'67 Radio Reception

[Michael J.]

I have gotten my '67's radio working (original Delco) after the antenna splice. The AM works great, but the FM isn't picking up very much. I live in a fairly remote rural area, even my garage radio doesn't pick up FM real strong, but my other new car's radios pick up lots ofstations. The question is how strong was the original radio reception on FM in these cars? I could drive closer to town, but my access road is full of ice, snow, and potholes, and I don't want to venture out now. Should FM reception be as strong as AM though? Or was AM always the strongest freq. for these old Delcos?

[Jack H.]

Re: '67 Radio Reception

FM has slightly different reception characteristics than AM does. On the older radios, it wasn't uncommon to lose FM lock when you could still pull in AM stations (albeit faintly) as you cruised cross country and got out into the 'boondocks'.

FM reception deals with a concept called capture ratio (how much larger is the incoming signal that surrounding noise). When you fall below a given receivers' capture ratio (sometimes called 'lock' range), there's essentially NO signal reception.

AM reception will continue, albeit laden with noise and signal 'drift' until the actual received signal strength falls into the 'mud'...

AM reception is quite finicky when it comes to antenna integrity while FM reception is less 'picky'. Essentially, so long as the FM signal strength at the demodulation is greater than surrounding noise by the radio's capture ratio, the radio will lock on and work.

The fact that you're getting AM signals implies the antenna is OK. You 'might' get a little bit better reception by 'tweaking' the radio's antenna matching circuitry. That's a variable capacitor hidden behind the front knob bezel that you access through a tiny hole in the chassis and turn with a miniature screw driver. The process is known as 't*******'.

The radio antenna is capacitively coupled to the radio demodulator and there will be unit-to-unit differences in the actual capacitance of the lead-in cable. So, by 't*******' a radio, you're tweaking it for the specifics of your car, your radio, your antenna.

Some make a big deal about t******* the radio, but it only delivers noticeable results under 'fringe' reception condition. Sounds like that's what you're up against, so you're a GOOD candidate for relief based on optimizing the radio by t******* it to the antenna!

Today's newer FM tuners are a LOT better than the early ones of yester-year. So, it doesn't surprise me that you seen better reception in your specific locale in your new car(s) than you do with your classic Corvette...

[Jerry R.]

Re: '67 Radio Reception

Michael, you should pick up many FM stations almost anywhere in the states. FM signals propagate line of sight, meaning they go to the horizon (approx. 30 miles), and drop off rapidly in strength after that. Even low power FM stations tend to operate with 5 KW of power or more. Local AM stations are often at only 1KW during the day and shut down at dusk. Out west, FM signals often travel even further due to the flat land, lack of obstacles and transmitting antennas positioned on mountain tops. I just don't know hw you are defining "rural remote".

The easiest test is to plug a $10 aftermarket AutoZone type antenna in during the day and compare the FM between your old antenna and the test antenna. That will tell you immediately.

If you do not receive lots of stations on FM with the temporary antenna, I frequently find that the first FM radio frequency amplifier transistor (and occasionally the tunable coil in the tuner hooked to that transistor) has failed in the 65-67's. Under that condition you will experience exactly what you described.

Regarding the antenna trimmer, it is only connected during AM reception. It's adjustment will have no affect on FM reception. To adjust it on AM, tune the radio to a very, very weak station (or even background noise) near 16 on the dial and peak it for maximum loudness.

The sensitivity of the 65-67's on FM is not bad at all when the radio is performing correctly. There's more but that will give you something easy to try.

_Jerry_

[Stephen L.]

Re: '67 Radio Reception

Michael, send your radio to Jerry. He'll repair it at a reasonable price. He has the equipment to check it out and knows his stuff.