Author Topic: Omnifex Stereo Chorus 704CH  (Read 6105 times)

tca

Omnifex Stereo Chorus 704CH
« on: October 31, 2012, 07:09:54 AM »
Hi,
 I've just found my 20 years Omnifex Stereo Chorus 704CH (forgotten in some box).



It had some wires disconnected, and after fixing that just plug it in. Do you have any experience with it? The chorus is very subtle almost unperceptive but is there! I know that it had been fixed at the time (probably some bad IC) but I can recall what was the problem with it. One can hardly tell the difference when changing the speed and depth controls. Probably is not working as it should (tried to find some info over the web, but nothing useful found). Any thoughts?

Cheers.
"The future is here, it's just not evenly distributed yet." -- William Gibson

Govmnt_Lacky

Re: Omnifex Stereo Chorus 704CH
« Reply #1 on: October 31, 2012, 07:21:07 AM »
Probably is not working as it should (tried to find some info over the web, but nothing useful found). Any thoughts?

If you cannot find any schematics or other info on it, try to post some good, detailed pics of the PCB. Both component and trace side. Then, someone can assist in finding the problem.
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of ‘up to and including my life.’

tca

Re: Omnifex Stereo Chorus 704CH
« Reply #2 on: October 31, 2012, 07:27:21 AM »
If you cannot find any schematics or other info on it, try to post some good, detailed pics of the PCB. Both component and trace side. Then, someone can assist in finding the problem.

I've just PM a DIYSB user (from Portugal btw) that made some mods on this, hopefully he still has the schematic. I'll post some pics later on.

Thanks.

P.S. (edit)

http://www.diystompboxes.com/smfforum/index.php?topic=21615.0
« Last Edit: October 31, 2012, 07:29:05 AM by tca »
"The future is here, it's just not evenly distributed yet." -- William Gibson

tca

Re: Omnifex Stereo Chorus 704CH
« Reply #3 on: October 31, 2012, 07:40:57 AM »
Here are the pics (it s not a trivial design)





Cheers.

P.S. (edit)
I know, check solder points! Just notice that a cap is messing (middle left), near the blue electrolytic and the first pin of the left IC, grrrrr!.
« Last Edit: October 31, 2012, 07:45:35 AM by tca »
"The future is here, it's just not evenly distributed yet." -- William Gibson

tca

Re: Omnifex Stereo Chorus 704CH
« Reply #4 on: October 31, 2012, 08:00:04 AM »
There are some similar circuits. I'll post here what I've found:

- Volz Chorus
- or the Locobox - Cosmochorus
- Small Clone
« Last Edit: October 31, 2012, 01:10:47 PM by tca »
"The future is here, it's just not evenly distributed yet." -- William Gibson

Mark Hammer

Re: Omnifex Stereo Chorus 704CH
« Reply #5 on: October 31, 2012, 07:30:30 PM »
Looks an awful lot like my Washburn SCX-7.  Exact same chips.  You may want to replace the MN3209 (256 stages) with an MN3207 (or equivalent), and crank up the clock frequency for better fidelity.

The lower capacity BBD on the 704CH means it has to be clocked slower in order to achieve the required delay time.  Clocking it slower means the sampling rate is lower and the lowpass filtering has to be lower.  By replacing the 3209 with a 3207 and increasing the clock frequency, you can get the same delay range and sound, but with nticeably better fidelity, less audible clock bleedthrough, and better high frequency response (assuming one also loosens up a bit on the lowpass filtering.

Alternatively, consider increasing the clock speed anyway, WITH the 3209 staying in place, for more of a flanging sound.

Govmnt_Lacky

Re: Omnifex Stereo Chorus 704CH
« Reply #6 on: November 01, 2012, 07:23:51 AM »
Any chance of a reverse on this? Schematic maybe?  ;D

I would attempt myself however, the PCB is too cramped and there is too much flux residue to get a good interpretation  :'(
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of ‘up to and including my life.’

tca

Re: Omnifex Stereo Chorus 704CH
« Reply #7 on: November 02, 2012, 06:29:53 PM »
Any chance of a reverse on this? Schematic maybe?  ;D

I would attempt myself however, the PCB is too cramped and there is too much flux residue to get a good interpretation  :'(
I have a few questions about your sugestion:

1. does it worth it? Why not take the enclosure and build a small clone? It is not easy to reverse engineering that PCB. I don't remember the sound of this Omnifex?!?! Last time I played with it was 23 years ago!
2. are the ICs still available? Are they ease to find (in Europe)?

Thanks.
« Last Edit: November 02, 2012, 06:35:06 PM by tca »
"The future is here, it's just not evenly distributed yet." -- William Gibson