Diverting from the System 700 obsession for a bit, I thought I’d try my luck at surface mount soldering with the OpenMusicLabs BA662 clone, in this case Altitude909’s version with the DC offset trimmer.
I’d already bought a few already built up from Synthcube, but they’re $12 each, and as I (possibly…) needed a big pile of them I was considering getting the boards made and soldering the parts myself.
The first attempt was going so well until I plugged it in, and nothing happened. Then I realised I’d got one of the transistors the wrong way round.
Next I yanked two of the pads off the board when trying to extract the transistor with my shovel sized soldering iron. Then I used tiny wires to reattach the legs to the connected transistor, which was tortuous, and on plugging it in all the smoke came out anyway.
It went from shit to worse when I found one of the Synthcube 662s, plugged that into my breadboarded SH-2 VCA test circuit and that promptly smoked.
Hmmm. All of a sudden now I’m not sure why I’m writing all this out ‘cos it makes me look like a total idiot – anyway, I needed a test circuit that didn’t actually fry the chip.
I went back to my stripboarded System 100 VCA, pulled the CA3080 out, wrote a pin mapping out…
and soldered up a new 662 (one hour with my fat fingers and vast soldering iron, more or less). Wobbly photo of the tiny fucker:
I plugged it in and it worked, in that the LFO varied the volume of the AS RS-95 oscillator. Whoop.
Not really sure if hand-soldering a shitload (however many that is – 8 x 4 plus a few more, possibly) is really do-able if I want to retain my eyesight. Despite smashing up a $12 “chip” and my £4-ish version, I’m still fairly chuffed to have managed a surface mount board for the first time.
SOT-363 is possible but feels like a step too far for doing it by hand on a regular basis, at least with my tools and skills as they are. I half-wonder if it might be possible to do a version of the 662 with a bigger surface mount package size, just to make it less of a nightmare. Having just done a test with an LM3046 I’d be tempted to use SOIC versions of chips in original boards.
Here’s my silly test setup in the meantime. This doesn’t use the 662 buffer, so that bit might still be broken. But I’m optimistic.
The other alternative might be to use LM13700 OTAs – in their SOIC-14 format they’re 65p in quantity of 10 and above from Farnell. There’s already another 662-pinout IC which uses it, I’m only wondering if it’s as simple as that, is there anything else going on under the IC on the synthnstuff version?