You can use the headphone jack and your soundcard to check the volume control linearity (and a bunch of other stuff) on the 500c. Much more accurate than the voltmeter.
Get the RMAA free software:
http://audio.rightmark.org/download.shtml
Install the software and set it up for your soundcard using a loopback from the line out to line in. This will usually be a 1/8" to 1/8" stereo cable.
After you are sure you have it set up correctly hook up your Fisher. Be forewarned the original Fisher power cable can be flipped and zap your soundcard. This is due to the computers power supply ground reference not liking the hot chassis ground when plugged in backwards. If your soundcard does not work unplug it from the Fisher, flip the Fisher power plug and give a minute or two to settle and try it again.
Use the headphone jack for the output. I use a 1/4" to 1/8" adapter and the cable used above for the input. For the soundcard output I use a 1/8" to RCA cable and input to the AUX input. Turn the volume down on the Fisher, turn it on and let it warm up.
In this case we are only interested in the test signal screen. You would have already seen it in the loopback setup procedure. Start cranking up the volume control. I usually turn off the speakers since the test tone is so obnoxious. The headphone output has load resistors so this is permissible.
The test signal screen has both numeric and graphic indicators for each channel. I usually watch the db numeric indicator as I turn the volume up. Be prepared for a surprise. Most of vintage units with this switch arrangement are out 6db or more at low volume.
