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Help needed, early analog stages
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Help needed, early analog stages

by ElvisMcPelvis on Tue Mar 31, 2020 3:20 am

Hi everyone,

I'm currently stuck with my build (from Willzyx) and if anyone could help me it would mean a lot. Sorry if the post is long, I'll separate it into parts.


The VCO works fine, I get saw and square waves when probing the waveform switch and their pitch changes with the pitch pot as well as the 2 trims. (I've decided to leave calibration for later once the 2V/3V are made switchable, because I've got a DIY power supply with a very sensitive trimpot for voltage, so it's always a bit of a bother to change between 2V and 3V).

The VCF is where I started to doubt things because the guide recommends probing the resonance pot and hearing the effect Resonance and Cutoff have on the sound.
I couldn't hear much more than a weak, low freq signal à d some noise, but maybe that's normal since the tests recommends using an oscilloscope and the freq in the pictures is quite low. Anyway I noticed that with Resonance all the way up, I could hear a swishy sweep while turning Cutoff, so I figured the filter was operating OK.
I don't know if I should have got something else here but I didn't find the sweeping sound weird in any way.

Next, the envelope.
Clipping R146 and R3 together and measuring at northern end of D35.
I get 0V when the clip is unconnected, but only ~9V instead of 12V when it's connected.
I notice the short voltage surge when measuring at D36, but I don't have a "hold max" setting to see if the peak is at a good voltage.

Finally, the VCA :
As you might have guessed, listening through the volume pot and connecting R146 and R3 doesn't result in any audible tone.


I'm currently looking around for misoriented components and stuff, nothing so far but I'll keep looking ; the problem's got to be somewhere.
I've checked the voltages going through J4 to the main board and they're OK.


If anyone has ideas as to where in particular I should look into, or can give any other insights, that would be awesome.

I can upload any pics and voltage readings needed, of course!



Cheers

ElvisMcPelvis
 
Posts: 2
Joined: Tue Mar 31, 2020 2:36 am

Re: Help needed, early analog stages

by ElvisMcPelvis on Tue Mar 31, 2020 1:19 pm

OK, progress made!

I noticed that the voltages aren't quite right at J4 : when both boards are connected and plugged into 9VAC, the voltages aren't 5.33V - 0V - 12V but instead, ~4.2V - 0V - ~9V.
Also, IC23 was getting hot.

Thing is, the voltages coming out of J4 are OK when unconnected to the main board and IC23 doesn't heat up, so I doubt the problem is in the power board.

When I only plug in terminals 2 and 3 of J4 to the main board, everything is fine. (0V in 2 and 12V in 3)
When I add the 1st terminal of J4, both voltages drop as mentioned before.

I measured resistance to ground for all terminals in J4 and I found a few KOhms (except of course 0 for the middle one which is 0V) so unfortunately, I don't think the problem's happening as early as J4.
I tried replacing IC23 and though the voltage drop in J4 was lesser, it's still quite a drop.

IDEA : I removed IC9 and J4's voltages went back to normal. I was using a 74hc174. Does this mean the problem was in IC9, or is it not necessarily an indicator? (like, could remove it be cutting the path towards the actual problem?) I don't yet have a substitute to try in its place.
Any ideas would be much appreciated!

ElvisMcPelvis
 
Posts: 2
Joined: Tue Mar 31, 2020 2:36 am

Please be positive and constructive with your questions and comments.