I've had a TR707 for about 18 years now. For over a decade of that time, it was in storage when I just wasn't doing any music.
A few years ago, I dusted it off and started messing around with it again. Same great interface, same 'ok' sounds, kinda
ho-hum but good for goofing off and working out ideas while sitting on the couch. Except for that damn wall-wart. I find
that when I want to be musical, if there's any setup or booting or too much cabling to be connected, I just won't make the
effort and I'll go do something else. So the 707's got great immediacy in the interface but I seem to always have to go find
the right wall wart, plug it in behind the couch, pull the cord around the...ahh forget it.
Basically, this mod adds battery power to the 707 by installing a battery holder to the internal space, and having an external
switch that allows you to route either the batteries or a wall wart to the circuits.
So, it appeared that AAA's weren't gonna cut it, but AA's would. If I used 8 alkaline AA's, I'd get my 10 hours.
But I thought rechargeability might be a better long-term option, both for overall costs and for the environment. One wrinkle was that rechargeable NiMH AA's don't run at ~1.3-1.5V, more like 1.2V from the get-go, so I'd have to use > 8 of them to maintain comfortable power supply headroom during the discharge. Luckily, Eagle makes a one-piece 10 AA holder sold by Mouser for a few bucks.
Most of the pictures are pretty self-explanatory. On the thriftstore 707, the previous owner/donor had hard-wired the wall-wart
to the PCB and made a mess of the soldering. So I cleaned up a lot of it and found there was no line filter inductor; no
big deal, since I was gonna use clean battery dc anyway. As far as the wiring, it's straightforward: wire the + of the power
switch to the center terminal of an SPDT slide switch. To one side of the SPDT, wire the + of the battery pack; to the other
side, wire the + shield of the power jack. All terminals get wired to ground of the PCB.
I found, as usual, that the hardest part was chopping out a square hole to fit the slide switch. Why the slide switch? Low profile and pretty much in-keeping with the design of the 707.
To make sure the batteries don't pop out from rougher handling and mobility, another simple solution: a slightly oversized piece of foam rubber that compressed over the top of the batteries up against the PCB when the thing was screwed shut.
Much better! Again the 707 sits well on the lap and it's nice to sit on the couch with headphones and no tethering cables. Too bad it'll never happen with the 808...
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