Around three weeks ago I won an IC-720A for almost nothing during a live sale
at our radioclub
(PI4RCG). It was presented defect, so I thought: “What the heck?”

Afbeeldingsresultaat voor ic-720a

The next day the rig was repaired within an hour. Main error was, guess what . . .
yes! . . . the rotary relay. After some fiddling, cleaning and lubrification of the
rotary switch everything worked as expected.

It appeared my IC-720A was enriched with a  FL-32 500Hz 9 MHz CW filter, so this good old
buddy may be a likely candidate for ‘permanent’ portable usage in our camper/motorhome.

(Note: I prefer 250 – 300 Hz bandwidth for CW with ~500 Hz pitch . . . )

Some googling revealed that this rig was a quantum jump in its time and (one of) the first rig(s)
able to be remote controlled by a predecessor of the current CI-V Icom (‘CAT’) bus/interface.

The IC-720A features a general coverage receiver in 1 MHz portions. Transmitting was limited to
HAM bands only. With nippers this limitation was ‘fixed’ within 20 msec ; -) (read on).

In order to ‘get the rig going’, N1MM+ and my Winkey compatible Arduino keyer forced the
IC-720A to give CQ in CW for around three hours with full power (100W).

After this endurance test everything worked well, but I noticed I had to enter around 50 msec
‘PTT preamble’ in N1MM+ . . . which is too slow for contest and/or pile up usage.

So, I decided to enrich the TX/RX relay in the FILTER unit with a relay accelerator.

After studying the IC-720A service manual (google on it) it appeared that the /PTT (‘SEND’)
signal towards the filter unit (containing the TX/RX relay) could be easily located and isolated.

The wire was cut and the relay accelerator was piggy bagged somewhere on the MAIN unit.

Below a picture ‘before’ is depicted, mods are marked with A, B and C.
(click on picture to enlarge in a new tab)

A. General coverage TX mod. Cut this wire. (period ; -)

B. /PTT (‘SEND’) wire to filter unit. Cut this wire.

C. μPC2002 (TDA2002) noise reduction mod: place 470n in series with 100 Ohm between legs 2 and 4

Ad. B
Cut the concerning grey wire around the arrow marked position (see picture above).
The piece connected to the connector on the MAIN board goes to the input of the relay accelerator.
The other side is connected to the relay accelerator output by lengthening the wire a little.

The results of B and C are marked B’ and C’ in the picture below (click on image to enlarge in a new tab)

To be continued . . .