c3 radio squeal

[Steve T.]

71 stereo radio has started making a high pitch squeal after playing about 15 minutes with the car running.
if radio volume is turned up it goes away for a few minutes and comes back louder
radio was completely rebuilt and bench tested by tech and working well a year ago. Checking the speakers I get +/- 5 ohms instead of 8 so I know the speakers are an issue. if I turn on the radio with out the car running there is no issue and plays fine over extended time. As soon as the car is started the squeal is back and does not go away. If I turn the car off and play the radio it will continue to squeal.
It is not an alternator noise and has no change in noise with engine rpm. I have tried touching a good ground to the radio, both amplifiers and mounting hardware at the console.
I am wondering if the radio circuitry is compromised having had wrong speakers installed
Open for ideas
Steve

[Joe L.]

Re: c3 radio squeal

[QUOTE=Steve Torp (37079);831402]71 stereo radio has started making a high pitch squeal after playing about 15 minutes with the car running.
if radio volume is turned up it goes away for a few minutes and comes back louder
radio was completely rebuilt and bench tested by tech and working well a year ago. Checking the speakers I get +/- 5 ohms instead of 8 so I know the speakers are an issue. if I turn on the radio with out the car running there is no issue and plays fine over extended time. As soon as the car is started the squeal is back and does not go away. If I turn the car off and play the radio it will continue to squeal.
It is not an alternator noise and has no change in noise with engine rpm. I have tried touching a good ground to the radio, both amplifiers and mounting hardware at the console.
I am wondering if the radio circuitry is compromised having had wrong speakers installed
Open for ideas
Steve[/QUOTE


Steve-------


The original speakers were 10 ohm rated, not 8. I know of no 5 ohm rated speakers so the ones you have are likely rated 4 ohm which are quite common. 4 ohm speakers should never be used with Corvette radios designed for 10 ohm speakers. However, if you have the radio problem only when the car is running and not with the engine off, then I highly doubt that some damage has occurred to the radio circuitry. If that had occurred, the problem would occur whether the engine was running or not.

[Joe R.]

Re: c3 radio squeal

Steve,

How did you measure the impedance of your speakers? If you used an Ohmmeter (or Ohms function on a Digital Multimeter Meter), that will give incorrect results. The Ohmmeter only measures DC resistance, not AC impedance. To measure the AC impedance looking into your speakers you would need to disconnect the speaker, inject an AC voltage using a signal generator with a known resistance in series. Then you would calculate it using the voltage at the source and the voltage across one of the resistors. A speaker will always measure lower in DC resistance than AC impedance. I suspect the speaker is fine, it sounds like the radio is at fault. I suppose it's still possible the speaker is not a good match to the radio. Did you replace the speakers or are they original?

Joe

[Thomas H.]

Re: c3 radio squeal

Steve,

How did you measure the impedance of your speakers? If you used an Ohmmeter (or Ohms function on a Digital Multimeter Meter), that will give incorrect results. The Ohmmeter only measures DC resistance, not AC impedance. To measure the AC impedance looking into your speakers you would need to disconnect the speaker, inject an AC voltage using a signal generator with a known resistance in series. Then you would calculate it using the voltage at the source and the voltage across one of the resistors. A speaker will always measure lower in DC resistance than AC impedance. I suspect the speaker is fine, it sounds like the radio is at fault. I suppose it's still possible the speaker is not a good match to the radio. Did you replace the speakers or are they original?

Joe

I agree. Impedance and resistance - while specified in Ohms - are measured differently. When you measure a speaker with an Ohm meter you are only measuring the DC resistance, not the AC impedance.

Since the radio plays fine when the car is off, I'm wondering if the increased input voltage when the car is running is causing a component inside to start breaking down, typically a filter capacitor or one of the output transistors.

When you run the radio without the engine running the input voltage is around 12 volts. When the engine is running it would be higher.

If you remove the radio to bench test it, run it at a higher voltage (13.5 to 14v) and see if you can reproduce the problem.

Tom

[Steve T.]

Re: c3 radio squeal

Gentleman,

Thank you for the valuable insight,

I did measure with a digital Ohm meter which I have now learned does not reflect the correct reading.
I figure with the car running it should have a higher voltage than 12v while charging, add in the wrong ohm speakers and the radio might be at its design limits.

I did speak with a radio tech and correct speakers are most likely in my future as a good choice in the right direction, as far as the squeal the transistors might be running hotter than normal or possibly a capacitor is at fault with higher voltage and wrong ohm speakers as mentioned.

With the radio on and the ignition switch in acc position the voltage should be lower where I am not getting the issue.

Turning the engine off to see if the squeal stops might not result in a squeal stopping if components have gotten hot until the components cool off which I have not waited to see if it stops.

Thank you all for your help.

The radio and amplifiers are out so they can be tested which is not my favorite task as well as getting at the speakers
I will share once the radio is tested as what they see is the issue.

Save the wave
Steve