need help troubleshooting VCO/VCF..can hear clicks, VCF..

[stinki2]

Hey guys,

OK I am stumped. This is my 2nd xox, my first went really smooth.

It seems on the verge of working. I've got a perfectly working xox to compare with, I've gone over the joints, checked and re-checked transistors and caps, checked for shorts, pulled chips out and checked for bent pins, etc...I'm really stumped as I have my first xox really modded out and had to troubleshoot it many times, but never got stuck like this...Have a feeling it's something dumb/simple but I don't know anymore...

Setting to random mode for testing, i have clicks and can hear the resonance creating a sort of whispery sound as well -- but you can't really hear the VCO's..Very, very faintly, you can hear the saw.

If I poke around in the VCO section just probing connections, the saw gets louder when I poke the legs of, for instance, Q25 and Q27.

It sounds exactly the same if I unplug J6 and J7.

I'm not knowledgeable enough to have any confident leads...wrong resistor somewhere in the VCO? or is this a problem in the VCF?

Any help deeply appreciated!

[antto]

it would help if you upload an audio sample

[stinki2]

OK, here's a little clip, rename the .zip to .mp3.. ? you can hear the oscillator come in during the 2nd half when i touch a solder joint in the r74-r90 section while also touching parts of the filter section..

so it seems the signal just isnt making out of the vco section into the vcf correctly...? i'm sure it's something simple i just can't freaking find it, and i don't know how to technically trace where things are hung up...i'm a quick learner i think tho, would really appreciate any help/instruction..thanks!

[antto]

weird.. the resistors there in that section are part of the resistor network that converts the PITCH signal from the cpu
something else that is involved in the PITCH is IC11

it's possible to have the vco at some uber-low or uber-high frequency, which would make it either sound like random pops (when it's at very low frequency) or simply inaudiable (if it's at some very high frequency)
i can't say wether it's that..

try this: (again, in random mode) without touching anything, just try to see if you have the VCO oscillating at "sane" frequency
if you have a scope, probe at the waveform switch, or you can try it with some cheap headphones

look around for proper values on these resistors, maybe rehit them with the iron, check also R105, R62, C17 for right values

[stinki2]

thanks so much for the tips!

R74-R90 are all identical & correct, R105, R62 and C17 are correct and well-soldered as well...I'll try hitting the R74-R90's again with the iron but I already have done this, probably 2 or 3 times, so it's unlikely there's a bad connection in there still..

when i do the headphone test with a pair of old iphone headphones i don't think i hear anything, just the "pop" when the headphone plug touches the pcb..

since i have another xox it was fairly easy to cross-check caps and resistors, fairly certain those are all the right parts in...

i have a feeling it's a wrong resistor, but i've cross-checked some with the other xox as well and havent spotted anything yet...but it's much more difficult to compare them

or a bad joint/connection/cable lead? but i've re-heated those solder joints fairly thoroughly, and i've jiggled the cable connections and didn't hear anything interesting...

let me know if you have any more ideas!

[antto]

hm.. i'm getting out of ideas

when you listen with headphones at the waveform switch, listen for a long time, do you hear a periodic pop/click?

[stinki2]

ok wait no those old ipod earbuds were junk, tried a different pair and there is faint, regular (periodic) clicking when i touch the saw pin of the waveform switch

[antto]

so that has to be the sawtooth oscillating at a low frequency
is your IC11 socketed? if so, i'd take it out and try some other dual opamp, lm359 or similar, they are cheap
also look around in the VCO section and check for right component values and proper orientation of transistors/diodes

[stinki2]

ahh cool, that's the saw..! so the problem is in the VCO for sure..whew that helps narrow it down at least..

my ic11 is socketed, and it is on my working xox as well, so i can easily plug in the known good one...we'll see..and will re-check for the right parts in the VCO, maybe I've missed something before..thanks!

[MMM]

Do you have a multimeter (or can you borrow one)?

I'd suspect IC9 or maybe IC11.

[stinki2]

well somehow i had a couple 200 ohm resistors where 10k should have been -- somehow they got in the wrong bag at some point..

now when i listen with cheap headphones at the waveform switch, the saw sounds normal..

but i still don't have sound from the outputs..

i don't see any more mis-placed 200 ohm resistors..

when i touch points in the r74-r90 array at the same time as some points in the vcf, the sound starts creeping in..my guess is that as before, there are some more wrong resistors?

but also i don't have a trimpot in TM4 right now -- while removing it to doublecheck it was a 50k, one of the legs broke off, so instead i have a 50k resistor between the two outside holes for TM4, and a 25k between one of the outside holes and the middle hole..is that right? (the idea being to mimic a centered 50k trimpot with the resistors)

well, any tips tracking down where things are going wrong now very appreciated...

ps yes, i have a multimeter..

[antto]

TM4 does the same thing the TUNE knob (on the front panel) does.. the idea is to have the TUNE knob centered and then adjust TM4 till the frequency of your A note matches 220Hz or so
later, if the tuning drifts over time, you can compensate that with the TUNE knob (instead of having to open the x0x and poke around..)

[stinki2]

Ahh, I knew it tuning, yes, and that TM5 does tuning width...

So my resistors in TM4 should only affect the tuning at most, I guess, not negate the sound? I replaced the resistors with a 50k pot for now anyways, until I can get a proper trimmer from mouser..no change though..

I swapped my IC9 and IC11 with my good ones from my working xox but it didn't make a difference either..

Just to recap, I have a good sounding saw from the waveform pins now, but still a similar sound from the outputs as I did before: whistley/wet "clicks" as in the file I uploaded already..And as before (and in the file I uploaded), when I touch points in the VCO and VCF, the oscillator comes through...It sounds better than it did before when the sound comes through, but that's not much consolation...It seems to me like the vcf is being triggered but the oscillator signal just isn't making it there...?

I've double-checked the components of the VCF and haven't seen any more wrong parts...Or could the problem be in the envelope or VCA? If I just knew better where to focus now I think that would help alot..

Any more tips on narrowing down where the problem could be?

[antto]

i think it's not the vca nor the filter, thus it's not related to the envelopes

[MMM]

That's not easy to debug without an oscilloscope; do you have one or can you borrow one?

Basically we need to trace the signal and within the filter it is very, very weak.