It's been a long time; but, I believe there is a 39 Kohm resistor that is in the B+ line to the tuner. If this resistor increases in value or opens, the tuner will not operate properly.
Or, the tuner itself could be bad.
Parts places like
www.matelectronics.com usually have these tuners at a reasonable price.
Another option would be to use the tuner from a DTV converter, CATV converter, or VCR. Just leave the TV on channel 3 and use the tuner in your VCR or converter box to change channels.