23 [R108] | o | MIC | O
VHF RX --> -[ []- D25 [R90 ] | o | J COMP | T
NOTE 2 --> -[ []- D22 [R91 ] | o | 1 ------ | T
[R92 ] | o | 0 | | | O
[R93 ] | o | | R101 []- | M
[R94 ] | o | | | |
[R95 ] | o | ------ |
| o | | E
----- | D
| G
| E
/-------------------------------------| |
/ | |
/ | | O
/ | | F
/ | |
| E N C O D E R | |
| | | C
| O P E N I N G | | A
\ | | R
\ | | D
\ | |
\ | |
\-------------------------------------| |
|
Note 1: The IC-820H Service Manual calls this board the "DISPLAY unit"
and the modifications call it the "logic board"; they are one
and the same.
Note 2: Diode D22 is listed as "[EUR],[AUS] only" in the IC-820H
Service Manual.
QRP mod for ICOM transceivers
-----------------------------
/
9-volt - o-------/ ------/\/\/\/\/\---------
alkaline S1 R1 |
battery + o-- |
| >
| < shielded cable
_____ variable R2--> > <------------------> ALC
--- < ------------------> JACK
- > | |
| | ground | ON
|__| | RADIO
| |
_____ _____
--- ---
- -
With this circuit connected to the radio, the power output
is adjustable between 0 ---> 100W. For best results. leave the
RF power control at maximum and adjust for desired power using
R2.
S1 is used to turn off circuit when radio is not in use or
QRP operation is not desired.
Radio Model: R1 value R2 value
IC-761, 765 47K 20K
IC-720(A), 735, 740, 745, 751(A), 275A/H 2M 1M
IC-725, 726, 728, 729, 736, 737(A) 220K 100K
06/01/94 CJR
For the IC-820H use the values for the IC-761, 765. The IC-781,
IC-765 and the IC-820H all have the same ALC characteristics:
Control voltage : -4 to 0 volts
Input impedance : More than 10 K ohms
----
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