It's a popular mod to bypass VR21 and control the receiver and transmitter frequency together from the front-panel clarifier.
Getting transmit and receiver frequencies lined up together just makes life easier when there is more than one other station in the conversation.
The factory setup controls the transmit frequency from VR21. If the radio were more stable, this would be okay. The receiver frequency is controlled by the front-panel clarifier. But the VR will never stay adjusted close enough to use.
And if it's JUST a two-station conversation, this works okay. The VR setting will never be perfect, but each station tunes in the other. All is well until a third station comes on the channel and wants to know why two stations are each transmitting on a slightly-different frequency. To make it a three-way is just easier if everyone is transmitting on the same "slot". Can only do this by modifying the radio to lock the receiver and transmit frequencies together.
But once this is done, VR12 just doesn't do anything any more.
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