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Modifications for the Icom IC-720
IC-720A
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| ICOM IC-720
extended xmit |
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Transmitter is unlocked by snipping the light blue wire that's at
the very end of the top hatch cover to the left middle side of the
transceiver.
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| Modifying
IC-720A to get rid of rotary relay |
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I have just finished modifying an old IC-720A HF transceiver to
replace the rotary relay filter control with a set of reed relays
and a small digital circuit to select the appropriate relay and
provide feedback to the existing CPU in the radio so it thinks
it's still on the rotary relay. The cost for parts was about $25;
the cost of my time was slightly more than that, but it was "fun".
The modification is so far successful, and the radio is much
quieter in band switching as well as being more reliable (the old
rotary relay was impossible to get mechanically aligned correctly,
leading to poor connectivity between the rx/tx and the antenna, as
well as many missed stepping pulses).
Summary: Use the step-pulse line to drive a small SPDT relay; use
the SPDT relay to drive an R/S flip-flop (built from two nand
gates) to provide a clean step signal (the existing step signal
has some kind of very high speed clock train superimposed on it
and therefore was not usable directly). Feed the pulse train into
a CMOS decade counter type 4017. Take the appropriate decade
outputs (active high) and recreate the switch-position feedback
signals using a small diode matrix ala the original design. Also
use the decade outputs through 33K resistors to turn on 2N3904 NPN
transistors, which are used to select the appropriate (1 of 7)
DPST reed relays. The CMOS chips (the nand gates and the decade
counter) can be had at Radio Shack and must be powered from 9V (or
so) because the radio does not use 5V logic (of course...)
I mounted the reed relays inside the original low-pass filter
subassembly, and the rest of the circuitry on a small perf-board
near the SWR module (connected by ribbon cable).
This is not a detailed design description but if you are
interested I will provide more details. This project did ssalvage
an otherwise-worthless IC-720A, but I don't think I'd do it again
(rather time consuming, but perhaps you can benefit from my
experience and save some time).
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I present here again the mods for amtor operation for the Icom
IC-720A transceiver.
I received this infos in the year 1984-1985 !
On my Icom IC-720A i did the first set of mods (by G4ICM) and now
my Icom IC-720A works well in amtor and packet !
I hope that this infos are useful for someone!
73 DE IK3BVD OP.ENRICO
Mods for the Icom IC-720A:
There are 4 sets of mods. Some duplication will be obvious.
By G4ICM
Main board:
Change C40 C42 C43 C54 to 0.1 uF
Change C69 TO 0.01uF.
I.F. Board:
C87 may be 33uF, but shown as 0.0047uF, which is on print side.
Change C87 to 2.2uF, leave 0.0047 on print side.
R.F. Board:
Remove C169 C127. Change C165 to 0.47uF.
Noise blanker update. (check if done)
Change C116 to 10uF
Change C171 to 0.01uF
If in difficulties call england 2273 63859
By KB9IS
Delete C40 and C43 on main unit
Change C42 and C43 (10uF and 2.2uF) to 1uF.
Disable A.F.Muting- link short-C69.
Delete cap. CT14 at pin 6 of IC3 on I.F. Unit.
Rearrange the R9V to TR Q22 on I.F. Unit so that it is connected
to unswitched "9volts supply.
Do a similar mod. To the "9 volt supply to the A.F. unit to
overcome surge pulse.
Good luck de " RON " GM3SAE
By PA3AXO
Connections to acc. Jack on the back of the tansceiver:
Pin 3 Keyline (note that 200 mA current capability is required so
a dil relay may give problems)
Pin 4 af to convertor
Pin 9 FSK input TTL level
Pin 8 ground
On main board:
Remove C40/42/43/54 and replace by 0.1 uF
Replace C69 by 10 nF
On RF board:
Remove C169 and 127 and replace C165 by 0.68 uF when desired the
relay to control an external linear amp can be disconnected to
reduce the clicking noises
Good luck and best 73 de Jack PA3AXO
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KB2LJJ