Layout for a micro-synth?

[Mark Hammer]

Likely whatever made the bridge rectifier fail.  BTW, I now have the octave-up working as well.  All I need to resurrect is the distortion voice, and we're good to go.  I did add the trigger indicator LED that Craig Anderton suggested in the 1979 DEVICE article, and also lessened some of the lowpass filtering on the sub-octave voice.

Your diagram has been of inestimable help.  I made myself a little probe to use with my little battery-operate Danelectro amp, and tapped the audio signal at various key points.  That helped to identify the problematic chips.  I hope the remaining missing voice does not require too many 3094s to rectify.

[Mark Hammer]

Done.  Required replacement of 5 chips, primarily dual op-amps.

Everything works well.  The only thing I'm unsure about is that the distortion voice is a LOT louder than the other 3.  I have no idea if this is normal or not.  That said, the individual sliders allow for it to be dialed down, as needed.

The key was the diagram Stephen supplied.  Thanks a million, pal.

[StephenGiles]

Done.  Required replacement of 5 chips, primarily dual op-amps.

Everything works well.  The only thing I'm unsure about is that the distortion voice is a LOT louder than the other 3.  I have no idea if this is normal or not.  That said, the individual sliders allow for it to be dialed down, as needed.

The key was the diagram Stephen supplied.  Thanks a million, pal.

My pleasure Mark, glad it's all working. Any thoughts on how to bend existing circuitry to make the trigger sweep respond to how hard a string is played?

[Mark Hammer]

It would depend on the issue.  Triggering seems to change over issues.  I did try installing the trigger indicator LED that Craig Anderton suggests in the DEVICE modification article, and it can be useful.  When the trigger control is set too high, the LED stays on.  One then rolls the trigger slider back until the light only comes on when you hit a note.  Personally, I didn't seem to encounter any difficulties getting reliable triggering.  I also installed the attack-delay sensitivity trimmer, and superglued a small piece of aluminum sheet-metal to the top of the 5554 regulator to sink off a little heat.  The sensitivity trimmer is currently up full, but I'll let the owner know that the trimmer can be dialed back a bit if the attack delay starts to sweep unpredictably or too often.

It also turned out that the stompswitch was shot, so I replaced it with a SPDT.  I could have installed a 3PDT for true bypass with an indicator LED, but quite frankly I doubt my drill bits would have been able to penetrate the folded-steel chassis.  Besides, the buffered output seems to function just fine.  I adjusted the gain trimmer to provide roughly equivalent effect/bypass levels, and was satisfied.  The owner will use it in his studio, so I doubt either of us are particularly concerned that the state of the pedal be discernible across the stage.

I have to say that I cannot recommend making an audio probe highly enough. I simple connected a wire with an alligator clip to the shielding braid on a guitar cable with the end cut off, and plugged the other end of the cord into a little battery-operated amplifier.  The hot lead in the cable is connected to a non-polarized cap.  In my case, I used a 220nf plastic cap - a big "greenie" that was awkward to used on boards in cramped quarters.  The free end of the cap is used as the probe.  The cap is necessary to block any DC that the signal is riding on in the circuit under test.  The alligator clip can be secured to wherever provides a nice reliable grounding point that doesn't physically interfere with probing points on the board.

Using a probe like this is equivalent to "plugging in" to the circuit at whatever point you want.  If the signal is transformed in some way at that point (amplified, distorted, or otherwise tone-shaped), you should be able to hear it through the mini-amp.  In my case, I could trace the signal's successful passage through this chip to that one, until I didn't hear anything.

What this method does NOT do is measure stable DC voltages, which can also be instructive in their own way  However, in this case a dual op-amp could show the appropriate supply voltages on pins 4 and 8, but one half of the chip was fried.  Getting an output on one side but not the other was identified with the probe.

I often recommend to folks trying to troubleshoot a pedal to measure the AC voltage at different points.  The audio-probe method can work just as well in many instances.  The advantage of the DMM method, as opposed to audio probe, is that the DMM can show you precisely how much a signal has been amplified or attenuated, whereas one is inclined to set the mini-amp volume to a level you can hear, and finer distinction are often not easily detectable.  As well, AC voltage does not distinguish between a full-bandwidth or severely treble cut signal, where the audio probe just might.

Why a mini-amp, you ask?  Good question.  Because you, your neighbours, and your partner/spouse/sleeping kids do not want to hear what an insufficiently attenuated signal sounds like through your Marshall stack (or even your Frontman 25R) at 1:00AM.

[Kevin Mitchell]

Why a mini-amp, you ask?  Good question.  Because you, your neighbours, and your partner/spouse/sleeping kids do not want to hear what an insufficiently attenuated signal sounds like through your Marshall stack (or even your Frontman 25R) at 1:00AM.

The last synth I've built had a bad connection in the VCA. I fixed it, plugged in the amp cable and pressed a key. I had grabbed the wrong cable and apparently forgot to turn off my amp head wired to a 4x12 cab I was running in the afternoon. It could have competed with a train horn. It was very late and the popping from the cab wasn't enough warning for me 

I've been neglecting eye contact with the neighbors ever since. I'm moving soon anyhow. Off to find new folks to torment. 

[StephenGiles]

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This is the VCO sweep generator from the EH Space Drum - which is no doubt familiar to some! My gut feeling tells me that suitably adapted for + & - 9v operation, with appropriate level shifting of the CA3094 output at pin 6, this should provide an alternative VCF drive for the EH Microsynth.

The circuit notes show that it works like this:

With the system at rest, no current flows through R4 into IC2 (CA3094) pin 5. This results in the transconductance amplifier portion of IC2 being effectively disconnected from its darlington emitter follower output stage, hence the voltage at pin 6 follows pin 1 which is at the voltage set by R7 (stop freq) and buffered by IC1B (LM324). When the drum is struck, IC2 is energised by the current into its pin 5 and becomes a voltage follower, forcing pin 1 to equal the pin 3 voltage, set by the Start Freqency control. The pin 5 current ceases to flow after about 5ms, and the voltage at IC2 pin 1 decays through R10-C4 to the STOP FREQ voltage.

I currently don't have a breadboard, so someone may care to play with this!

[Mark Hammer]

Hmmmm, could I use that to drive the sweep of an OTA-based phaser?

[StephenGiles]

Hmmmm, could I use that to drive the sweep of an OTA-based phaser?

Yes, I did that back in the 1980s one morning at 3am or so, but my then dog managed to tug the tablecloth and shed that work in progress out of the breadboard and on to the carpet. I never got it working properly again unfortunately!

[Mark Hammer]

I have some populated Ropez boards I can play with.  I may just take a stab later in the week.

[Mark Hammer]

Well this is exasperating.  Had the Micro-Synth working just peachy-keen.  Re-packaging the damn thing has actually been harder than the troubleshooting and repair.  First off, it uses solid-core wire, and there are a LOT of them.  They do not take the sort of bends required to fit the two big boards into the case all that well.  So I've had to deal with multiple fractures.

Additionally, lining up the standoffs and bolts, through the two boards has been tricky, and I think I may have shorted out somethng last night while trying to line things up.  I may very well be back to square one in repairing this beast.

You'll excuse me while I go do something that manages to displace my frustration with this pedal.  I just heard the snow plow pass by.  It has left a wall of ice at the end of our driveway, making it impossible to leave the house until the shovelling is done.

[StephenGiles]

Mark, I've been there and bought the teeshirt!! The big problem is that you don't know that one or more of those solid core wires have come adrift when re-assembling until you fire up the unit.

[Mark Hammer]

Currently wearing that tee-shirt.