VCF broken in HF mode?

Discontinued DIY products Anushri
#1

1. hold button pressend for drone mode
2. all 5 tuning pots in minmal position no osc audible at this moment
3. filter resonance in max position for filter self oscillation

now the frequency range for sweeps (with cutoff pot) is very small and in a very low frequency range. days before, i used to adjust the frequency range from very low to very high with the “vcf env mod” pot. but this isn’t working anymore. so high frequency sweeps aren’t possible now. this stopped working two days ago.

is there something broken?
thanks in advance!

#2

IC6 pin 14 should be between 0V (highest cutoff) and 2V (lowest cutoff). What do you get?

#3

Thanks for the fast reply!
I got 1.3V at highest cutoff and 3.15V at lowest cutoff on pin 14 at IC6…

#4

Voltage on each side of IC13?

Voltage on each side of R69?

Voltage on IC20 pin 6?

#5

IC13: 4.09V and 0.0V
R69: 0.0V
IC20 pin6: 0.0V

#6

4.09V or -4.09V for IC13?

Does the voltage on IC20 change when you enable the envelope or LFO on the filter?

#7

IC13: -4.09V and 0.0V
(forgot the minus sign in the previous post)

#8

yeah voltage on IC20 pin 6 changes (at LFO rate i guess)

#9

Try swapping the two DACs (4822). Does it change anything?

At rest, the DAC should be near 2V (or lower if a lower note is pressed). Not as low as 0V - this is what causes the low cutoff.

#10

swapped IC 19 and IC 20. synth clicks and pops in drone mode when i turn the 5 scale pots. no holding tone is audible… swapped the ic’s back in original position - same behaviour -> no tone, but pops and clicks when scale pots turned.

#11

Does the VCO work?

#12

the VCO seems to work but at very low volume. the funny thing is: when i turn the volume knob, i get the loudest signal at middle position. if i turn up the volume on max position. the VCO disappears completely and i hear only a high frequency humming.

#13

I mean: do you get a clear and loud signal on points F, G H, and K?

#14

sad. i don’t own a oscilloscope…

#15

what i can say: swapping the both DACs changes how the synth behave. in one configuration (swapped) it pops and clicks when turning the 5 scale knobs and the volume knob acts in a linear way as usual. in the other configuration (original) the VCO produces short notes and a humming when i turn the 5 scale knobs and the volume is at maximum in middle volume knob position as i described above.

i don’t have a midi keyboard ready at the moment. all the tests are in drone mode.

#16

What I don’t understand: you were describing a problem with the filter. Now you have put back the DAC in its original solution and what you describe sounds like a totally different problem.

#17

sorry for this confusion. it seems things are getting even worse with the device. 2 days ago i got the hf filter issue. today i tested the unit with DACs swapped and recognized this “new” problem.

#18

the problem seems only to occur when in drone mode!

yesterday evening, i had the chance to test the device with a midi keyboard: the anrushi sounded normal! the HF worked, everything worked fine as usually! but the drone mode shows up the problems described above.

#19

Hmmm that’s strange, I’ll try to reproduce the problem… Maybe something is wrong with one of the settings (maybe velocity -> VCF mapping?)

#20

wait a moment. i guess i have it…