I don't know if this problem is common across all Nakamichi DR series cassette decks or just in the DR-2 model but here is what I found in one and how I fixed it.
When measuring various aspects of the deck I noticed this harmonic noise comb in the source monitor loop. Not evident in the tape monitor loop. Either because absent, line muted, or tape noise covered.
After examining it I understood the ground-loop's root issue and decided a least intrusive solution. To simply insulate the RCA quad's grounded face plate from the chassis.
I punched a white Post-it paper with a BIC ballpoint using the back of the chassis as template for the RCAs to feed through. It worked... Here are the FFT results.
I believe this fix will prove useful to Nakamichi DR owners. It increases the bass to mid resolution (less grunge noise) in the source loop during the recording process.
When measuring various aspects of the deck I noticed this harmonic noise comb in the source monitor loop. Not evident in the tape monitor loop. Either because absent, line muted, or tape noise covered.
After examining it I understood the ground-loop's root issue and decided a least intrusive solution. To simply insulate the RCA quad's grounded face plate from the chassis.
I punched a white Post-it paper with a BIC ballpoint using the back of the chassis as template for the RCAs to feed through. It worked... Here are the FFT results.
I believe this fix will prove useful to Nakamichi DR owners. It increases the bass to mid resolution (less grunge noise) in the source loop during the recording process.
Attachments
Good detective work! Mouser sell nylon washers for insulating common DIY chassis mount RCA sockets (like the picture) from chassis; you need one each of these part numbers for each RCA: 534-7687 for the shoulder washer, and 534-3182 for the backing washer.
Nice, but that Nak's in/out RCA quad has full face conductive plate. Covers the plastic right angle mounting base's front. It's one and the same thing electrically with the barrels.