ba662 clown eyesight destruction test

bare BA662 clone board

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…

ba662 ca3080
2 2
3 3
1 5
6 6
9 7
5 4

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:

ba662

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.

ba662 test circuit

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?

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3 comments

  1. 30th December 2024Maik says:

    Hello uA726,

    I would like to ask you a question because you are probably one of the few who know Roland quite well.

    I wanted to build the Roland SH-2 filter according to the original circuit diagram.
    I wanted to use the BA6110 as a replacement for the OTA BA662.
    It’s a bit new territory for me though, I don’t know how I translated the pins from BA662 to BA6110!
    There is a circuit diagram for the translation on the internet for e.g. B. Roland TR-808, but none for the SH-2!

    Maybe you know what to do, I would be very happy to hear back!

    Best regards

    Mike

  2. 31st December 2024ua726 says:

    At this point, if I didn’t want to make my own 662s (which is fairly painful, and error-prone) then I’d just get some Alfa AS662s. They’re probably the closest thing to the original, and Rob Keeble (who is probably the expert on these sorts of things) says that the Alfa chips have tighter tolerances than the other alternatives, which means it’s easier to get a working filter.

    As far as the BA6110, Curetronic used it in their version of the 100M 121 VCF way back – as far as I can remember they didn’t do anything special apart from matching the ICs and translating the pinout. I haven’t checked it myself but this (probably the one you mention) looks alright? https://obsoletetechnology.wordpress.com/repairs/roland-tr-808-repair/

    Also I’d forgotten I’d made an adapter myself out of stripboard for a 100M VCA.

    FWIW, my first 100M clone had one VCF made out of a Curetronic 121 board (with the 6110s) and another with my homemade 662s in, and I struggle to tell the difference between the two.

    Sorry, that’s probably muddied the waters further 🙂

  3. 31st December 2024Maik says:

    Hello,

    Thank you for your feedback!
    The note about the BA622 is very good, I had read about it briefly before but was fixated on the BA6110.
    On AliExpress. com I would have
    I got 10 pieces for €10 including shipping, but I don’t know how to compare them, but according to your information, I did
    Rob wrote to me, hope there is a reply!

    I’m now trying to concentrate more on the AS662 or the latest clone BA622.
    I was able to find some dealers:
    Synthcube, Erica Synths, Thonk etc.
    But I need two that have to be coordinated, that’s the biggest problem!
    Soldering SMD is always a bit of a struggle, but it’s the lesser evil.
    By the way, I’m on Reverb. com found another provider that offers coordinated BA622:

    https://reverb.com/de/item/23089381-roland-ba662-clone

    … I wrote to him, hope there is an answer. Offer officially ended.

    By the way, I’m still excited about your System 100 stripboards: VCO, VCF, VCA and System 700C VCF.
    Always happy to use them 🙂

    Thank you.!!!

    Happy New Year!

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