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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Mini Synth, No resonance (Solved)
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Starspawn



Joined: Jun 14, 2013
Posts: 49
Location: Oslo

PostPosted: Fri Jun 14, 2013 1:25 am    Post subject: Mini Synth, No resonance (Solved)
Subject description: Help to troubleshoot
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Hello, recently I got a failed build of a Minisynth and proceded to check it over and fix it.

Mostly it works fine now, except I get no resonance at all except when I crank up the noise in addition to oscillators so that inputs gets just a little voltage.

I dont understand what voltages are supposed to be where in that part so was hoping someone could help me test by measuring a working one.

At the pin 8 output where I figured I would get positive voltage theres only 1.1x V although pin 11 is good at near 9V

Likewise both the points where negative voltage enters are dropped to zero after the 20K resistors, but as I want to just add gain (positive voltage), not find a new balance point I suppose thats normal and we only want the positive?

Its no wonder the resonance does little when all Ive got to work with is a volt that drops away to 0.16V along the way, so is the problem at the pin 8 output?
Whats normal voltage there?

I have changed the IC though, as I suspected it, but still the same problem.

All parts and points should be correct, so probably looking for a faulty component in the resonance voltage path.

Any ideas?

Thanks for looking Smile

Edit: Voltage at the switch is weird. On LP setting I get 0.16V middle and top (M and N) connection, and on BP I get about 0.40V on the same ones, the bottom J connection gets no voltage on either setting.

I used a middle off switch (on off on ) instead of just SPDT on/on, but that worked fine other places?

Also the 13700 chips are 13700N, but the diagrams are identical, so just mentioning it in case it matters.

Last edited by Starspawn on Mon Jun 17, 2013 6:17 am; edited 1 time in total
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elmegil



Joined: Mar 20, 2012
Posts: 1535
Location: Chicago
Audio files: 14

PostPosted: Fri Jun 14, 2013 6:18 am    Post subject: Reply with quote  Mark this post and the followings unread

I've done a fair bit of troubleshooting with the Mini Synth, and I have a working one here that I will measure for you.

Unfortunately, I also have a crazy busy day and weekend, so I can't make any promises as to WHEN I can do that.

But I know I've posted similar requests many times, and it's disheartening to get no response at all, so I figured a response saying "you will get it eventually" is better than none Smile
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Starspawn



Joined: Jun 14, 2013
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Location: Oslo

PostPosted: Fri Jun 14, 2013 6:41 am    Post subject: Reply with quote  Mark this post and the followings unread

That is indeed better than nothing, and thanks Smile
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Wobuse



Joined: Dec 07, 2011
Posts: 33
Location: Lyons,il

PostPosted: Fri Jun 14, 2013 6:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

I was looking at the schematic ,& I think you should be measuring current??
Also ,check the pin out of the transistors in that part of the circuit.
I am assuming that you are using a +\- power supply...

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elmegil



Joined: Mar 20, 2012
Posts: 1535
Location: Chicago
Audio files: 14

PostPosted: Fri Jun 14, 2013 9:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is not my strongest area in Ray's circuit Smile but I'll give it a shot.

It seemed to me that the easiest thing to measure would be the voltage from ground to the wiper on the resonance pot. That definitely is a voltage and not a current, as it is being summed back into the positive voltage input of the OTA.

I've attached the result. The waveform you see there changes amplitude with the setting of the resonance pot (as you'd expect) up to the point of self resonance at which point it becomes a sine wave. The period of the waveform changes with the frequency pot, again obviously enough.

Pin 8 is connected to the top of the pot, and at very fast scope speeds you see a "bump" so I decided to measure the voltage and got photo 2 at minimum resonance and photo 3 just shy of full self oscillation.

