I'm starting to wonder if the 12v DC input on the right side of the DR-680 is really, really, really picky about what voltage it gets.
I've just experienced my first problem while using a 12v SLA, which is discouraging, since it had been working fine since I switch from a DVD battery. I'm using my DR-680 as a bit bucket lately for transferring my cassette masters to WAVs. When my SLA voltage dropped to 11.65v today a gstarted getting a reaally nasty squeal on the recording. I also had 8 NiMH AA batteries in the battery compartment on the bottom. The only way I could get the squealing to stop was to remove the NiMH batteries.
When I tested the NiMH batteries they were all pretty well depleted. So, I turned the unit on again with the NiMH batteries removed and the 12v (also depleted) SLA connected to the power jack on the right side of the recorder. Now, after a few minutes using it this way, the record meters just stopped working and I got no audio recorded. The next thing I did was to leave the batteries out and connect it to the Tascam AC/DC power supply. It started working properly again and I've no problems again.
I suspect that the battery compartment isn't as picky with voltage. I'm going to see what happens if I wire the 12v battery (after recharging) to go in through the bottom battery compartment instead of the regular input on the right side.
Since the manual says the battery compartment can take NiMH batteries (which are 1.2 v ea... = 9.6v DC) I assume it has lower battery voltage requirements than the 12v input on the right side.
I'll report back what I find....
Funny thing is that I just posted today that I would give up on this thing if I had one more problem.
But, I am still compelled to figure out how to make it work, because I still like it so much.