YUSYNTH VCO strange startup behavior

[smokestacksproductions]

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?

[Sebo]

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

[smokestacksproductions]

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

[Sebo]

I mean the 79L15 in the VCO itself.

[smokestacksproductions]

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?

[prgdeltablues]

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

Peter

[Sebo]

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

[smokestacksproductions]

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" ?

[Sebo]

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

[smokestacksproductions]

That's ok,

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

Thanks!

[Cardinen]

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.

[yusynth]

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

[smokestacksproductions]

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:

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Description:	this psu provides bipolar 5v and bipolar 15v
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[Pilastik]

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?

[smokestacksproductions]

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

[Sebo]

I think they also provides extra stability to the VCO.

[Pilastik]

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

[ranioItaly]

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

[Sebo]

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

[ranioItaly]

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.

[marvkaye]

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

<marv>

[ranioItaly]

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.

[marvkaye]

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>

[ranioItaly]

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

Ciao

[littlem4tt]

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?