SOLVED Neurotron (Ring Stinger) sawtooth waveshape : low volume, no modulation.

[eh là bas ma]

Hello,

I built the Neurotron recently, from a musikding's kit.

https://aionfx.com/app/files/docs/neurotron_documentation.pdf

Based on what i could listen in the utube demos, it sounds like everything is working on my build (VCO, LFO, etc) except the sawtooth waveshape.

According to Lovetone's original manual, Triangle waveshape should be lower in volume compared to the other waveshapes. It's not really the case on my build, Triangle works but the level is quite the same as the other waveshapes.

However, sawtooth waveshape is lower in volume, and i can't hear any effect. Drive, Timbre and Blend affect the signal, but not the other controls.
Looks like the LFO isn't working neither when VCO waveshape is set on sawtooth.

I tried to reflow the waveshape rotary switch pads (main board and sub-pcb) and all the suspicious pads i could find, but no luck so far.

I'd like to do some continuity tests or some audio probing to narrow it down, but i am a bit confused with the schematics :


The most difficult and useful question for me is : what is the signal path when we are set on sawtooth waveshape ?

I am lost with this wave shaper schematic, the rotary switch routing is quite confusing for me...

Edit : surely the 4 waveshapes are B1, B2, B3 and B4... I wonder which one could be the sawtooth ?

Every observations and suggestions are welcome !

 

[chris.knudson]

Do you have the previous page of the schematic that you can share?

 

[comradehoser]

documentation including schematic

OG manual

 

[chris.knudson]

Lots to read there, but a couple of things I've found that may or may not be helpful...

It appears that the Sawtooth is not modulated by the LFO PWM as the other shapes are. From the OG Manual : "All the VCO waveforms change shape when PW modulated by the LFO, EXCEPT sawtooth (where subtle amplitude modulation will result instead)."

Based on this, I'm assuming that the sawtooth is waveform C, but honestly, this circuit is a bit beyond my skill set.

 

[comradehoser]

I agree it's very unclear without knowledge of how wave shapes are produced by circuits other than clipping--this circuit really brings out my ignorance of basic electronics.

If the switch numbers correspond to the a/b/c 1/2/3/4 designations [edit for clarity: it's a 3 pole/12 throw switch limited to 4 throws--the 3 poles correspond to a/b/c, in which it looks like b is the active selector and a and c are grounds (?), so if the switch designations 1, 2, 3, 4 correspond to the schematic designations of 1,2,3,4, ...] then sawtooth would be position 2. This is further correlated by looking at what's going on with position 4 which skips ahead and joins the d13-16 diode ring most directly, therefore making it most likely the sine wave, which would match the aion build docs.

But you could always shoot Aion a quick line

 

[eh là bas ma]

More than a year later, i finally fixed this build.

I suddenly decided to desolder the VCO Shape rotary switch, and resolder it again. Just a random attempt in the blind, hoping for the best...

During the process, a soldering eyelet came off the pcb, pad 1, connected to R44. I made a jumper do restore the connection, and it works, the sawtooth waveshape works now, and it sounds great !

I probably overheated that pad during the building process, and the connection was broken all this time. It looked ok but it wasn't.

Never give up !

So in the wave shaper schematics, B1, B2, B3, B4 are the shapes in the same order as on the enclosure labels : B1 is sine wave, B2 is sawtooth, etc. That's what i would guess, at least. R44 is connected to B2 and restoring the connection there fixed the sawtooth wave shape.