TRC-503
I thought this was an FM Walkie-Talkie, not a CB?
Don't know specifically what the DEVIATION control is in this unit, but most of these FM units of similar design and origin have two adjustable POTS in them, most probably labeled RV1 and RV2. One is usually the squelch and one is usually the deviation. Mark where they are from the factory and commence to turning one. If that's not the deviation control, put it back from whence it came and turn the other one.
'Fuzz.