RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?
Well, I took a quick look at my parts radio, and the signal line to the RF module appears to be driven thru a transistor, so is probably open collector - that would explain my low single level with no module installed.
The ground for the RF module on the 8U is the (-) battery ground plane - all the lines to the module plug, and the trainer jack for that matter, go through inductors to keep the RF out of the lines. The case on my radio is mostly plastic, so no grounding there.
Does the 127DP have a connection for the DSC cable? That would at least let you know if it would accept the 512 PCM coding.