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 Forum index » DIY Hardware and Software » YuSynth
YUSYNTH VCO strange startup behavior
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smokestacksproductions



Joined: Aug 24, 2012
Posts: 18
Location: USA

PostPosted: Fri Aug 24, 2012 8:23 am    Post subject: YUSYNTH VCO strange startup behavior
Subject description: I have to momentarily short out the negative rail to get the VCO to work
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Hello,

I recently completed a YUSYNTH VCO. When I turn on my system, the VCO does not work, I measure less than 1 volt on the pins that should be -14.3.

If I short out the negative rail momentarily it comes to life and I can use it regularly for the rest of my session.

What could be causing this? Is it the 7915? Or a bad Capacitor?
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Sebo



Joined: Apr 27, 2007
Posts: 534
Location: Argentina

PostPosted: Fri Aug 24, 2012 9:38 am    Post subject: Reply with quote  Mark this post and the followings unread

I had the same problem once, change the 7915 for one of a diferent brand.
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smokestacksproductions



Joined: Aug 24, 2012
Posts: 18
Location: USA

PostPosted: Fri Aug 24, 2012 1:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Sebo, I'll try this and get back to you.
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Sebo



Joined: Apr 27, 2007
Posts: 534
Location: Argentina

PostPosted: Fri Aug 24, 2012 1:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

I mean the 79L15 in the VCO itself.
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smokestacksproductions



Joined: Aug 24, 2012
Posts: 18
Location: USA

PostPosted: Fri Sep 28, 2012 8:12 am    Post subject: Reply with quote  Mark this post and the followings unread

I built another one of these modules, this time I made the lm394 version. I get the same strange behavior. After more investigation I learned that it works fine if I apply power to the positve rail first, and then the negative rail. I have not had problems with any other modules, I am using a power supply that I built which is based on the standard 7815/7915 circuit with center tapped transformer. Can anyone offer some idea as to why this is happening?
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prgdeltablues



Joined: Sep 25, 2006
Posts: 171
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PostPosted: Fri Sep 28, 2012 8:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, it's a known problem with negative voltage regulators (some, not all, perhaps certain brands). Goggle latch up.

Peter
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Sebo



Joined: Apr 27, 2007
Posts: 534
Location: Argentina

PostPosted: Fri Sep 28, 2012 10:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Just change the 79L15 in the VCO for another one (of another brand).
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smokestacksproductions



Joined: Aug 24, 2012
Posts: 18
Location: USA

PostPosted: Fri Sep 28, 2012 1:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

I replaced the 7915L on the firts VCO like you suggested, but the problem remained. I first used a generic "multicomp" brand, and then replaced it with one from NTE which still did not work.

Can you reccomend a known good brand or part number please?

What about adding extra diodes to prevent "latch-up" ?
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Sebo



Joined: Apr 27, 2007
Posts: 534
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PostPosted: Fri Sep 28, 2012 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry, I can't remember what brand worked and what brand not.
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smokestacksproductions



Joined: Aug 24, 2012
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PostPosted: Fri Sep 28, 2012 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's ok,

I really appreciate your help with this. I will try to order some different ones soon.

Thanks!
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Cardinen



Joined: Sep 15, 2009
Posts: 26
Location: Italy

PostPosted: Sat Sep 29, 2012 6:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, is a common problem sometimes with some brands of 78xx/79xx.
I suggest to use regulators from ST or Fairchild, never had a problem with these.
Anyway you can try a schottky diode or germanium (i've used 1N5819 with success for preventing latch-up).
Another method could be add a small resistance in series with the negative regulator, so the negative rail will delay up a bit so the positive rail can start without latch-up.
Usually latching problems comes when there are rail-to-rail decoupling caps.
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yusynth



Joined: Nov 24, 2005
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PostPosted: Mon Oct 01, 2012 6:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I have question what power supply are you using ? Have you tried with another PSU ?
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smokestacksproductions



Joined: Aug 24, 2012
Posts: 18
Location: USA

PostPosted: Tue Oct 02, 2012 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

I am using a power supply I built myself. It is actually two isolated power supplies on one board, the smaller one is driven by a smaller (and seperate) transformer. I have actually thought about using a normally open relay on the negative rail so that the positve rail must start first and this will cause a small delay before the relay is closed and the negative rail is powered up. Here is a quick view of said power supply:


modular psu +-5v +-15v.png
 Description:
this psu provides bipolar 5v and bipolar 15v
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modular psu +-5v +-15v.png


