Factory Radio Trouble Shooting

[90BlackMJ]

Hey guys, I’m in the process of putting my Comanche back together from restoration and upon testing the radio, I get nothing. 
 

The radio is a unit I bought from Jeremy (Minuit) so I think it should be good. All 4 speakers are brand new from crutchfield. 
 

I get power and can scan through channels, but no sound. 
 

Any help is appreciated! Would like to troubleshoot this before I get too far back together with the interior. 
 

thanks. 

[MagicJames]

Pull the nob off the volume button and crank it to max.

 

Do you hear any hiss at all or complete blank?

Do you have the corrct +/- pairings, so two greens or two purples for example example?

 

[90BlackMJ]

8 hours ago, MagicJames said:

Pull the nob off the volume button and crank it to max.

 

Do you hear any hiss at all or complete blank?

Do you have the corrct +/- pairings, so two greens or two purples for example example?

 

I will try the knob thing and see what happens. 
 

also the polarity should be correct as I used adapter harnesses at each speaker. 
 

I will double check and send info on what I come up with. 
 

thanks!

[90BlackMJ]

I took a video of what’s happening, which is basically nothing. Too big. 
 

I can turn radio on. Scroll through all the presets, turn volume up and down, but I get zero sound or zero static. 

[90BlackMJ]

[MagicJames]

You said your speakers are new and that it's all wired correctly. So if you have completely no sound, not even static, then its pointing towards the power amplifier. But you said you got the radio back from Jeremy so it should be good. I put a photo of the amplifer below circled in green, its behind the heatsink cover at the back. So if you want you can test it with a multimeter 

But first, I hate to go condemning the amplifier straight away, so check with one individual speaker buy placing a AA battery between negative and positive ends, it should crackle loudly when you connect it to a power source. Then make sure volume is all the way up, even try a casstette if you can. 

If it's still dead silence, then pull the heatsink at the back and test the amplifier chip, its a few small screws but they're different lengths so make sure you remember where each goes.

 

If the chip doesn't look burnt, then put a multimeter negative probe on the ground and the positive probe on the output pin. Our radios take a 17 pin motorolla chip, we don't want this to show the full 12V, it should be about half so around 6V is good. I tried to take a picture for you but its hard holding the probes and the phone at the same time. If you get 12V at the chip speaker pins then the chip is closed and shorting the full battery power straight to your speakers. 0V means the chip is blown open. In both cases the chip is bad.

 

I don't have the exact pin lay out for the chip on hand, its in my notes and i can get it for you.  But I know pins 9 and 14 should be 12v main power and speaker pins should be about half that at 6v. I think pins 11/12 and 15/16 are the 4 speaker pins but i could be wrong.

 

 

i hope this helps 




 

[90BlackMJ]

Thanks for info, I will test and return with results. 

[90BlackMJ]

This baby looks brand new in here.

 

checking on the pin u have pictured to ground I have 5.48v and checking on the speaker pins on that chip I have the same. 

 

I'm not sure what the speaker outputs are supposed to read where the body harness plugs in, BUT checking between FEED and RETURN between each speaker I get ZERO. 

[MagicJames]

Sorry for the late reply.

When you test the terminals at the speakers, the difference across them when sound is playing will be zero. When music is playing, the two speaker wires move in opposite directions, so as the feed rises, the return falls. To test the speaker, put the negative probe on the radio ground itself and you should get 4/5V at each speaker from the positive probe. 

 

As for the amplifier chip, i can't seem to find my diagram, but i went and tested a good radio I had for you to compare with. Below are the results at each pin. The circuit board itself is labelled for pin 1 and pin 17 so you know what direction to go in. 

 

[90BlackMJ]

Thanks for the info! 
 

Do you happen to sell refurbished radios like Jermey did? I may be interested. Thanks 

[MagicJames]

I don't sorry, I'm just hoarder 

[ghetdjc320]

Out of curiosity, does this radio have an aux input mod? Take a quick picture of your preamp circuit board. It’s the one with the volume and tone controls potentiometers. 
 

If you’re amp is testing ok (which it should) check for a 3 pin jacketed cable (typically grey) that runs from the preamp board to the main circuit board to ward the rear of the radio. Make sure it is plugged in well onto the main board. This line carries the line level audio signals from the preamp stage. 
 

The aux input mod uses a pair of factory test points to inject audio into the signal pathways. A simple short to ground here will knock out the audio entirely for the am/fm tuner. 

[90BlackMJ]

It does not have that mod done to it. 
 

I was just wondering myself, but what happened to Jeremy?