Radio output volts AC DC

1970FuryConv

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1973 Dodge Monaco factory radio question.
Input voltage is 12.6V direct voltage
Output voltage at left rear speaker is 31V to 0V alternating voltage. There is no direct voltage at speaker or fader.
Does the radio have a transformer internally that converts DC input to AC output?
I am a stereo neophyte so all help in understanding is appreciated.
 

Mike66Chryslers

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All audio is generated by frequencies in a certain range*, so effectively yes. *Human hearing is considered to be in the range 20Hz to 20kHz. The receiver takes the AM or FM radio signal and extracts the audio frequencies from it. Then the amplifier takes those small voltages and boosts them so that they will be strong enough to move the electromagnetic coil at the base of the speaker, so the speaker will generate sound waves in the air that you can hear.

If you're connecting a voltmeter set to AC to the speaker terminals, that won't give you a valid reading. Voltmeters are calibrated to read AC RMS value for 60Hz sinusoid. Audio frequencies will just confuse it.
 

1970FuryConv

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AC voltage was the only difference between left rear speaker which has sound output and right rear which does not. I could not get a reading with direct voltage or amps on 0-10amp scale on either speaker.

What is correct way to test input to speaker?
 

CBODY67

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Are you doing the checking with the amps connected to a speaker, at the speaker terminals, or just checking the speaker wire outputs not connected to anything? I.e., "load" or "no load".

Reason I ask is that some types of amps do not like being powered-up and unhooked from a load. Sometimes, "the smoke escapes".

Just curious . . . why the curiousity on this issue?

Enjoy!
CBODY67
 

Ross Wooldridge

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Yes, a standard passive speaker operates from a signal from the amplifier which creates something one could term "fluctuating voltage and amperage alternating current" - to make the speaker oscillate back and forth the current has to alternate between + and -, and while it's not true single frequency alternating current, a lightbulb hooked up to a speaker wire coming from an audio amplifier will light up. In fact, light bulbs are used as protection fuses in high output music systems that typically run in the area of 1000 watts or so. Whatever the power source to the radio and amplifier (whether it be DC [car]or AC [home audio]), the final output to the speaker is a form of alternating current.

Regardless, from your description of no output on one side and lots of output on the other, your radio sounds like the "smoke has already escaped" and needs a visit to the repair man - being a mono radio, BOTH left and right speaker wires should be producing an identical output that your volt meter will read (whether or not it's confused, the fact that it's reading something says there is an output). One side only indicates a problem somewhere.

Test the radio by hooking up a known good working speaker.

Test the speaker by hooking up a known good radio or amplifier.

Don't turn radios on without a speaker hooked up - as noted above, it can damage them. They need a load (speaker) on them when turned on.
 

The Goose

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Yes, a standard passive speaker operates from a signal from the amplifier which creates something one could term "fluctuating voltage and amperage alternating current" - to make the speaker oscillate back and forth the current has to alternate between + and -, and while it's not true single frequency alternating current, a lightbulb hooked up to a speaker wire coming from an audio amplifier will light up. In fact, light bulbs are used as protection fuses in high output music systems that typically run in the area of 1000 watts or so. Whatever the power source to the radio and amplifier (whether it be DC [car]or AC [home audio]), the final output to the speaker is a form of alternating current.

Regardless, from your description of no output on one side and lots of output on the other, your radio sounds like the "smoke has already escaped" and needs a visit to the repair man - being a mono radio, BOTH left and right speaker wires should be producing an identical output that your volt meter will read (whether or not it's confused, the fact that it's reading something says there is an output). One side only indicates a problem somewhere.

Test the radio by hooking up a known good working speaker.

Test the speaker by hooking up a known good radio or amplifier.

Don't turn radios on without a speaker hooked up - as noted above, it can damage them. They need a load (speaker) on them when turned on.

We always had good luck using a Lionel train power box as a remote power source when testing things we didn’t want roached by a car battery. Now that I typed that I’m wondering if you can even find those anymore...
 

1970FuryConv

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Are you doing the checking with the amps connected to a speaker, at the speaker terminals, or just checking the speaker wire outputs not connected to anything? I.e., "load" or "no load".

Reason I ask is that some types of amps do not like being powered-up and unhooked from a load. Sometimes, "the smoke escapes".

Just curious . . . why the curiousity on this issue?

