Ibanez CP-9 Troubleshooting

[Gahoo]

Howdy

I've got an Ibanez CP-9 needing repair on my bench. Signal comes through the pedal both when it's off and when it's on. The Volume control works fine. When I adjust the Threshold control the sound will cut out within some section of the pot's rotation. The Attack control doesn't appear to have any effect on the signal. I'm not getting any compression.

I've checked both the Threshold and Attack pots, both are good.

Here's a schematic that someone drew up. Apparently it has errors but I haven't noticed any. It's a lot cleaner looking than the factory schematic anyway. http://www.cruachan-audio.pwp.blueyonder.co.uk/schems/Ibanez_CP9.gif

And here's the factory schematic http://i.gyazo.com/94e44b418e552813df37292bee369e24.png

So it seems to me that the problem is somewhere between the phase splitter transistor (Q102 below) and the Threshold pot. Here's that area with the transistors labeled. http://i.gyazo.com/ae84f7d442e4307090c1ac82e62282ea.png

I can see DC voltage rising and falling with the Threshold pot if I scope at CP4.

Anyone have any suggestions?

Thanks in advance

[Mark Hammer]

1) If the 10uf cap just ahead of Q105 is shot, then you won't really hear any audible compression because the rectified voltage will drain off too quick or stay level.  Try replacing it and see whatthat does.

2) The circuit uses only half an LM13600.  Those puppies can fry if too much current hits the control pin.  The pedal uses only half of a dual OTA, so try the other unused one and see if that makes a difference.  Alternatively, unsolder it and try a new socketed one.

[Gahoo]

That 10uF cap tested fine, replaced it just in case and still no dice.

Is running the circuit through the other side of this OTA as simple as putting jumpers from one pin to another?

I'm also a little concerned about blowing out the other side of that chip if something else is failing.

I just checked the emitter and collector of Q102 (phase splitter) and noticed that the signals weren't out of phase on my scope. Would this point to Q102 being faulty? And might this explain the lack of compression?

[Gahoo]

Replaced Q102 with a known good transistor. No change in the lack of phase splitting. No change in the Threshold-knob issue.

Is my next move trying the other side of this OTA?

[Mark Hammer]

There is nothing exotic about the chip.  They can be easily found for a modest price.  Maybe one of the other folks here would be able to provide some insight into whether both sides blow at once or whether one side could be defective but the other preserved.

In some respects, the least destructive strategy would be to desolder the chip and replace it.  Indeed, that would be less destructive than what you'd have to do to make use of the other half.  DO NOT expect that running jumpers from the pads on one side to the corresponding pads on the other, will yield normal performance, without disconnecting the pins currently in use.  That's why I say it may be less destructive to gently desolder the chip, clean up the mess (with solder wick, followed by flux cleaner) and replace it.

But just to confirm, before you go any farther, you ARE getting audio signal out of the chip?

[Gahoo]

But just to confirm, before you go any farther, you ARE getting audio signal out of the chip?

Correct, which is part of the reason I was hesitant to think the chip has failed. I get output at pins 8, 9 and 12.

[slacker]

You said adjusting the threshold pot changes the volume that suggests that the LM3700 is working. Whilst playing through it what sort of signal do you get on the emitter of Q105 and the collectors of Q102 and Q103?

[Gahoo]

You said adjusting the threshold pot changes the volume that suggests that the LM3700 is working. Whilst playing through it what sort of signal do you get on the emitter of Q105 and the collectors of Q102 and Q103?

Adjusting the threshold pot does effect the output of the pedal but only within a certain range of the pot's rotation. Or at least it did. Last night I mistakenly left the pedal plugged in and on (no signal running through it) while I was asleep. When I checked it this morning the threshold pot no longer cut out the volume in the range of it's rotation where it used to. Still, I'm not getting any compression.

I don't get any signal on the emitter of Q105 nor the collectors of Q102 and Q103. I measure 9V DC on all three though.