C1 wonderbar radio reception problem

[Philip P.]

Re: C1 wonderbar radio reception problem

I agree with Tom, I have never seen anything like a capacitor in a cable connector type used on a car. I would like to know however what they are referring to, I have worked on these radios and have just used a spare antenna with less cable than the car has and do not remember any reception problems. But then I was a Radar guy in an earlier life.
Phil

[Ken A.]

Re: C1 wonderbar radio reception problem

Talked to steve at ford & garland radio repair, and he suggested that the replacement antenna leads do not have a capacitor built into them like the original. He is sending me one to lay outside the car to antenna to try. The capacitor fools the radio into thinkin the antenna lead is only 4 ft long. Will let everyone know results.

This is correct all the way up to 67 and only affects the AM reception function. The FM signal is not affected .

[Dan D.]

Re: C1 wonderbar radio reception problem

I have been following this thread, and it is time I chimed in.

Yes, these lead-in cables do have a series capacitor built in. It is necessary in order for impedance matching the antenna to the radio. This is mentioned in ST-12 (page 13-2), and can be confirmed by measuring each end of the cable with an ohmmeter.

Not all repro-cables have this capacitor, although they say they do. I have purchased some from different suppliers and they are different. If the capacitor in not there, you will get the half dead symptoms you are experiencing. If the trimmer capacitor in the radio will not tune (noticeable by a sharp volume increase and a peak in the middle of the trimmer range) then the cable either does not have the capacitor, or the right value capacitor. Best thing to do is install your old cable outside of your car and see if it will tune. Just because it is old does not mean it is bad. My original 57 cable looks faded and worn and such, but it works just fine. Measure your new cable with an ohmmeter. If you get continuity between both ends of the center conductor, then there is no capacitor and it will not tune. It must measure open circuit. You can even buy a el-cheapo aftermarket replacement antenna with short cable for passenger car use just to try it to see if it will tune.

If these tests all fail and you are sure - to the best you can determine - that the radio is bad, then Jerry Rudbeck, mentioned in this thread, is an excellent source for information and repair. But I will tell you, poor reception is not a common failure mode with these radios, Usually they either work, or they don't. -Dan-

[Philip P.]

Re: C1 wonderbar radio reception problem

Wow, its a good day I learned something.
Thanks
Phil

[Ken A.]

Re: C1 wonderbar radio reception problem

The radio can be slightly modified internally to boost the AM signal & you can then use a new coax. Very few "repair shops" know how to do this.

[Thomas H.]

Re: C1 wonderbar radio reception problem

I have been following this thread, and it is time I chimed in.

Yes, these lead-in cables do have a series capacitor built in. It is necessary in order for impedance matching the antenna to the radio. This is mentioned in ST-12 (page 13-2), and can be confirmed by measuring each end of the cable with an ohmmeter.
-Dan-

Now that is interesting. I too learned something today (Philip, that will teach you to agree with me..........). I have a fair amount of experience with antennas, Ham radio operator for over 30 years and I worked in an antenna lab developing antennas for use on the EA6-B aircraft. True, capacitors as well as inductors are used to help match the feedpoint impedance to the load. They can also be used as part of the antenna itself to help make it look "electrically longer or shorter" to help bring the feedpoint impedance up for a better match to the cable / receiver front end.

The more I think about it, the front end of these older radios (probably using Germanium diodes / transistors) probably had a much higher matching requirement not like the 50 ohm impedance matching requirements of the modern radios in use today. Therefore so some sort of feedline matching was required, thus the capacitor in the cable concept to help transform the impedance seen by the radio to something it could easily match.

I'm open for a bit of education on this subject as what I wrote above is just my brain trying to understand and is not based on any knowledge or experience with these older systems (so I could be totally off base in my theory).

Tom
Neat stuff.

[Rob M.]

Re: C1 wonderbar radio reception problem

I feel a Restorer article boiling out of this thread...

[Dan D.]

Re: C1 wonderbar radio reception problem

Hi Tom,

You are spot on with what you wrote. The coax is not 50 ohms - it is 93 ohms. The front end of these radios is not even germanium - it is tubes. The short whip antennas are far, far from being even close to 1/4 wave at 550-1.6MHz.

I worked for 42 years for a company that designed and manufactured radio communications systems for the US and Canadian armed forces, and many other countries as well. Our radios are the primary means of communications for our forces in Iraq and Afghanistan. I'm very proud of what we do. -Dan-

[Philip P.]

Re: C1 wonderbar radio reception problem

Dan/Tom
With this type of antenna and cable, how will this work a new replacement type radio that may be installed in the car, is this antenna compatable with these type of receivers? Back in the day we had some equipment that required 92 ohm coax it was in some video circuits as I remember and was not compatable with 75ohm that was normally used, and caused problems.

Phil

[Dan D.]

