Boss CE-2 help

[nosamiam]

Hopefully someone can get me pointed in the right direction...

A friend of mine has a Boss CE-2 chorus pedal that he has given to me to troubleshoot.  I'm very much in the learning stages of pedal-building and -I've never tried to fix an inoperative pedal, so I really don't know where to start.

The pedal passes signal both when the circuit is engaged and when it's bypassed.  But when it is engaged, there is no chorus effect.  There is a trimpot inside that, from what I can tell, is supposed to vary the amount of effect, but it doesn't do anything all no matter which direction I turn it.

Do I break out the multimeter?  Any suggestions on how to make myself look like a hero in this situation?

[TheBigMan]

You should make yourself an audio probe.  There's been a few threads about it recently, and there's an article at www.geofex.com.  An oscillator like the Geofex one is also very useful to save you trying to play guitar and probe at the same time.  

Q9 is the FET that controls whether or not the wet signal gets passed to the output.  Check that the wet signal is reaching it, if not then the problem lies before that in the circuitry around the BBD chip.  If the wet signal is reaching the FET but not the output jack then either the FET is faulty or poorly biased so it doesn't switch.

[Kent S.]

Hopefully someone can get me pointed in the right direction...

A friend of mine has a Boss CE-2 chorus pedal that he has given to me to troubleshoot.  I'm very much in the learning stages of pedal-building and -I've never tried to fix an inoperative pedal, so I really don't know where to start.

The pedal passes signal both when the circuit is engaged and when it's bypassed.  But when it is engaged, there is no chorus effect.  There is a trimpot inside that, from what I can tell, is supposed to vary the amount of effect, but it doesn't do anything all no matter which direction I turn it.

Do I break out the multimeter?  Any suggestions on how to make myself look like a hero in this situation?

That trimpot doesn't look to be a direct/effect balance control. It looks like it control the gain of the second section of the 4558 opamp, so it looks as though it regualtes the gain of that stage, as the direct is connected to pin 2 of that amp, the source or drain of Q9 (2sk-30A-Y), I forget the pin out of that, connects to that same pin. That trim also looks like it effects the bias of that Q as well as other stages as well (but is not a bias control), my bet is either the 2sk-30 itself, the switching line going to it's gate, or something in the effect path itself ... like the guy before me stated, if you got effect signal before that fet, then you know it's not getting thru. Make sure that gate (switching line ramps to negative 9V)when the pedal switch is depressed back and forth, one state should show -9V and the other state something higher (it drifts a bit).The input is essentially hardwired (more or less) to the output opamp section after some first stage buffering. Anyway, that's were my money is ...

[nosamiam]

Thanks a lot! I'll get on it as soon as I unbend my brain from reading those two replies.  Actually, I must be getting the hang of this stuff because I get the gist of what you guys are saying.

Thanks again! I'm sure I'll get it working.

[Mark Hammer]

(You could be the 1 millionth customer on this one!)

Every BBD chip, hence every BBD-based effect, needs to provide a bias voltage for the signal to "ride" on.  When BBDs are not involved, the bias voltage that most circuits using a single 9v battery will use is simply one half the supply voltage or 4.5v, and this is usually obtained from a pair of equal value resistors that chop the supply voltage in two.

In the case of BBDs, the bias voltage is NOT the "middle".  You can "guesstimate" it and use a pair of selected fixed resistors to provide it, but better results tend to come from using a tripot and more accurately nailing it.  When the bias is wrong, you will tend to hear no output from the BBD when it is biased too high or too low, a distorted output from the BBD when biased just a little too high or too low, and a normal clean delay sound when it is biased just right.  You can do it by ear and probably get close enough.  Note that over or underbiasing will not damage the chip, so tweak away.

As it happens, although the bias voltage needed for the BBD is not the optimum for the rest of the audio path, it is not so far off the beaten path that you can't use it for that.  Many pedals using BBDs will use one bias voltage (+v/2) for the audio path and LFO, and tap the supply voltage with a trimpot for the "special-purpose" bias.  In this instance (CE-2), Boss has used +V/2 for the LFO, but has opted to use one global bias voltage for the entire audio path including BBD.  If you look at the schematic, you'll see a path from the trimpot going to many different stages, each with a different resistor to adjust current.

Bottom line: if the bias is adjusted poorly, you can probably still get "clean" audio, but will not get the delay signal to mix in with it to produce chorus.

[Ansil]

OK one last post, then off too bed.   and i mean this the way i type it.

"READ MARK HAMMERS SITE!!!!!!!!!!!!!!!!!!!!!!!!!!". This site contains nearly every question you could think about when it comes to time based fx.  all the coolest tricks and different mods and explanations of it.  

when i need to know something that i couldnt' find on the board. i just look for marks site or for one of his posts.

of course dont' get me wrong i dont' mean don't ask  just saying that if you do like i did and print off his whole website everything that is there and put it in a 3 ring binder.  (use the 3" once as it will fill all of it. depending on how large you print it.)i have mine now in one 4" folder rather than two 2" ones.  you will undestand a lot quicker on bbd's and such.  

again mark thanks for all your help in all my questions too.