[sdiy] 4069 vco gives v+ noise?

[René Schmitz]

Hi Karl,

> I use the same pinning as in the schedule you sent but it's still 
> giving noise (in the v+ line). The noise seems like a 50hz wave with 
> some minor disturbances riding on it that seems to be the 
> osciallating tone.

One quick test if it is related to line frequency is to
set the scope to trigger on the line frequency. (Or trigger
on the AC output of your transformer, if your scope has none such thing.)

> The minor noise can at high frequencies be eliminated trough adding a
>  10uF cap at the v+ line but I can't reduce the major noise (the 

That is possible. There is likely some dirt introduced when the CMOS 
gates switch.

> probable 50hz) without putting a resistor inbetween ground and gnd 
> pin on the 4069. And that - as i said - makes the square out die.

That naturally reduces the load on the PSU. (Two of the six
gates are used in class A, so they have substantial
quiescent current.) A 100k resistor reduces the current that the 4069
can draw, and also changes all the biassing. That is why the pulse out
doesn't work anymore.

> The odd thing is that the noise only occurs if i add more modules to 
> the same powersupply, for example a ASM lfo. The powersupply should 
> be able to drive both modules (at least i think so) without any 
> distrubances. It's based aroun 78xx, 79xx and 6800uF caps and uses 
> the standard layout found in most powersupplies.

Hmm, 6800uF sounds like a lot. What transformers/rectifiers
do you use? 6800uF would be adequate for a 3amp supply.
You are likely reducing the conduction angle of the rectifier,
if you don't nearly draw that much current. And possibly even
destroying the rectifiers with overcurrent.
Less is sometimes more. And that certainly goes for PSU caps.

All in all this sounds like the transformer has not enough
margin for the drop over the regulators. (Like a 15V type for a 15V PSU....)

I would try to isolate the problem by using a dummy load on your PSU.
Say use a lightbulb or (hi-power) resistor that draws the nominal
current that the PSU is designed to handle. And then you'll see wether
the regulation is ok or not.

Cheers,
  René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159