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Troubleshooting haraldswerk VC LFO
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Troubleshooting haraldswerk VC LFO
unvrsnyrgi
Hi all!

So I ordered PCBs using the provided gerber files.
https://www.haraldswerk.de/LFO/VC_LFO/VC_LFO.html

I've build one LFO but the problem is that it's not oscillating. There's no triangle wave in IC4 pin 1 but constat negative voltage. I'm running +/-12V system. I've build the module using 1/8W resistors (since I had them already). Nothing hasn't fried so I'm thinking that's not the problem, or is it?

I couldn't source MPF102 so they are substitued to Tayda's 2n5458 (They have the same pinout). I've checked power pins from every IC and they're ok. Also no shorts between GND, -12V, +12V. I changed all the ICs and it didn't help. Resoldered every joint didn't help. I changed BC560C to new ones - nope.

I'm clueless how to continue troubleshooting and all help is very much appreciated. Thank you!

Here's more info about same circuit but the flat version. It's the same but some labels are different:
"C1 and associated components comprise a linear voltage to log current converter. IC1A sums the control voltages. IC1B provides the temp compensation realized with KTY81-110. TR3 adjusts the V/Oct characteristic. Q1 and Q2 forms the log converter with IC1D as constant current source. IC1C scales the control voltage for the linear FM input. The transconductance of IC1OTA1 controls the frequency of the oscillator. IC2C, C1 and associated components comprise an integrator. When current flows into IC1OTA1 output the integrator ramps up, when current flows out of IC1OTA1 the integrator ramps down. When the integrators output goes above the threshold of comparator IC2D its output goes high. The output of IC2D is fed to the non-inverting input of IC1OTA1 OTA through D1, D2, R1, R2 and TR1. TR1 balances the current flowing during the high and low periods of IC2D. With TR1 you can adjust the symmetry of the triangle. While IC2Ds output is high current flows out of IC1OTA1 OTA and the integrator ramps down until the voltage at the input of IC2D goes low enough to overcome the hysteresis provided by R13 and its output goes low. When this happens the comparator starts to ramp up again and thus we have a triangle wave at the output of IC2C. The bias of the comparator IC2D is controlled by the current generated by the linear voltage to log current convertor. This controls the current that flows in and out of IC1OTA1 and thus the frequency of the oscillator.

The sawtooth is created by mixing portions of the original triangle wave and an inverted version of the triangle wave. N-FETs Q1 and Q1 are used as analog switches."
SphericalSound
That´s a hell of schematic for a LFO...

If IC4 is not producing a triangle it may be for the 100nf cap is you are super lucky or something related to the 13700 or the rest of the circuit.

I would recheck that c3 is 100nf, is not shorted and doing good conection.

Then, I would try to feed some volts at the FM_LIN and recheck if theres triangle in IC4.

Then I would try to feed some volts at the V/OCT and CV1 and recheck if theres triangle in IC4.

Then, post results. Maybe another user can give you more hints
unvrsnyrgi
Thanks for the hints! I've checked C3 and tried to feed CV inputs but no luck.

I'll put this project on the side for a while. It's too frustrating to put more hours into this smile Earlier I've succesfully build Bastl Kastle so I'm thinking of making multiple Kastle LFOs instead of these.
Haralds:Werk
unvrsnyrgi wrote:
Thanks for the hints! I've checked C3 and tried to feed CV inputs but no luck.

I'll put this project on the side for a while. It's too frustrating to put more hours into this smile Earlier I've succesfully build Bastl Kastle so I'm thinking of making multiple Kastle LFOs instead of these.


Remove the connector to SW1. It is possible to shut off the LFO with wrong adjustments of TR1 or TR2. Measure the voltage drop over R42. It should change when you turn the coarse pot. If there is no change your problem is in the CV source. Otherwise it is in the core.
unvrsnyrgi
Thank you Harald! I removed the SW1 connector and now I get all the waveforms. w00t
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