Background

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.

Well, I decided to fix that. The 707 is feather light yet sits on the lap nicely. If you open it up, it's all digital and there's plenty of room (for bending and mods, but that's another day). So, why isn't it battery driven? To make it more professional? It's no 909, with that steel case and the built-in transformer, nor the wonderful battleship that the 808 is. It may have been that battery technology was pretty lame in the 80's (NiCads...ugh) and this digital drum machine had just a bit too much mA draw to make batteries a feasible thing.

But it's 2007, the age of portables, Li-ion and NiMH in many, many consumer items, so perhaps the 707 can join this new world. I recently picked up a dirty-but-fully-functional-with-all-slider-caps-and-knobs 707 at a Goodwill ($70, which I felt was a savvy and high price for a thrift store, but it's still cheaper than ebay).

The Mods

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 first I had to decide on the batteries. This depended upon the mA draw of the machine while it was running. I plugged in the wall wart and routed the current through my DVM and, with the thing driving headphones at full blast, on a pretty complex pattern at 265 bpm, the draw was about 180mA.

Next requirement was determining the minimum necessary supply voltage. Instead of the wall wart, I hooked up a variable bench dc power supply to the power jack and, with the 707 running that same complex pattern, I reduced the voltage until the 707 stopped working. I found that the 707 would run pretty reliably down to 7.4 V.

Ok, next requirement was my own: I wanted the thing to able run for about 10 hours between battery replacements. Any less than that, and I'd be opening it up too often.

Another requirement: the batteries had to fit fully inside the 707. No external battery packs because that's just an anchor, worse than a wall wart.

And the final requirement was that the batteries had to be easily available

So, faced with the above, I decided upon multiples of either AAA or AA cells. These are available everywhere and can be found reasonably priced in bulk. I thought about Li ion camera battery cells and the like, but these can be harder to find the exact model and can be quite expensive when you do.

Note: one elegant solution I thought about was to try to repurpose the 707's own 2x AA battery-holder for the supply batteries and replace the memory battery supply with a C23032 coin cell. The main issue was this was trying to find the batteries that would fit that 2x AA form factor: half-size AA Li ion cells that could be 4-stacked (28XL) into the space didn't have the mAh rating; full size AA Li ion cells were only 3.6V, so 2 stacked wouldn't provide enough supply voltage. And ALL of those batteries are not common and usually more expensive.

Next came the experiments: I connected some battery holders together to stack up the voltages, then soldered the terminals to a plug that would fit the power jack on the 707. I ran 3 experiments: three different stacks of AAA and AA cells, powering the 707 running continuously with a very complex pattern running at 265 bpm and driving headphones at max output. I connected the battery supplies up, set the beat going, and then for the next several hours, measured the battery voltage periodically and checked that the 707 was running as programmed. The battery voltage as they discharged for each experiment is shown below.

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.

Installation

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.

Final Thoughts

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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