VCO CEM problems.

[Paul Schreiber]

Have to take cat to vet. Here are 2 quick things to check.

1) LF347 is the most sensitive, spastic op amp ever invented. CMRR is awful, has loop stability problems.
Try subbing a TL084/74. The company I used to work for recalled all equipment with LF347s and swapped
them all out! They are also "batch to batch" twitchy.

2) Look at CEM3340 data sheet. Notice there is a reference current set resistor Rr on pin 15 to the positive
supply. Now, if this 'supply' moves, the VCO moves as well (which is why my designs use a constant-current
source instead). Perhaps trash is coming in through pin 15. This will definitely cause headaches.

Let me know! Off with the feline!

Paul S.

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From: 	Ric Miller[SMTP:rmiller at pangea.ca]
Sent: 	Wednesday, May 14, 1997 5:48 PM
To: 	Paul Schreiber
Subject: 	Re: VCO CEM problems.

> From: Paul Schreiber <synth1 at airmail.net>
> To: 'BJ' <zzynt at algonet.se>; 'synth diy general post'
<synth-diy at horus.sara.nl>
> Subject: RE: VCO CEM problems.
> Date: Wednesday, May 14, 1997 5:05 AM
> 
> Greetings from the CEM Master!
> 
> Are you saying that if you connect the LFO board and the VCO board to the
supply, it causes the VCOs to drift? 
> If you removed the LFO board, the drift stops?

Exactly.  This is the problem that I'd posted a question about, 2 DYI, about a
month or so ago.  It doesn't matter WHICH of my LFO types it is (Sine, Variable
Symmetry or VCLFO), each one of them causes the VCOs to change pitch by a few
noticable cents, when the LFO switches to the Positive half of it's voltage
travel.  This has no difference, with the LFO speed, or output amount of the
LFO, as determined by the output pots.

But .... this problem has ONLY been happening on my Condor power supply!  It
NEVER happened on my original GFC/Teccor/Hammond power supply, that I'll have
back next month.  :(  

I've tested for improper grounding between the modules in question; I've tested
the output voltage from the Condor; I've added 0.1uf caps to the pins of the
LF347 quad opamp that is the main portion of the LFOs ..... and still, I have
the problem.

Any thoughts?

Take care,
Ric