VCLFO ~ Ramp/Saw, Square not working

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

Hi,
The VCLFO is my first MFOS build.

Sine & Triangle outputs work just fine. CV inputs, Coarse & Fine all work properly. As does the LED.

However, the Saw & Ramp outputs sit at an alarming, steady +10.5V and -10.5V respectively, and the Square output stays at +4V.

I've wired up an attenuator (100k) to the PWM input (wiper to PWVX, CW to socket, CWW to GND).

I am a relative beginner, and not very well experienced at debugging (yet)... Where should I start looking ? I suppose I should concentrate on U5 & U6, so what voltages should I see at which pins ?

I've already checked for the typical basic-level debugging stuff like shorts, bad solder points, etc...

tia !
_guy

slacker · Posted: Mon May 31, 2010 4:25 am Post subject:

The first thing I would do is check that Q4 and Q5 are in the correct way round. If they were backwards that would stop any signal getting to U5 and U6.

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

After a lot of head-scratching, I found the problem.

A faulty TL082 (U5).
This happened before with the TL07/8 series.

I've got two VCLFO's happily pulsating now...

_g