ua726 replacer quick test

My attempt last year to build the Doepfer A-110 heater circuit on a small daughterboard for my 702 VCOs went up in actual smoke. It took me a while to realise I’d used a bad CAN-10 footprint for the ua726 connection, so now we have this

ua726 replacer board

Is it gonna work? Is it gonna fill my room with smoke?

ua726 replacer board on 702VCO

…and nothing is on fire! it works!

It gets vaguely hot – putting my finger on it, it seems warm – and pointing my (newly acquired, slightly shit) laser thermometer at confirms this.

It was a drag to get it in the dodgy socket I’d fashioned out of bits of a standard DIP one. Next time I’ve resolved to build some sort of temporary jig (inevitably out of blu-tack) to make it easier. Getting the legs the same length would make it rather less difficult.

While breadboarding the circuit I tested both surface mount and through hole versions of the 3046, and I’ve gone for surface-mount – partly due because it’s being readily available from the usual places, but also because the SOIC version seemed to get up to temperature more quickly. I suspect this was because the through-hole version is bigger and takes longer to heat up anyway, but also perhaps because those massive legs conduct the heat away into the breadboard, though I can’t prove that.

ua726 replacer board on 702VCO

The components are a bit of a tight fit on the board (which measures 46 x 20mm), but I had to get it small enough to avoid covering over the existing trimpots. I like Dr Korg’s attempt at this but that’s beyond me at the moment.

Bunging my makeshift frequency counter on the output showed that it took longer than I expected to get up to a stable temperature – after about four minutes, it was still slowly dropping. When I returned after wrestling my son about ten minutes later, it had stabilised – a watched pot, and so on.

I’ll write up some numbers another time, and compare it to the original 726s inside my System 100. I’m interested to see if insulating the 3046 might help.

If you’re going to be replacing a ua726 in a circuit (I’ve got an original SH-2 oscillator board which needs a couple) you might need to change a resistor on the feedback of the CV input opamp to give the scaling trim pot enough leeway to tune the oscillator. I had to alter R9 on the 702 from 50k to 68k.

Here’s the schematic – should be familiar enough from the Doepfer version.

ua726replacer schematic

If you want to build it, I can’t guarantee it’s going to fit on your board, whatever it is, but I’ve shared it on oshpark, where it’s $7.20 for three. There’s a BOM and further instructions there too.

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9 comments

  1. 21st December 2016Bob Missingham says:

    Thank you for making this available. Will now have to pull my finger out and build the 700 vco, and the Formant vcos that I have been musing over for the past 3 or 4 decades! A long pop etching session is foreseen. Any chance of sharing further files? (He asked hopefully)
    Cheers,
    Bob.

  2. 12th February 2017kev says:

    has this been tested and working?

  3. 12th February 2017ua726 says:

    It works, but I don’t have any numbers to compare it to an original ua726 at the moment. I did start doing it but it took a long time and was interrupted.

    To my ears my (original) System 100 sounds stable, but I found that it quickly rose to a pitch then drifted slowly downwards, presumably as the heat builds up inside the box, measuring 1Hz down over thirty minutes.

    Just to reiterate the point in the post that it might not be a drop-in replacement – you may need to adjust supporting resistors in the CV amp to get the 1V/oct range you need.

    Someone at Muffwiggler is working on a THAT300-based version which might be closer to the original but a lot more expensive than the 3046 used here.

  4. 18th May 2017Karl says:

    Howdy,

    Where does this connect to ground? I’m going to replace a busted 726 on a Teisco SX-400 and pin 6 of the 726 goes to ground, but on this PCB, pin 6 is not connected.

    Here’s the schematic for the circuit I’m replacing the 726 in: https://www.gearslutz.com/board/attachments/geekslutz-forum/657149d1493633483-synth-vco-troubles-what-voltages-should-i-getting-sx400-vco.pdf

    Thanks! And good work!

  5. 18th May 2017ua726 says:

    On the original ua726, pin 6 is the temperature adjust pin, but we’re setting the temperature through the trimmer, so it doesn’t matter about pin 6.

    (ua726 datasheet – http://pdf.datasheetcatalog.com/datasheets/105/303692_DS.pdf)

    There might be a clever way of changing it so the transistor heater temperature can be set from the external resistor (so we could ditch the trimpot) but I leave that as an exercise for the reader 🙂

    It might be worth you setting the trim pot half-way (measure with a multimeter) before you start, that way it shouldn’t be at too extreme a temperature to begin with.

    good luck! (are you sure the 726 is busted?)

  6. 18th May 2017Karl says:

    Hi,

    Aha, I see. So there’s no ground for this daughter board? Floating ground only?

    I’m not completely sure, but by process of elimination it seems that the 726 is at fault. Its 1st transistor doesn’t seem to be producing enough gain to pull down the collector. I’ve measured all the stuff surrounding it in the circuit and unfortunately it’s the only thing acting anomalously. Which is a bummer!

  7. 18th May 2017Karl says:

    Have you ever tried replacing a 726 with an LM394, direct drop in? Seems like it should work.

  8. 18th May 2017ua726 says:

    Haven’t got the schematic in front of me at this second but that’ll tell you if there’s no ground – guess it’s just +V and -V then.

    I’ve tried with a pair of transistors (http://ua726.co.uk/2014/11/25/diy-roland-system-100-vco/) sorta-kinda-matched, legs akimbo across the socket, and that worked OK as a test but it drifted all over the place as the temperature changed. LM394 is the same thing, just matched better.

    Someone made a Eurorack version of the 100m VCO without a heater but people found it drifted, so they retrofitted a heated version similar to this.

    This setup with the heater takes some time to warm-up, ymmv. (“how long?” “it depends”)

  9. 18th May 2017Karl says:

    Ah, I see.

    I like this little daughter board. I’m going to get some and give it a try. Fair dues on making it and making it available to the community.

    As for warm up time. It doesn’t really matter. As long as it’s stable!

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