Bench testing my AM radio - Team Camaro Tech
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post #1 of 20 (permalink) Old Dec 2nd, 06, 10:41 AM Thread Starter
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Bench testing my AM radio

I have an extra auto battery, and my AM radio out of the car.
My radio seems to have 3 prongs on the back of it. I would imagine one is power, one is ground and one is for the speaker? Any idea what goes where? I don't want to fry it (if it even works) How does the speaker ground itself?

Jason
2009 CTS-V - 1968 Camaro 327/M20 Sequoia Green
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post #2 of 20 (permalink) Old Dec 2nd, 06, 10:54 AM
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Re: Bench testing my AM radio

I can't help with the wiring but definately know which wire is what before connecting it up. I would also add an inline fuse to the power wire while testing. The size, maybe a 10A.

As far as a speaker grounding itself some speakers relied on the mounting of the speaker to take care of the negative terminal on the speaker. If the speaker had one wire connected to it you need to run a jumper wire from the frame of the speaker to the chassis of the radio to complete the circuit.

Chances are a terminal on your radio was "Y"'d off to where one wire went to ground and the other to the negative on the speaker.

To bench check the radio you will need some type of antenna. Without it you may hear some background hiss when the volume control is turned way up but since AM relies 100% on an antenna you need something to check for a signal. The test antenna doesn't have to be pretty but take a chunk of 16-18 gauge insulated stranded wire (like a speaker wire) and remove about 1/2" of insulation from the wire end. Carefully insert it into the antenna hole in the radio chassis to where the bare wire hits the center pin connection in the hole. Have the chunk of wire maybe 35-40" long and positioned vertically if possible.

Once you get it back into the car there is probably an antenna trimmer screw either on the chassis of the radio or behind the tuning knob. This is to match up the antenna to the radio. You tune to a weak station up high on the dial and then turn the screw one way or the other until you get the loudest output from that weak station.

Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #3 of 20 (permalink) Old Dec 2nd, 06, 03:36 PM Thread Starter
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Re: Bench testing my AM radio

well, it works. I hooked it up. BTW, for anyone wondering a 68 factory am radio has 3 wires on the back. If you are looking at the back, from left to right, the first wire is the power, the second two are the speaker wires, the radio grounds itself against the dash.

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post #4 of 20 (permalink) Old Dec 2nd, 06, 05:10 PM
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Re: Bench testing my AM radio

Quote:
Originally Posted by blue89bird View Post
well, it works. I hooked it up. BTW, for anyone wondering a 68 factory am radio has 3 wires on the back. If you are looking at the back, from left to right, the first wire is the power, the second two are the speaker wires, the radio grounds itself against the dash.
The terminal connections are stamped into the radio at the plug hole
12V SPK GD


Also, stamped into the top of the radio is " Do Not Operate Without 8 to 10 OHM Speaker"

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post #5 of 20 (permalink) Old Dec 2nd, 06, 08:17 PM
 
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Re: Bench testing my AM radio

When I turn my radio on it crackles for a second. Any one know where to get some type of spray to clean the volumne connection. Thanks
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post #6 of 20 (permalink) Old Dec 2nd, 06, 08:42 PM
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Re: Bench testing my AM radio

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Originally Posted by Flynn69 View Post
When I turn my radio on it crackles for a second. Any one know where to get some type of spray to clean the volumne connection. Thanks
You can see if Radio Shack has contact cleaner. If they do, unhook the radio from power so it is dead and then spray the cleaner into the potentiometer and work it fully clockwise then fully counter clockwise a bunch of times. Let it sit to allow the liquid to evaporate then power it back up.
I can't think of the name of the stuff we use at work but to help slow down the oxidation this stuff is then applied. It's a red liquid in a very small container with a small steel tube to dispense it out. I did a search and I think it's called Cramolin ?.

Jim

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post #7 of 20 (permalink) Old Dec 3rd, 06, 12:57 PM
 
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Re: Bench testing my AM radio

Jim, Nice info, but I,m lost. Where is the potentiometer located,
Thanks Errol
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post #8 of 20 (permalink) Old Dec 3rd, 06, 03:18 PM
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Re: Bench testing my AM radio

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Originally Posted by Flynn69 View Post
Jim, Nice info, but I,m lost. Where is the potentiometer located,
Thanks Errol
The volume control shaft passes into the inside of the radio and is connected to and is part of a larger assembly that this potentiometer is used to vary the volume of the radio. It also contains an on/off switch. Going off of memory the volume control shaft should be maybe 3/8" diameter and the potentiometer inside the radio will be maybe 1" in diameter and maybe 1/2" or so thick.

