Some of the System 700 modules use Toshiba TA7136 SIP opamps (datasheet), which are long obsolete and semi-sought after by DIY guitar types because it was used to apparently pleasing effect in a Boss distortion pedal. An eBay seller is trying to get the best part of twenty quid for one.
A while back I bought a bunch from Utsource, and when I eventually got round to making a board to test them I found they were fake…
I made up this adapter for an alternative op amp, ‘cos I don’t think using this particular chip is that important (for now, anyway).
Here’s a strangely moody photo of the resulting stripboard.
After this I cut one chunk out of one side of the board to indicate pin 1.
I’ve had this working in the 708 noise/ring mod and the 711 output module, and it works fine, although I’ve not had a chance to compare it directly to the 7136.
The original envelope on the System-100 keyboard wasn’t really one of my favourite things about it. There’s precious little resolution on the decay slider for tuning in those tight bonky basslines, so I tend to end up moving the slider by infinitesimally smaller amounts to try and get the sound I want.
I should point out that it’s a different circuit to the possibly legendary System 100m ADSR envelope (“the snappiest adsr eg of the world…”). For once the hype is deserved, my stripboard 100m 140 clone felt super-snappy, with satisfying control ranges.
Let’s build the System-100 version anyway, maybe we can use different potentiometers or swap the timing capacitor size for something a bit smaller.
The rare-ish programmable unijunction transistor N13T1 was swapped for a 2N6027, although it has since been possible to get hold of the originals on eBay. The 700 LFO worked better with the N13T1, so that might be case here. I used 2SA733 and 2SC945 transistors as per the schematic, albeit the -GR variant rather than -Q, with 1N4148 diodes standing in for the 1S2473s that are splattered throughout.
The original never quite fully opened the VCF so I’ve added a single opamp to optionally boost the level from 6v to 10v.
Here’s a video of the envelope in action – I left the text on there to keep the camera from trying to going apeshit trying to autofocus on the trace. Not being a storage oscilloscope makes it trickier to track slow moving signals but you get the idea.
In this test I’m using a 3.3uF tantalum for the timing capacitor as standard but smaller pots than the original: 500k for the attack, 100k for the decay (which is too small, really), and 500k for the release, all audio taper. At about 0:43 the sustain pot is turned to 100% which causes the voltage to ramp up slightly rather than staying level. I’ve not looked very hard into fixing this – I’ve just been turning it up to just below maximum.
Here’s the layout and the DIY LC file (without the 6v to 10v boost) should you feel in the mood to torture yourself with some stripboarding.
Having been through all that, if you want tight and snappy then the 100m envelope is probably a better bet.
Due to other things (work, house move, more work…) the System 700 envelope PCBs I made have been left untouched in a box since March, I’m looking forward to getting those going and comparing them to the System 100 and 100m… eventually.
For the stripboard perverts, here is a version of the System 700 703a/b/c/d 12db state variable voltage controlled filter, which has high-pass, band-pass, and low-pass modes. It should use matched CA3080s, although I haven’t got round to that yet.
According to analogue-heaven lore, it was the only type of VCF included with the first System 700 models, with the 703e/f/g/h 24db low-pass added later.
I spent a fair while wondering why it was so horribly distorted, before I realised that R33 in the feedback loop should be 1K rather than 10K. Here’s a detail from Yves Usson’s photo of the board:
…yes, I have spent a fair amount of time staring at pixels wondering if they’re red or brown, how exciting for me.
The other problem was the orientation of the stereo resonance pot. Having built it up as suggested by the schematic, the level into the filter gets boosted as you turned the resonance up, which makes sense, but it amplifies the input so much that the resonance struggles to be heard over the original signal. The System 100 VCF has a similar arrangement but boosts in a rather more subtle manner. Here’s the relevant bit of the schematic re-arranged a bit to fit into the page.
Flipping the wires round boosts signal as the resonance is turned down and lights the overdrive LED for a 10v peak-to-peak input; not the correct behaviour according to the manual, which states that the red LED should light for an input of 13v p-p.
For now I’ve just bridged the wires going to the boost side of the resonance pot. It’s possible there’s something wrong in another part of the circuit, given that I’ve laid the board out manually in DIYLC, but I’ve been over it several times and been unable to find anything different.
Before I do a PCB for this I’ll experiment to see if it’s worth adding a resistor across the boost side of the pot to tame the amplification somewhat. I’ve stared at Yves’ photo of the 703 trying to work out if there’s anything extra going on but to no avail.
When listening to the first demos for this on headphones I noticed some familiar fizzy distortion on the lowpass output, which turned out to be caused by the connected LED level indicator circuit, just as with the 704 VCA. I re-recorded the demo with the LED disconnected, but fixing this should just be a matter of adding the 1.5nF capacitor from BL+ (the positive supply to the U8) to ground, marked as C17 in the track layout in the service manual.
There are a couple of versions of the 12db 703, one with LM301s and (according to the schematic) 1.5nF mylar capacitors, and another with CA3140s and 470pF polystyrene caps. The 301s need compensation capacitors, and the 3140s don’t. I started out intending to build the LM301 version, but in the course of debugging the distortion I ended up swapping to the CA3140s (The LM301s should work fine, it wasn’t their fault).
Here’s a quick demo of the System 100 square wave into the 703 VCF – first is the bandpass, second is the lowpass, all sequenced by the MC-4, no VCA – these were done using 1.5nF mylar caps.
And here’s the sound of the different modes using the sawtooth from a different oscillator into the 703 VCF, with cutoff initially modulated by a dubious MFB Dual LFO and increasing amounts of resonance, and then some fiddling about with the CV from the MC-4 and the clone System 100 envelope, all with no VCA. They’re hardly the most musical of demos (all in the key of predictability) but it’s more to get an idea of the character of the filter. Warning: nasty high pitched resonant sounds!
Possibly unpleasant highpass:
And the bandpass:
…not really your classic fruity Roland sound, but then you should expect that from the 12db-iness of it, and to my mind that’s a good thing to have a different flavour to the usual 24db sound. It sounds slower to react to the envelope than the 5-stage (30db?) System 100 VCF.
Here’s the layout out of DIYLC, including the C17 LED noise fix, but with the resonance pot boost as described in the schematic – should you build it you might need to experiment with this. I’ve been over it a few times checking for errors but it’s always possible I’ve missed something, let me know in the comments below if you spot anything.