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VCF no output on resonance
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VCF no output on resonance

by puddyman on Sun Sep 24, 2017 3:25 pm

so i've just built up to the VCF.

all tests have worked so far. if i probe the bottom of 105 i get a saw wave on my scope, but if i probe the left pin of the res pot like in the instructions pic i get nothing (is it the far left pin you're meant to probe? it doesn't actually say).
I am getting 3v from the variable power suppy and 3v from pin 7 on IC11.
are there any tests i can do to figure out where i'm losing the signal or which part may need looking at?
I'm just following the guide and don't have much electronics experience so i'm dead in the water when it comes to troubleshooting on my own, i've been very carefully following along, triple checking each part etc.
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Re: VCF no output on resonance

by antto on Tue Sep 26, 2017 12:38 pm

the resonance pot is dual, so it's like two sepparate potentiometers with a shared "position" (but not related to each other electrically)
one of those potentiometers is used for the actual "resonance" so it's part of the audio feedback path of the VCF
the pins for that potentiometer are located like so:
W A - - - B
while for the other potentiometer (but you don't care about that one):
- - A W B -

A and B are the end points (you get 50k resistance across them), W is the wiper, which travels along the resistance

now, specifically in the 303/x0xb0x audio path, you need the first one
its A terminal is connected to ground, so you can identify it by looking closely at the PCB - the copper pour on the back side stays away from all other pins, except that one, since it's ground
if we assume that's pin2, then pin1 is the wiper of the same pot, and you're interested in pin6, which is the terminal B where the audio comes from
if you happened to measure at pin1 (the wiper) you may get a signal or not, depending on the position of the knob
so measure at pin6 then, and you should definately get a signal there
note: the signal level may be low there.. it won't be a problem if you're probing with a scope, but if you're using the headphones method - you might have to listen carefully

also, since this is a lowpass filter, the cutoff frequency might happen to be low enough so that the oscillator is very "filtered" out and hard to hear too

EDIT: i actually found a pic i've made some time ago:
Image

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Re: VCF no output on resonance

by puddyman on Sat Sep 30, 2017 7:42 am

thanks for the reply antto!

That makes sense, your digram clears it up nicely. however probing any of the pins 1-6 at various positions of the pot still gets nothing on the scope. the scope picks up signal at the bottom of R105 so i know the scope must be working.
shall i carry on with the build or try and troubleshoot this VCF bit first?
any suggestions on how to find the fault?

heres a pic of the front VCF
https://photos.app.goo.gl/yY7jxGrjd6eKaveq2

and heres the back
https://photos.app.goo.gl/av1t89INo9jYHNoD2
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Re: VCF no output on resonance

by TobiasBassline on Sat Sep 30, 2017 12:12 pm

I've pretty much the same problem here. The VCO part worked just fine but I don't get a signal on the scope from the VCF part.
On pin 5 (according to the pic from antto) I get a little current on the scope, but that's not a wave, just a flat line.
On all other pins I get exactly nothing.
I heard there are fake 2SC2291 floating around. Could this cause this problem? mine is labeled with C2291 6XG.

Update:
I measured some voltages according to this thread: viewtopic.php?f=12&t=12453

Especially on VR4 and Q22 and they are all more or less exactly like on the reference.

I probed it again with the oscillator but still a flat line. I tried also to zoom in but it stays flat.

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Re: VCF no output on resonance

by puddyman on Sun Oct 01, 2017 6:58 am

so My Q22: 2SC2291 is also labelled like yours tobias with C2291 6XG.
maybe this is our problem?
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Re: VCF no output on resonance

by diplowave on Sun Oct 01, 2017 7:37 am

I have already used 1583 silkscreened XG, they have not posed any problems...
For the 2291 you can test them with the diode tester of your multimeter.
They are NPN ECBCE

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Re: VCF no output on resonance

by TobiasBassline on Sun Oct 01, 2017 9:27 am

I think it works now (without changing anything though)!

the problem was: the Oscilloscope!

I've read a bit into the manual.
they suggested to connect the probe and press Auto Setup. I always tried to measure with the same parameters which worked fine before R62.
After R62 it was only a flat line and so it was across every point in the VCF which I tested.


I tried Auto Setup and got this after R62:
20171001_141857small.jpg
20171001_141857small.jpg (158.12 KiB) Viewed 168 times


Then I tested VR4 again with Auto Setup and got this:
(VR4 set to min)
20171001_150433small.jpg
20171001_150433small.jpg (164.38 KiB) Viewed 168 times

(VR4 set to max)
20171001_150524small.jpg
20171001_150524small.jpg (158.05 KiB) Viewed 168 times


I'm not 100% sure if it is supposed to look like this, because it seems to be a very dirty wave, but there is definitely a signal and I can alter it with the knob.

Can any1 confirm that my waveform is how it is supposed to be?

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Re: VCF no output on resonance

by puddyman on Sun Oct 01, 2017 12:00 pm

I'm glad you've got an output now.
my oscilloscope is considerably older than that. and doesn't seem to register anything.
perhaps mine is working and i just can't see it due to a problem with the scope (or my lack of scope understanding) if i continue with the build could i troubleshoot this section later?
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Please be positive and constructive with your questions and comments.