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Hi. I am fixing up my friends old MXR OMNI rackmount effect box. Aside from usual interconnect problems, all effects function except for the chorus/flanger. I hear the gain increase of the guitar when i enable the chorus or flanger block (push button swithhes between chorus or flange), but there is no effect. Can someone advise me of a starting point to debug this. I don't have a schematic for the OMNI, which is my first problem. the fact that both modes don't work yet there is signal passthru indicates a common element... maybe there is a clock signal to check for? thanks for any help.
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The first thing to always check with a nonfunctional BBD-based effect is the bias. The BBD requires the audio signal to be riding in on a bias voltage (which will vary depending on BBD type used). Slightly higher or lower than the optimum bias and you get weak and distorted effect signal. Even higher or lower, and the wet signal disappears completely. You can happily play with the bias trimpot without fear of damaging anything, although you probably want to take notes about where the sweet spot is.
Likely to be located near the HF clock (which will be an MN3101/3102, or CD4046/4047) or as part of the lowpass filtering just ahead of the BBD. -
I own two of these and Mark is right, check the clock to the SAD512 or R5106. The units I have use these types of BBDs.
Does the Chorus/Flanger LED on the footswitch come on when pressed?
As you probably have seen, these units have lots of mechanical problems like jacks that are worn out and pots that are scratchy or are ready to fail.
Post back with your findings, I'm sure we can get your OMNI up and running.
CJComment
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Thanks for the tip. Yes the led comes on and ther is nominal gain in the stage but no effect. It has the sad1024 chip and the 571. i'll drag out the scope later and let you know what I see... thanksComment
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Slightly higher or lower than the optimum bias and you get weak and distorted effect signal.
Yes i found the trim pots broken from the wiper to the resistive element, so this will create a Open/break or operating DC bias in the signal path which will cause no effectComment
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