what changes and switch did you use fir tor the envelope length mode switch, if I might ask, love the look and have 2 of my own w/o mods
Hi there,
used an mini on/off switch and put a larger Polyprop-Cap in parallel with the 1uF Env-Cap @ C14
(switch on position enables the 2nd Cap and adds (my end) 2,2uF to the existing 1uF Cap.
(One could even go mad, and use a smaller Cap for really short Env´s… and/or a rotary switch with as many positions one likes to enable different Caps in parallel/switching from one to the next,
for supershort/short/medium/long/ultralong Envelopes)
(ignore the multiturn trimmers on that pic,
i skipped the idea of having the osc-scale/tune accessible from the front Panel.
cause it was kinda hard to make sure that the needed tiny holes for the trimmer will match their position)
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Heya, as far as i understood it…
these 2x 20k Trimmers for OSC 1+2 Scaling.
the 2x 100k Trimmers for Tri-Wave Offset of OSC CrossModulation.
But i guess tuning that thing accurately is pretty hard, since nearly everything modulates everything.
thanks for that update , will try on one of mine , great news
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Heya, I built it following the stripboard layout and it works but the volume is super faint and I don’t know why. I have to turn my speakers all the way up and I can barely hear it. I’m not sure what’s wrong. 
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All I can think of is the BC107 is backwards but I think I wired it correctly? It’s hard to tell by the stripboard layout which way it should go.
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Emitter of the transistor go to output (33)
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Ahh am I blind or is the schematic listed somewhere? I put the 1M resistor in but they’re saying it’s a 10k from -12
have a listen to the audio via the legs on thevbc107. if thats the problem it will be loud on one of the legs. the 1m not sure where they are. ill find the schematic when I get back . follow the signal path with a wire wired into a jack socket going into a crappy hifi amp and see at which point you stop hearing the audio. kablammy thats where the problem is
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I do something similar but I add a capacitor to the wire so that DC offsets will not make any problems (like loud plops from the amplifier while probing the circuit).
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So I’ve poked and prodded around, found one or two things in the wrong spots, but it all seems to boil down to the LM13700 chip not outputting for some reason. I’ve got strong signal on the input then just nothing on the output. My board matches the stripboard layout but unless that’s wrong this should be working. I’ve also tested multiple LM13700 chips, didn’t make a difference.
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it could also be the opamp after the lm13700. the strip board works unfortunately its been built a few times by people. problem areas to look at pin 1 an d 16 on the lm13700 these should give the same signal and they should also receive a different voltage when you twist the cutoff knob, if it doesn’t then there is a problem with the 2n3906’s or something around the voltage inputs for the lm13700.
also check afterwards. as in the opamp after the lm13700 as I don’t think you’ll hear anything directly from the lm13700 as it wont be buffered.
hope that helps. my bet is the problem is around the bc558’s. Its been a problem for me before
twist the cutoff knob and see if the voltage changes on pin 16 and 1 pin of the lm13700
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Are you sure the LM13700 is powered correctly? Try measuring the current into the Iabc pin, which is a sort of control over the scale of the output*. You can measure this by measuring the voltage across each of the 2 10k resistors.
Which input do you see the signal on, and which output are you expecting it on? The outputs are very low, they are boosted up by the TL074 after filtering.
*I don’t exactly know how OTAs work, here I am guessing the theory but the practical implications should work. Too little current = no output. Too much current = £3 down the toilet.
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Going off http://circuits.datasheetdir.com/37/LM13700-pinout.jpg
I have input signal going to pin 3, and if I read it correctly should be getting output on 5. Pin 1 should be the amp bias, connected to the 558’s, maybe its the 558’s acting up. I’m connecting the output to an amplified speaker so I can hear it, which is why I can hear the input. My voltmeter is acting up and not giving me correct readings so I’ll have to get another one.
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OTA output is current, not voltage, so looking directly at it with something that expects a voltage input is likely to cause confusion. You can use your audio probe to check the LM13700 inputs and the outputs of the TL074 it connects to, but it’s probably not useful to try to use it on the LM13700 outputs directly.
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yes it is current control however it does fluctuate and fluctuates correlating to the input. so it is still a way to see if anything is happening
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also if you wanna be completely sure, just get a banana cable and just pop it on the 10k’s the side of the 558’s and connect that to 12v see if anything comes out the speaker
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also @analog output mentions that what if you try and bypass the lm13700??? that’ll show you if its that I guess!
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I measured the voltages on mine(not fartbox, MS-20 filter) just now, and the voltages across those 10k resistors swing exponentially from 0-10.8V when I turn the cutoff knob.
If you are able to check that those voltages are consistent, then the issue is not in the exponential converter. In this case, here’s a badly annotated diagram of where to try probing. Use a >1uF NP capacitor if you have one, to stop any pops or clicks with DC offsets. Here’s a badly annotated diagram showing where to probe, tracing the signal around the circuit.
If the issue is in the exponential converter, then try reflowing and checking all your connections around the 558s.
Wait… In mine, the input is going to pin 4. Pin 3 is grounded via a 220Ω resistor. Right?
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