The original 40-channel (ONLY) Colt model 485 dates to 1978. It's covered in Sams volume CB-227. Has a PLL02A pll chip. This is the "unlocked" type chip that was outlawed a year or so later. Made this radio popular for folks who wanted lots of extra channels easy.
The AM carrier is set by a big white ceramic power potentiometer. It's mounted on a bracket to the side rail, about halfway to the rear of the radio behind the channel selector. If it fails, you get no AM carrier. Simply turning the shaft a bit to see if maybe it is set to a "bad" spot on the resistance element is where I would start.
The two wires that connect to this control are a good place to test for DC voltage. Should have at least 12 Volts DC on one of the two lugs when you key the mike in AM mode. The other lug will have roughly half that, depending on the setting of the pot.
And if one lug shows zero volts, the control could have failed altogether.
Not a common thing to see, but not impossible.
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