Icom IC-740

Volatile VOX modifications

I noticed rather volatile behaviour of the VOX. When the VOX button is on and the POWER button is depressed the TCVR switches to TX momentarily. When the TCVR is already on, it switches to TX also when MODE is changed.
It could be possibly dangerous as you may transmit to a detuned antenna. It is even worse when PA linear is in use.
I didn't find any common reason for this flaw so I decided to fix step by step.

TCVR power-on transition

The FET tranzistor was inserted into the line to the VOX button. The gate is connected to 100uF capacitor. The capacitor charges slowly through a rezistor from +8V. This causes that after the POWER button is depressed it behaves as if the VOX button is not depressed for about 4 seconds. .

Schematics diagram

FM/SSB vox disabled in CW/RTTY modes

In CW and RTTY mode +8V is fed to the base of NPN trazistor that pulls down the voltage on the C63 capacitor on the main board. This way it actually diables VOX (more exactly its SSB/FM part). See the schematics diagram and description on VOX in your service manual.

Schematics diagram

Modification the optional keyer (IC-EX243)

Even with these two mods it sometimes happened that the TCVR switched to TX momentarily when MODE was switched to CW. I found that for this last case the optional electronic keyer is to blame.
Go to
Electronic Keyer (IC-EX243) Modification for more details.

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