Think of the volume control (potentiometer) as similiar to a combination headlight/instrument panel light dimmer control. It pretty much works the same way. To vary volume or lighting brightness inside of a potentiometer there is a wiper arm that slides, glides, contacts different points on a section or two of wire in different points depending on the arms position. Think of this section of wire that the arm contacts as like a slinky toy in a circle shape. When the volume is low then the wiper arm connects towards the end of this long piece of wire to keep the volume low and when it's at maximum volume the arm is connecting to a shorter section of this long wire to give maximum volume. Anywhere in between and the volume will be somewhere between loud and soft. Since the connections points are between the arm and the wrapping of wire is variable the material used for the connections corrode. When this happens you get scratchy output or in the case of a headlight dimmer switch a spot that might be dead or should be half bright but it is only a quarter bright. Another similiar potentiometer is the fuel sending unit.

Here's a picture of a potentiometer from the backside:

http://www2.eng.cam.ac.uk/~dmh/ptialcd/trf/VR.jpg

Another but from the side:

http://www.satcure-focus.com/hobby/images/volume.jpg

I have some times dribbled the cleaning solution down the shaft to allow it to get inside the housing and I've also seen the service tech at work drill a very small hole in the potentiometers case to get the solution in but I worry about metal shavings doing it that way.

Hope this helps.

Jim

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post #9 of 20 (permalink) Old Dec 3rd, 06, 05:57 PM
 
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Re: Bench testing my AM radio

Jim, I want to say thanks for this very valuable info. I will make a copy of this and post it in my shop for other radios I have. I have had other nice AM/FM radios go cheap because I thought they were just worn out.And most of them had this problem. Thanks Again and Happy Holidays. Errol
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post #10 of 20 (permalink) Old Dec 3rd, 06, 06:16 PM
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Re: Bench testing my AM radio

Where does one pick up a 8 - 10 ohm center dash speaker?

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post #11 of 20 (permalink) Old Dec 3rd, 06, 09:10 PM
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Re: Bench testing my AM radio

Quote:
Originally Posted by Flynn69 View Post
Jim, I want to say thanks for this very valuable info. I will make a copy of this and post it in my shop for other radios I have. I have had other nice AM/FM radios go cheap because I thought they were just worn out.And most of them had this problem. Thanks Again and Happy Holidays. Errol
Classic-Camaro
N.Y.
Errol, Your welcome. The contact cleaner is not a cure-all but it's worth a shot. Like anything else parts do wear out. I've never looked at a volume control up real close but I would think the contact arm is actually hitting two windings of the wire in that if it didn't you could get dead spots while turning the volume control very slowly. I would think they would be sorta like a make, before break setup.
Since we've been talking about potentiometers and other places they are used in a vehicle I had to troubleshoot a customer's fuel sending unit on his car and on it the customer bent the contact arm some to give it more tension against the windings. It worked however I told him he might have put too much pressure on it to where it may wear out prematurely.
Again, glad I could help and Happy Holidays to you also.

Jim

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post #12 of 20 (permalink) Old Dec 3rd, 06, 09:17 PM
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Re: Bench testing my AM radio

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Originally Posted by X33D80 View Post
Where does one pick up a 8 - 10 ohm center dash speaker?
Someone else may help with specifics but I know there are new ones out there. A person could take two 4 ohm speakers and wire them in series and the resulting resistance would be 8 ohms. To do this you take the positive speaker output wire off of the radio and wire that to the first 4 ohm speakers positive terminal. This leaves a negative terminal which then has a wire connected to it to go to the second speakers 4 ohm positive terminal. You are now left with a negative off of this second speaker and this then goes back to the radio's negative speaker output wire. You add the 4 ohms to the 4 ohms and get 8 ohms total.

While a person could use a high enough wattage resistor added in series to a lower resistance speaker the resistor itself will eat up power limiting how much the actual speaker gets.

Jim

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post #13 of 20 (permalink) Old Dec 4th, 06, 03:32 AM
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Re: Bench testing my AM radio

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Originally Posted by X33D80 View Post
Where does one pick up a 8 - 10 ohm center dash speaker?
SS396.com Ground Up Catalog page 120 has correct fit center dash speakers as does Ebay. You should find them easily at a local radio shop too. More info on Ohms etc...

Kev


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post #14 of 20 (permalink) Old Dec 8th, 06, 07:11 PM
 
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Re: Bench testing my AM radio

Want a replacement speaker for your camaro or Chevelle. 8-10 ohms . Try
http://www.turnswitch.com .I shopped around and I,m fussy. i Tried these guys and they are good. You tell them what car you have,and they will tell you what you need. I bought a 4x10 Delco special 10 ohm for my 67 Chevelle SS. PERFECT AND REASONABLE. Sent to me in 3 days. I put them at the yop of my list for speakers. Big plus is ,they have all the delco books from 1949 on up....................No Problem

Last edited by Flynn69; Dec 9th, 06 at 10:41 AM.
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post #15 of 20 (permalink) Old Dec 9th, 06, 04:16 AM
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Re: Bench testing my AM radio

Flynn69,

Your link is wrong and doesn't work. It should be..

http://www.turnswitch.com


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