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Pilastik



Joined: May 17, 2010
Posts: 5
Location: Greece

PostPosted: Fri Oct 26, 2012 10:03 am    Post subject: Reply with quote  Mark this post and the followings unread

I have the same latch up issues with the regulators. Definitely the Motorola ones don’t work, some only after shorting out the negative rail. Is there any recalculation on the resistor values to power the vco without the 78L../79L.. ones?
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smokestacksproductions



Joined: Aug 24, 2012
Posts: 18
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PostPosted: Fri Oct 26, 2012 2:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

The regulators are there to provide a reference voltage of 14.2v (this is what you get after voltage drop) so use that figure when you calculate
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Sebo



Joined: Apr 27, 2007
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PostPosted: Fri Oct 26, 2012 5:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think they also provides extra stability to the VCO.
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Pilastik



Joined: May 17, 2010
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PostPosted: Fri Oct 26, 2012 9:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes I know that and when it works is amazing and very stable (LM394 version). I’ll try to find some ST regulators…
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ranioItaly



Joined: Feb 18, 2013
Posts: 13
Location: Italy

PostPosted: Mon Feb 18, 2013 9:29 am    Post subject: same problem with 78L15 Reply with quote  Mark this post and the followings unread

hallo, I am new in this forum, I have built a VCO but at first time the 78L15 puts out less than 1v.
I try to change it with other brand but it is the same.
I read from Cardinen post the possibilty to add a resistor or diode.
Can somebody post the schematic for this modifies?
Many thanks
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Sebo



Joined: Apr 27, 2007
Posts: 534
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PostPosted: Mon Feb 18, 2013 1:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

If your 78L15 put out 1V probably you have a short somewhere.
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ranioItaly



Joined: Feb 18, 2013
Posts: 13
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PostPosted: Tue Feb 19, 2013 8:02 am    Post subject: Reply with quote  Mark this post and the followings unread

thank you for your answer Sebo, but I have missed to tell: if I switch down the power supply twice or three times the 14,2 volt appears and the VCO works normally.
I think the 15Volt (measured with voltmeter) on input of 78L15 isn't enought for start it.
I would try to insert a big value resistor, or a diode, between input and output of 78L15 for put high the 14,2 volt at start-up , but I prefer ask to the user of this forum if is a wrong idea.
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marvkaye



Joined: Mar 14, 2011
Posts: 225
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PostPosted: Tue Feb 19, 2013 8:20 am    Post subject: Reply with quote  Mark this post and the followings unread

The 78L15 needs at least 16.7 input volts for reliable regulation. It would prefer 17.5. See the data sheet.

<marv>
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ranioItaly



Joined: Feb 18, 2013
Posts: 13
Location: Italy

PostPosted: Tue Feb 19, 2013 8:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Marv, thank you.
I understand what you mean, but the Yusynth VCO wants +15v/-15v for supply the board and inside it has one 78L15 and one 79L15 for increasing stability. these is the original schematic and should works fine.
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marvkaye



Joined: Mar 14, 2011
Posts: 225
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PostPosted: Tue Feb 19, 2013 9:34 am    Post subject: Reply with quote  Mark this post and the followings unread

I see what you mean. I read Yves notes regarding them on his website... looks like the 14.2v is crucial to the design. It'll be interesting to hear how you get to the bottom of your issue...

<Marv>
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ranioItaly



Joined: Feb 18, 2013
Posts: 13
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PostPosted: Wed Aug 28, 2013 6:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Hallo, new update after extensive testing:
after trying different brands of regolators, I've found one that does start the VCO , however, the voltage at the 79L15 is -14.3V while the voltage at the 78L15 is just 13.6V.
however the VCO seems to work very well , and since I will use it for fun and not for work I am very satisfied Very Happy

Ciao
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littlem4tt



Joined: Jun 30, 2013
Posts: 40
Location: uk

PostPosted: Wed Oct 30, 2013 10:01 am    Post subject: Reply with quote  Mark this post and the followings unread

damn, i measure the same voltages as you but i still have no outputs.
I thought my problem was the -13.6v was not correct but if yours work with same voltages maybe my problems are elsewhere. Or is it just lucky that yours work.

Any thoughts anyone?
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