Let me preface this thread by saying that Im not a radio guy. I have no formal training in it. I know enough to be dangerous!
With that being said, I'll try and show you a few tip and tricks Ive learned and pass along some information Ive gleaned in fiddling with these.
I want to touch on bench testing. Its not that difficult to set up a bench testing station. I have a Samlex adjustable DC power supply I use. Its adjustable from 0-30 volts. For bench testing radios, I set it to battery voltage- 12.75. You can use any transformer that puts out 12 volts DC. Test it first though, dont assume! You'll need a radio harness pigtail cut from a parts car or a set of test leads. A cheapo 9.99 aftermarket radio antenna. And lastly, an 8-10 ohm speaker. I have an old home stereo speaker that just happens to be 8 ohm. make sure you verify the ohm rating as many modern speakers are 4 ohm which cant be used.
If the radio crackles when you turn it on, the tuner is dirty. You'll need to get a can of tuner cleaner. Radio Shack still sells it. You want to spray the cleaner in between the sleeve and the shaft. Make sure you put a rag or a paper towel over the straw as it comes out with a bit of force and may spray back in your eyes. So be carefull! Work the volume control back and forth a little. You make need to give it a few applications before it clears up.
Next you'll want to adjust the antenna trim. Tune the radio to a station near 1400 khz that can barely be heard with the volume on full. Adjust the trimmer screw back and forth till you find the spot that gives you the maximum volume. The antenna trim gets adjusted on AM only.
Another adjustment you can make is a bias adjustment. The bias should be adjusted to 1.5 volts. Get a digital volt ohm meter, place the positive lead on the FM output transistor and the black to case ground. You can also check this voltage at the speaker wires. The FM output germanium transistor is on the black heatsink and is usually marked "Delco DS-501"
If its out of adjustment, adjust at the control pot on the bottom of the radio. You dont even have to take the bottom panel off the radio. There is a hole for a small screwdriver. Adjust to as close to 1.5 as you can. This one's a little out of focus because I needed three hands.
There are more in depth adjustments to a factory radio. The Delco service manual outlines the alignment procedure to adjust all the trimmers. But for that you need a signal generator like a Sencore SG-165. In a nutshell you inject a specific radio signal through the antenna socket, attach an analog multimeter like a Simpson 260 to a specific area of the curcuit board and adjust the specific trimmer for peak. I'll be teaching myself how to do that in the near future. This pic is what the radio looks like from underneath. The tall square things are the trimmers.
If anyone has any other knowledge that they can share, please add to this. I'd like this thread to become a reference for factory radios