On Sat, 22 Apr 2017 08:21:01 -0500, Foxs Mercantile
wrote:

On 4/22/2017 1:54 AM, Phil Allison wrote:
The amp uses bridge mode output - right ?
So both speaker terminals are amp outputs.

I'm going to go with this as the correct answer.
The audio output of a Motorola Spectra is like this, and
there are warnings repeatedly through the service manual
NOT to ground either side of the audio.

This has nothing to do with "hot chassis" or floating
grounds.

I just hate to agree with you, but y're right. Nobody builds AC-DC
transformerless radios and audio amps these daze.

It's probably a bridge amp:
https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers
That looks like this:
https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers#/media/File:Bridge_amp.GIF
Notice that neither speaker wire is grounded.

If you want to look at the output of a bridge amp with a scope, you
need to have a scope with an A-B (that's channel A minus channel B)
input. Connect one probe each from each scope channel to each speaker
lead. Connect both scope ground leads to ground.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558