Okay. I'm at my basement workbench and have just ran some tests.
First, Gadget, I tried your quick check of running a cable from the MPX OUT jack to the left channel MPX IN jack. The sound did indeed have a bit too much treble...but
it was the same amplitude as listening to straight FM mono without using the MPX jacks.
My oscilloscope is old and does not have any useful calibration settings - I can adjust gain, sweep speed, attenuation and "range" but I cannot set actual voltages with the knobs. So anyway...I took a measurement of the MPX output and took a mental note of the image.
Next, I tried the eBay MPX decoder again...and I owe you guys an apology...I misspoke when I said "tape head" inputs. I should have typed "tape
monitor" inputs.
Again you were right, Dave; an input into TAPE HEAD
does result in too loud, distorted sound.
So anyway...
I plugged the output of the eBay MPX decoder into the MPX IN of the 600. Result - sound was definitely lower, bass lacking, flat, lousy.
I then unplugged one channel from MPX IN and connected that to my scope.
The waveform was clearly lower!
Further tests indicated the following:
MPX to AUX IN - sound was low, bass lacking, flat, lousy.
MPX to TAPE MONITOR - sound was low, bass lacking, flat, lousy.
MPX to TAPE HEAD - way too high, distorted, until I turned the input adjustment of the eBay MPX decoder waaay down until I reached a point where it actually sounded pretty good!
I'm now convinced the TA-600 is likely OK, and this MPX decoder is definitely crap.
Sure, it decodes stereo audio. But if its output is below line level, what good is it?
Now I'm going back upstairs and bring my Technics SL-D3 TT down, hook it up, and try it out. Will report back soon...