[sdiy] lfo

[TheMysticSatin]

thanks for the reply, this is so weird!  All of a sudden while iwas
following the current path with the meter leads and checking the voltages at
all different places it started to oscillate.  I built four and one by one
they started to oscillate, the fourth one wouldn't at first but after fixing
a small connection problem, it too began to oscillate.  So i was pretty
psyched so i hooked up the LED's and mounted the lfo's in a panel.  Now only
two of them work and the voltages I'm measuring are similar to what I was
getting before - either -14 or +5 on the out put of the square producing op
amp and small <1v voltages elsewhere or nothing anywhere except at the
supply inputs.  Here is my question, these are 4 lfo's built around two
tl084's, am I overtaxing the op amp by running two oscillators off the same
chip?  the op amps are pretty hot but I can't tell if that's because there's
a connection problem I can't find or because that's normal or because of the
above, any way any suggestions or troubleshooting advice for this circuit
would be appreciated thanks, 
Gavin 
----------
>From: Jim Patchell <patchell at silcom.com>
>To: TheMysticSatin <elmystico at earthlink.net>
>Subject: Re: [sdiy] lfo
>Date: Wed, Oct 24, 2001, 3:13 PM
>

>    It is always tough to diagnose a problem like this remotely...but, you are
>right, this is a very simple circuit...(I am looking at the ASM-1 schematic).
>The most likely mistake is how you have U1A connected (the comparator).  Make
>sure you have the +/- inputs of that amplifier connected as shown.  The feedback
>should be from the output to the + input of the amplifier (positive feedback),
>and conversely, U1B the feedback,through C1 goes from the output to the - input
>of the amplifier (negative feedback).
>
>    Also, are your power supplies connected up correctly?  For a TL082 (or
>TL072), +15 to pin 8, -15 to pin 4...I know this may seem basic, but remember, I
>can't see what you have built...for this circuit, power supply bypassing and
>compoent layout will not be at all critical...in other words, if every thing is
>connected correctly, it should work.
>
>    There is also the posibility the op-amp is bad...pop a new one in the socket
>if you can't find anything else (I hope you used a socket :-) ).
>
>    Sorry I couldn't be of much more help...
>
>    -Jim
>
>
>TheMysticSatin wrote:
>
>> I'm building an lfo out of electronotes that is pretty much identical to the
>> asm lfo.  I've wired it up and, not to pose the stupidest point in the world
>> but, it doesn't work.  I can never measure any voltage on the triangle outs
>> and the square out produce all kinds of voltages based on the settings of
>> the pot but none of them are useful.  occasionally i can measure oscillation
>> but it is not controlled in frequency by the pot it just appears at certain
>> pot settings and is not really consistent with the circuits purpose,
>> occasionally it is close to supply (15v) in its amplitude.  The thing i
>> can't get my head around is that this is such a simple circuit it doesn't
>> seem like anything can go wrong.  The supply is bypass capacitored and all
>> and geez, what else is there?  Is the distance of the cap from the op amp an
>> issue?  or is there something that needs do be done that i just don't know
>> about? any leads much appreciated.
>> gavin
>