AM Radio Schematics

[geunther]

I am looking for schematics for a 72 stock AM radio if anyone happens to have a set. All I get is a hum out of the speaker and have done all the troubleshooting I can without schematics.

Any help would be appreciated.

[ray_mcavoy]

This site has some schematics for older radios --> http://techpreservation.com/schematics/ Look under both the Chevrolet and Delco listings.

You can also try searching on the SAMS Technical Publishing website at: https://www.samswebsite.com/en/photo...GENERAL_MOTORS

If your radio still has the paper tag / sticker with the model number, that will help in the search. If it's missing, I believe it would likely be a 21TPB1 for a 72 truck AM radio. That one is covered in SAMS AR-110. Copies of these old SAMS Photofacts manuals often show up on ebay and many are available from www.findatube.com

[hamjet]

I had the same problem. it ended up being the speaker. test with another speaker first before ripping apart the radio. Plus they claim you can burn out the final power resister having the radio on for a long period of time with a bad speaker..the speaker should be 10 ohm.

[geunther]

Thanks for the responses. I had already found the speaker to be bad and was testing with an 8 ohm speaker (close enough for testing). The original had pretty much dry rotted and tested infinite ohms.

I found one of my problems to be my power supply, which was inducing the hum. Once I cleaned up the power, the radio is mostly working. There is some static while adjusting volume so Ill attempt to clean the volume/power switch.

I found a vendor for original type speakers which use the original mounting bracket.

[ray_mcavoy]

Originally Posted by geunther

There is some static while adjusting volume so Ill attempt to clean the volume/power switch.

Yes, cleaning the volume control will often take care of the static. I prefer to use Caig DeoxIT F5 ... it's safe for use on plastic and carbon based potentiometers.

Originally Posted by geunther

I found a vendor for original type speakers which use the original mounting bracket.

These guys ? --> http://www.turnswitch.com/speakers.htm

If so, I have used their speakers in a few of my vehicles and have been very satisfied with them. As advertised, they fit perfect in the stock mounting brackets and they sound great.

Originally Posted by hamjet

Plus they claim you can burn out the final power resister having the radio on for a long period of time with a bad speaker..the speaker should be 10 ohm.

Yes, operating the radio for a long time and/or at high volume levels with a bad or incorrect speaker can damage the output transistor and/or the .5Ω fusible resistor. Speaking of which, these older Delcos used a germanium output transistor with a biasing circuit that places about 1.5 Volts DC across the speaker output. So it's normal for them to make a "thump" noise when the radio is tuned on and that DC bias causes the speaker cone to lift away from the magnet.

[geunther]

I ordered today from turnswitch. Pictures looked like good quality. Resistance is a little off but should be no issues.

[hamjet]

Originally Posted by geunther

I ordered today from turnswitch. Pictures looked like good quality. Resistance is a little off but should be no issues.

The one I ordered 3 years ago from them was 10 ohm which is correct, did you get the right one? 4" x 10" Delco Style Special Rectangle - Newest addition to our high quality line of 4 x 10 speakers. This speaker is an exact replacement for original Delco speakers used starting in 1965 Chevrolet and Chevelle and by 1968 almost all GM cars. This speaker is correct for 1967 - 1972 Chevy pickup trucks as well. This speaker is very light weight and will mount with all original Delco/GM mounting brackets. The magnet cover block on the back of the speaker can be configured in either direction to match your original application and is user attached with an EDPM double backed tape ring. Full installation instructions included. Fabric cover on all 4 x 10 speakers.

[geunther]

I ordered the correct one (pictured). Their site states, "Most of our speakers are true 8 ohm impedance and are fully compatible with 10 ohm Delco speakers", so thats why I was thinking it is an 8 ohm. Later in the web page it states 10 ohm specifically for the 10" special rectangle, so I was wrong (cool).

[gmc 2500 4x4]

nice