Vintage Synth Repair

Mark-

yes Pin 10 is the output (well, the final stage of 4)

Here's a block diagram of the P-One filter:

http://www.electricdruid.net/images/cem3320/larger/ProOneCEM3320Filter.gif


Here's some ideas-
First make sure the chip is getting power +15V at pin 14, -15v (about) at pin 13


Put a 1Khz ( 2 octaves above middle C) square wave into the filter, turn resonance to min., filter cutoff to max. Are you seeing a (sort of) square wave with slightly rounded corners at Pin 10? Note that there are phase shifts being applied at each filter stage so the wave isn't exactly shaped as it went into the filter, ( but it would sound the same if not swept by the EG.)

Continue watching the signal at pin 10 and rotate the cutoff knob CCW . Is the signal getting "rounder"? If so the filter is working, and the signal is getting at least that far in the chain.

If not check each filter section (pin 7, pin 6, pin 15) and see if the signal is getting to each stage. If not check for bad solder joints, bad resistors, etc, and the 150p caps at pins 5,4,16, 11. 

Also the 2.2uF DC blocking electrolytic C143 might be bad is the filter is working as expected.

Seeing any signal at pin 7 of U110?


--- On Mon, 5/9/11, Mark <casiotone1331@...> wrote:

From: Mark <casiotone1331@...>
Subject: [vintagesynthrepair] Sequential Circuits Pro One schematic help
To: vintagesynthrepair@yahoogroups.com
Date: Monday, May 9, 2011, 10:43 AM







 



  


    
      
      
      I recently posted about my nonfunctional Pro One. I'm getting nothing out of the output, except for some noise.



Anyway, the transformer is ok, and the PSU caps are brand spanking new. I also replaced all the regulators, so there's no problem there.



I got it hooked up to an oscilloscope and tested all the oscillators. Good news is I'm getting saw, square and triangle perfectly. All the functions work including the envelope and pulse width. I've narrowed it down to a critical point in the board.



The CEM3320 filter IC is being fed with both OSC A and OSC B through pin 1. When I put my oscilloscope leads on pin 1 of CEM3320 and ground, I get a perfect reading of whatever wave I am pushing through it. I've done all my testing on this pin so far, and the synth appears to be completely functional.



Here's where it gets weird. (I'm not really hugely experienced in bench diagnostics, I only have the o-scope to calibrate the few YuSynth modules I've built, so diagnosing isn't my strong suit, so your help is appreciated).



According to the schematic, pin 10 is supposed to be outputting everything plus the filtered signal (am I wrong?). Here is a pic of where in the schemos I'm at:



http://i483.photobucket.com/albums/rr195/casiotone1331/SequentialCircuitsPro-OneSchematic2of2.jpg



When I probe pin 10 I get an odd wave with a slight curve, nothing short of key presses or cutoff and resonance adjustments change the wave at all.



Does this mean my VCF IC is bad? Hope not! I might try to change the caps or check the resistors. What do you guys think?



Mark