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 Forum index » DIY Hardware and Software » YuSynth
Yusynth minimoog VCF trouble
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alexmillman



Joined: Dec 11, 2016
Posts: 5
Location: London

PostPosted: Tue Dec 13, 2016 2:26 pm    Post subject: Yusynth minimoog VCF trouble Reply with quote  Mark this post and the followings unread

Hi everybody, i've just joined this forum after my first endeavours into the wonderful wold of DIY modular synthesisers. I have successfully built my first module, a Yusynth VCO, and I am now looking for some insight into a problem I'm having with the VCF; I just can't get the damned thing to work. Not getting any sound at all.

Ive checked all the components and my soldering. Im getting a signal out of the TL072 amp, but I cant seem to find anything worth noting in the ladder.
Ive replaced all the silicon apart from the transistors in the ladder, so I can be fairly sure its not a chip that has gone. Ill attach some pictures of my build. Any help would be very much appreciated!

Top side of the board:


IMG_1661.JPG
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Top side of the board:
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IMG_1661.JPG



IMG_1663.JPG
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Signal going into the VCF:
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IMG_1663.JPG



IMG_1664.JPG
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Signal at pin 4 of U1:
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IMG_1665.JPG


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alexmillman



Joined: Dec 11, 2016
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Location: London

PostPosted: Wed Dec 14, 2016 1:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just double checked component polarities. All correct. Any ideas?


IMG_1668.JPG
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solder side of the board:
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IMG_1668.JPG


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blue hell
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PostPosted: Wed Dec 14, 2016 2:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Can you point me to a schematic?

also, IMG_1665.JPG does not have a description, what signal is it?

Edit : and welcome

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Jan
also .. could someone please turn down the thermostat a bit.
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alexmillman



Joined: Dec 11, 2016
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Location: London

PostPosted: Wed Dec 14, 2016 3:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the welcome!!

That picture is the signal at the collector of the 1st transistor in the ladder. All of the info for the build can be found here:

http://www.yusynth.net/Modular/index_en.html
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blue hell
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PostPosted: Wed Dec 14, 2016 3:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

IMG_1664.JPG shows clipping, which should not happen .. what does U3b pin 7 look like?
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Jan
also .. could someone please turn down the thermostat a bit.
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alexmillman



Joined: Dec 11, 2016
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PostPosted: Wed Dec 14, 2016 3:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Pin 7 of U3 is identical to that picture. The 120K resistor in series with the input signal is trimming the top off
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gdavis



Joined: Feb 27, 2013
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PostPosted: Wed Dec 14, 2016 3:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

alexmillman wrote:
Pin 7 of U3 is identical to that picture.

The amplitude of the signal at U3 pin 7 should be much greater than at U1 pin 4, about 4V peak to peak based on your input picture.

Quote:
The 120K resistor in series with the input signal is trimming the top off

It shouldn't be.

Measure the input with the scope in DC mode, not AC, and make sure that the signal is centered around ground.

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alexmillman



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PostPosted: Thu Dec 15, 2016 6:13 am    Post subject: Reply with quote  Mark this post and the followings unread

So, I think I may have found the issue; the +ve voltage is shorting to ground. Im getting 600ohms of resistance between +ve and ground, while theres a good 1Mohm between the -ve and ground.
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blue hell
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PostPosted: Thu Dec 15, 2016 10:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Heh, probably not, better do what gdavis suggested :-)

600 Ohm is hardly a short, and also when swapping the pins for ohm measurements you'd get something entirely different as there are lots of semiconductors in the path which will usually conduct one way and not the other way.

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also .. could someone please turn down the thermostat a bit.
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gdavis



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PostPosted: Thu Dec 15, 2016 3:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

600 ohms at 15V gives you 25mA which is just about the expected current draw as I recall. It's also common for the positive rail to draw more than the negative rail. However, reading the resistance like this isn't really accurate as Blue Hell mentioned because of the semis.

If there was a short between power and ground, it would read very close to 0 resistance and the power supply would react when you powered up the module. When you measure the voltage on the power pins, it would be very low, and if the power supply wasn't current limiting, stuff would probably be blowing (this is why I like to use a current limiting bench power supply to power up new modules Wink)

This isn't to say there couldn't be a short somewhere else in the circuit. Once the obvious has been eliminated, the best way to attack the problem IMO is to systematically go through the circuit step by step to narrow down potential problem spots to look at more closely.

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Grumble



Joined: Nov 23, 2015
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PostPosted: Fri Dec 16, 2016 12:46 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm sorry to tell you this, but you should realy work on your soldering skills.
Looking at the solder side of your board I notice several possible shorts and bad connections...
Take a magnifier and some good light and doubble check your solderings!
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