Enjoy!
CBODY67
Right rear speaker inoperative.
I checked with amps inline, wire disconnected, meter inline between wire and terminal.
 

1970FuryConv

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Regardless, from your description of no output on one side and lots of output on the other, your radio sounds like the "smoke has already escaped" and needs a visit to the repair man - being a mono radio, BOTH left and right speaker wires should be producing an identical output that your volt meter will read (whether or not it's confused, the fact that it's reading something says there is an output). One side only indicates a problem somewhere.
  • Test the radio by hooking up a known good working speaker.
  • Test the speaker by hooking up a known good radio or amplifier.
  • Don't turn radios on without a speaker hooked up - as noted above, it can damage them. They need a load (speaker) on them when turned on.
Hi Ross,
Condition
  • Factory Radio is factory am/fm stereo multiplex (not mono)
  • Left rear speaker: violet wire, operates normally
  • Right Rear speaker, dark green wire, inoperative, no AC Voltage with radio on and input and ground wires connected. Right front speaker which also has green wire does operate. Center front speaker also works. Left front does not operate.
  • Right and center front and left rear speaker both get louder/more static when stereo right/left knob is turned all the way right. Opposite happens when turned left.
  • By diagram below: violet to left rear speaker, dark green to right rear speaker. Dark green to right and left front speakers, violet to center speaker. Can't see front center and left speaker wires on car.
TEST, Continuity Right Rear Speaker Dark Green Wire
· I do not have a known good speaker to substitute. You say I should not test output with loads disconnected, so continuity test for wire to right rear speaker is only other test I can think of.
· Disconnect green input wire at right rear speaker. Jumper from green input wire connector. Clip jumper wire to black cable on meter
· I disconnected speaker 3-terminal connector at rear of radio while lying on driver floor
· Red cable on meter at green wire terminal: 1.0 ohms resistance
· Red cable at purple wire terminal: 43.0 ohms resistance
· Red cable at black wire terminal: 39.0 ohms resistance
· Cletron Crossover box and under dash thumb wheel fader still connected
· Should I condemn the radio and say it has no right channel output, such that it needs rebuild?
· Are there other tests I should perform? If I have to buy a speaker, what should I buy?
· Are the continuities to purple and black wires a concern?

Stereo Radio-Fader wiring diagram.jpg

Stereo rear speakers.jpg
 

Ross Wooldridge

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Thanks for the clarification - I thought your radio was mono - my bad.

Regardless - it sounds like it needs to go to a repair shop to diagnose the non-op side.

To rule out the speaker being bad, and since you have no additional speaker to test with, try using the speakers in the car that you know are working - try hooking the speaker that you get no sound from to the side of the radio that IS making sound,

Disconnect the left speaker, and run some jumpers from the left speaker's wires coming out of the radio to the right speaker's wires going to the speaker. If the right speaker is actually good, it should make sound using the left side of the amp - if not, then perhaps that's your issue. Reverse to test the right side of the radio - run jumpers from the right side of the amp to the left rear speaker which you know is good. If the radio is good, the speaker will make sound.
 

1970FuryConv

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2021-10-23 Rear Speaker Jumper Wire L to R

· Jumped violet wire at left rear speaker to terminal for green input wire at right rear speaker.

· Right rear speaker worked perfectly

· @Ross Wooldridge Excellent solution, thanks!
 

Ross Wooldridge

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Switch the connections after the switch, keeping the radio's inputs into the switch the way they should be, and see if the switches function turns to the other speaker - if so, it's NOT the radio, it's the switch.

Also, you can jump directly out of the radio to each speaker to see if you've got output from the radio.
 

1970FuryConv

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Switch the connections after the switch, keeping the radio's inputs into the switch the way they should be, and see if the switches function turns to the other speaker - if so, it's NOT the radio, it's the switch.

Also, you can jump directly out of the radio to each speaker to see if you've got output from the radio.
Is this true, even if the fader looks excellent and works properly with left rear speaker?
 

Ross Wooldridge

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Looks aren't everything... it's 50 years old.

I believe those switches use carbon-based rheostat type guts, and by nature they wear out. It's the same as vintage phono cartridges - they have to be "recarboned".

Sometimes they just fail...
 

1970FuryConv

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It may be a couple days. I've got a leaking 727 shift seal that's going to command my attention. National 8609 in hand.
 
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