703e with LM13700s

Someone asked on electro-music about subbing a LM13700 for a couple of CA3080s in a Roland-style filter, so I thought I’d have a go. I levered out the 3080s in my 703e clone and wired up three LM13700s (two for the four poles, one-half for the resonance control) and to no-one’s great surprise, it works.

Here is some quick fiddling with an out-of-tune clone 700 oscillator into the 703e VCF.

It sounds a bit overdriven to me – I was trying to avoid any red lights, but being in a hurry to get something down it’s possible I got something wrong along the way.

Now I think about it, it could be that I didn’t recalibrate the DC offset trimmers from when it was set up with the 3080s.

The wires strewn all over the board probably aren’t helping either, so I’m tempted to build up another 703e and make some LM13700-to-CA3080 adaptors to test this further.

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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 100m. 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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System 100 LFO fixing

System 100 102 module VCF/VCA board

While making a godawful racket with the System 100 on Saturday, I switched to PWM on the 102 VCO and the pulse width stayed unmodulated. The LFO light didn’t light, no matter how much I waggled the slider. Hmph.

Should be easy to sort out a broken LFO, right? I’ve built all the bits of a 102 on stripboard, so surely I should be able to fix it?

Fretting that it wouldn’t be as easy as all that, I peered at the schematic.

102 LFO schematic

None of the outputs were working, so I reckoned the problem was in the tri-core, either the capacitor or the op-amp. Just to check, I measured the output of IC301, and it was pinned high.

At this point feeling strangely optimistic, I enlisted the help of my son, who was bored enough to come and watch/help/encourage/take the piss as appropriate. Is dad gonna be able to fix this thing?

I yanked out the CA1458G in IC301 with a desoldering gun:

Dead CA1458

I bunged a socket in, and replaced it with an MC1458 I had in the bits box. So far, so good.

Replacement MC1458 in place

Switched it on, and…

…it worked straight-away. WHOOOP.

I probably should have measured all the voltages and given it a bit of a once-over before I put it all back together, but my son was keen to start making some horrible noises, so I just had time for a couple of slightly wobbly bonus photos: here’s the VCO board – see all the keyboard specific components left unpopulated:

System 100 102 VCO board

and here’s the ring-mod/S&H board

System 100 102 ring mod board

It’s missing the circuit board annotations that my 101 keyboard has, and the soldering on the back is without solder resist (more like earlier System 700s, or my SH-5). The serial number starts with 46, so according to that Boss serial number lookup chart, it was made in May 1976. My 101 keyboard serial is 60xxxx, which works out to be July 1977.

Here’s my Sys101 VCF/VCA for comparison.

System 100 101 keyboard VCF/VCA board

I’ve not gone over the board to see if there are any differences in terms of components; there’s nothing extra soldered on to the back of the 102 VCF/VCA board, which suggests probably not. I pulled out the circuit board for my 104 sequencer recently, and it has a different circuit board layout to the one in the manual, with a load of bodged extra capacitors soldered to the back.

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