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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
No output from LM13700 MFOS Dual VCA
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Bogus Noise



Joined: Jun 03, 2009
Posts: 59
Location: Sheffield

PostPosted: Sun Jan 08, 2012 3:29 pm    Post subject: No output from LM13700 MFOS Dual VCA Reply with quote  Mark this post and the followings unread

I'm trying to build up a dual VCA, and have breadboarded this one:

http://www.musicfromouterspace.com/analogsynth_new/OLDIESBUTGOODIES/VCA/dualvca-2.html

Problem is, I'm not getting any output! I've checked and double checked and triple checked the connections and everything seems fine. So I decided to try stripping it back completely and breadboarding just one LM13700 section, and tying the CV In at pin 16 to the positive rail. Still no output. Confused

Posted Image, might have been reduced in size. Click Image to view fullscreen.

The current build just has the lower section of the above schematic. Anything above the Lin/Log switch has been omitted, except for the power connections. I've tried a few LM13700s, but the result is the same - there is no result!

I'm thinking there's something obvious that I've missed but can't figure it out. I know there's other VCA designs out there, but this one seems to best suit my needs in this project, and I'd really like to understand the reasons why I haven't been able to get it going.

So, can anyone help me out? Smile

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prgdeltablues



Joined: Sep 25, 2006
Posts: 162
Location: UK
Audio files: 10

PostPosted: Mon Jan 09, 2012 5:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi, bit difficult to tell without a few more details, but I've built one of Rays' dual VCA's (not sure if it is precisely that version, but certainly that module) and can confirm it does work.

When you say 'tied the CV in at Pin 16 to the positive rail' I hope you meant through a 30k resistor. The maximum allowed input current is 2mA there, so without a resistor you have probably fried the LM13700.

What signal level are you putting in - the circuit expects to see +/-5V? If you could give us some voltage readings at various points, that might help pinpoint the problem.

Peter
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Bogus Noise



Joined: Jun 03, 2009
Posts: 59
Location: Sheffield

PostPosted: Mon Jan 09, 2012 11:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi, thanks for your reply!
Yeah, I'm absolutely certain that Ray's design is not at fault here, I've ruled that out from the start Smile

At the moment I'm powering it from a +/-9v supply. The signal in is a quick 555 osc I also breadboarded for the testing, and is running off the same supply.

A fried LM13700 is possible. I had it connected through the resistor at first but through frustration and trying to find the fault that might've gone out the window!

The 555 is swinging between 0v and 7.33v, but it's going through a 100k resistor as per the schematic, which only gives me 0.016v. Would that be too low? And would 7.33v without the resistor be too high for the 13700?

I have more on the way anyways as they're useful chips, so will test that out again tomorrow or Wednesday.


Edit: tested it out with some other 13700s I had ordered, and the circuit seems fine now. I probably fried the other ones in this and a previous project while testing a bit too blindly... thanks for your help though!

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