Re: C1 wonderbar radio reception problem

That's a good question Phil. I do not know the answer. I don't know if new design radios will match to it or not. But most traffic these days is FM and that will not matter. Perhaps Jerry can answer this.

Just thinking about this now, I have some newer radios in the shop that I can hook up and see if they will work. Maybe I can try this in the next few days. Got my interest up now. -Dan-

[Jerry R.]

Re: C1 wonderbar radio reception problem

That's a good question Phil. I do not know the answer. I don't know if new design radios will match to it or not. But most traffic these days is FM and that will not matter. Perhaps Jerry can answer this.

Just thinking about this now, I have some newer radios in the shop that I can hook up and see if they will work. Maybe I can try this in the next few days. Got my interest up now. -Dan-

_______________-

Dan, do you use magnets or mental manipulation of minds? I haven't been on here in ages (repair overload) and you type my name and I log in and happen to see your questiion.. Spooky!

I haven't read the whole thread but: 1.) yes there is a very low picofarad series capacitor in the radio end connector, 2.) it is there to satisfy the requirement that a certain value of capacitance and inductance are necessary to create a resonant circuit for effective signal transfer (an electronic equivalent of the tuning fork), 3.) as you said, the whip, which is the inductive component in the resonant circuit, is a few hundred feet short of a desirable length on AM and therefore must be made up for with capacitance (in general terms), 4.) the ratio of the inner conductor diameter to the outer conductor (shield) has been constructed to provide 93 ohms (RG-62), 4.5.) and that's why the center conductor is so thin and frail (velocity factor noted) 5.) the radios input circuit has been designed to maximize transfer of radio band energy when it looks into that 93 ohm load, 6.) the feedline is actually a part of the antenna itself (which was dramatically obvious in car antennas from the late 30's and early 40's), 7.) the only vendor who is savvy enough to make a correct antenna cable for 58-66 vettes is Paragon, 8.) unless you use this cable on your Wonderbar it will be as deaf as my pet rock and the signal seeker won't stop on many stations thus losing points (save those original GM cables!), 9.) and the antenna trimmer will not peak the radio within its range for maximum signal transfer without the correct cable (that's the proof of correct performance), 10.) the cables from Long Island Corvette, Corvette Central, etc are made from RG59 (cable TV wire - 75 ohms; hey, it's cheap and easy to get) and are as wrong as the electrical disaster area speakers they sell for these same year cars - except for CorvetteAmerica and now ZIP Products who showed they care about their customers by switching to Greg Thompson's correctly designed speakers, 11.) why aren't the FM signals affected by all of this? The whip on my 67 looks like a 1/4 wavelength on the band to my eyes and the series cap in the line is electrically huge at those frequencies. That's my guess.

Your real question related to the new era radios is that they use new technology (GASFETS/MOSFETS/etc) in the RF stages and those tolerate impedance variances better. They give new meaning to broad bandwidth, function at a lower Q and can deliver gain with significantly reduced signal to noise ratios. Now all of that could start a whole new discussion so remember, I'm being general in the details. EE401 this isn't.

Thomas, congrats on 30 years a ham! I'm at 52 or 53 years now (K9JR) and it was the best choice of an exciting hobby, leading to fascinating jobs and a life of electrical wonder. Bless you Nicolas Tesla and whomever put your biography in my 6th grade library.

Sorry for the rambling Dan and thanks for the opportunity to comment, I hope there's something of use in my offerings. As guys are finding out, a correctly working original vette radio hooked to an electrically correct antenna system driving an electrically correct speaker can sound pretty darn good! Amazing what a couple watts of audio can do. Use the aftermarket stuff, including the high power/low efficiency speakers and half kilowatt amps, in the resto-mods.

Back to the bench - taking May off to drive the vette and go flying. Be well all! _Jerry_

[John F.]

Re: C1 wonderbar radio reception problem

Another great job, Jerry! Just got my radio back from you today as well.

[Dan D.]

Re: C1 wonderbar radio reception problem

Hey Jerry, good to hear from you again. Spooky, but I use a MOSFET to home in on South Carolina, and then my Wonderbar to tune in on you! Some day I am going to drive to South Carolina just to meet you! Someone mentioned a Restorer article on these things. They certainly are black magic if you don't understand them. Hey, enjoy your time off and - DRIVE THAT VETTE. -Dan-

[Bill W.]

Re: C1 wonderbar radio reception problem

To all you people who recommended Jerry Rudbeck for radio repair, a big "THANKS!" this guy is great, resonably priced, quick turn-a-round did the job right! Back to the garage. Bill

[Gary L.]

Re: C1 wonderbar radio reception problem

Man, have you guys got big words! For us grease monkeys who don't speak that language, looks like I need the one from Paragon. Jerry's correct about the Corvette Central will not work. Just got to try their new one and it is the same. I can plug a cheap antenna into the radio with a 4 ft lead and it fixes problem. Wonder bar stops on stations and picks up stations every 16th inch on the dial.