Here a description how to activate the 1750Hz tone call with a US IC-820
I've made those modifications with a IC-820 #13 (written on the cover)
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remove the top and the back cover of the 820.
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remove the 2 screws on the left and on the right on the front panel
Now you see the printed circuit board which is on the other side of the
front side of the 820.
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locate the 8 diodes located near the U-shape hole on the circuit.
starting from the hole they are :
/----- I
I I D D D D D D D D I
I I 2 2 2 2 2 2 3 2 ---
I I 2 5 3 6 4 7 0 8 all diodes : /
-----
I
I
D22 : When soldered, Europe version, activate the 1750
D25 : When not soldered, RX on VHF from 136 to 174 MHz
D23 : When soldered, USA version, activate the Sub-tone on TX
D26 : When not soldered, RX on UHF from 420 to 460 MHz
D24 : ? who knows ?, not soldered
D27 : When not soldered, TX on VHF from 136 to 174 MHz
D30 : When soldered, activate the transponder
D28 : When not soldered, TX on UHF from 420 to 460 MHz
The initial matrix on a US-820 (from D22 to D28) OFF ON ON ON OFF ON OFF ON
on a European 820 ON ON OFF ON OFF ON OFF ON
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to activate the 1750, remove the diode on D23 and put it on D30.
You don't need to either reset the 820 or remove the battery. You have now a
quiet 1750Hz when you push the TONE button, and a standard Europe bandwith on
VHF and UHF (144-146, 430-440 MHz)
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Now we need to boost the modulated 1750Hz, to do so, you have to short two
resistors : R80 100k, located near J16,J1 which is between R79 and R29. Also
R17 1k, located near IC11, IC8.
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Now the level is near to correct. If you whish, you could trim with R64.
It's an adjustable resistor marked STBN located on the large printed circuit
board on the bottom of the 820.
Remember that you lose the capability of sub-tone on TX (CTCSS), but not on RX,
if you have the optional module installed.
What to say more ? It's works fine on my old-US-820 !!!
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