Some questions that occur to me:

You say the resonance pot does nothing. Does the cutoff frequency pot do anything? Do you notice any change when you switch between low pass and band pass? Have you looked over the resistors and capacitors in that section of the circuit to make sure

1) no obvious cold solders or unsoldered bits (ask me about my threeler question in the DIY section Smile )
2) all component values correct
3) no obvious short circuits

I need to spend some quality time with Electronotes and other info about state variable filters before I'm going to be able to say much else intelligent about the circuit Smile

Edits: problems with photos, realized I said something foolish early and then reversed it later.


IMAG0126.jpg
 Description:
center tap of resonance pot, short of self oscillation
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IMAG0126.jpg



IMAG0128.jpg
 Description:
pin 8, resonance minimum
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IMAG0128.jpg



IMAG0129.jpg
 Description:
pin 8 just short of self oscillation
 Filesize:  1.77 MB
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IMAG0129.jpg


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Starspawn



Joined: Jun 14, 2013
Posts: 49
Location: Oslo

PostPosted: Sat Jun 15, 2013 2:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Great! Thanks Smile

As I said everything else works fine as far as I can tell, and components have been checked many times.

And the resonance pot does do something when the noise helps boost the signal a little, but its just so low that it doesnt work as resonance at all, no oscillation, you barely hear it.

Looking at the waveform on my (very old so no pics) scope, there is clear change when I turn the knob. But after R47 when the audio sources are collected the level drops a lot so I can barely see at the same setting.

I dont have 2.6V though, I got 0.10 on pin 9 and nothing on pin 8.

Kind of brings me back to suspecting the chip, because the voltage going in is right, and I cant see anything that would affect what it puts out other than its internal parts?

Could the internal diodes be the first things to go if the previous builder shorted the voltage input there or something?

Since there was two 13700N I changed them with each other and assumed that since both worked in VCA they were ok, but if the VCA doesnt use that buffer circuit ... ?
Edit:
Other than to get a little gain it seems, and my mini isnt that loud, so might very well not be working that stage on the VCA.
(also getting a low reading on pin 9 at VCA, just 0.6)

On both chips the voltage at the top of the buffer triangle is pin 12 and 10 = 1.24-1.26V and 0.6 - 0.10 on the 9 pin, the lower pin 5 and 7 on VCF is just 1.15 which explains 0v at pin 8 I guess if the drop is the same)

If you agree Ill try replacing both chips before bothering you any more.
Could it be a problem with the LM13700N vs D though?
Whats the pin readings on 12-10 and 5-7 on yours?

Hmm, also these voltages are dropping steadily it seems?
Ill get new batteries even before changing chips.
Theyre also a bit unequal as it was not a similar pair of batteries, at -8.96V vs +8.74V ... might also be a problem?
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elmegil



Joined: Mar 20, 2012
Posts: 1535
Location: Chicago
Audio files: 14

PostPosted: Sat Jun 15, 2013 5:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, batteries below 9V can cause all kinds of strange things. More than one of us has been frustrated with a problem only to find new batteries solved it.

I will look at those voltages tonight, I'm just doing my morning reading before the "crazy day" starts. I will say that the voltages I was seeing were very low. I had my scope set for .1V and 50mV per division in most of those screen shots.

So maybe the problem is with the VCA or the linkage to the VCA?
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Starspawn



Joined: Jun 14, 2013
Posts: 49
Location: Oslo

PostPosted: Mon Jun 17, 2013 6:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Bah, the problem was the resistor for the sound inputs being the wrong value.
As they sounded correct I probably got lazy after having checked all the other color codes, and my brain told me I had already done it.

Always double check when stuck, even when you think you have done so already Very Happy
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elmegil



Joined: Mar 20, 2012
Posts: 1535
Location: Chicago
Audio files: 14

PostPosted: Mon Jun 17, 2013 2:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yep Smile

I had the same experience when I couldn't get either of the expo converters to track worth a darn--turns out one resistor of each of the main pairs in that part had been swapped between the two inputs, and it wasn't until I was just about ready to completely give up that I spotted it on one last check through.
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