Question for Amos: fried phaser fixes??

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eric coleridge
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Joined: Fri Mar 24, 2006 3:46 am
Location: NYC

Question for Amos: fried phaser fixes??

Post by eric coleridge » Fri Dec 24, 2010 2:51 am

Not long ago, I inserted a control voltage, a positive going envelope, into the 'sweep in' input on my 103 Phaser. I think this CV may have been too high a voltage (maybe as high as +15v perhaps), because it seems to have fried part of the 103. Almost immediately after using the external ENV CV on the sweep, I began noticing a static/distorted quality to the phased tone where there was only clarity before.

I dont have a schematic for the 103, so I can't really say what component I may have overloaded-- I'm hoping that someone reading this, who has a schematic, perhaps, might be able to make an educated guess as to what component(s) I would have damaged with that CV. Theres probably an opamp for the summed 'sweep' voltages-- that may likely have been damaged--but I'm not sure why that would effect the audio signal. Also, I don't know which opamp it would be.


Can anyone suggest a component I might try replacing?

eric coleridge
Posts: 574
Joined: Fri Mar 24, 2006 3:46 am
Location: NYC

Re: Question for Amos: fried phaser fixes??

Post by eric coleridge » Fri Dec 31, 2010 8:44 pm

Anyone? Has anyone had any experience working on a 103 Phaser. It sounds to me like the CV signal is bleeding into the audio chain (I don't know why this should be happening)-- the distortion/static I'm getting seems to be rising and falling with the LFO. I'm guessing ill need to replace a damaged op amp-- but there are so many, and I don't even have a schematic. I thought I might follow the traces that lead from the Sweep Input, and possibly follow it to whatever summing amp/buffer I find. I'm not sure how I would track down the problem, other than just guessing though-- I don't think I'll find a short. Maybe I could put a sine wave through the circuit and look for where it starts to short.

Any suggestions? I really want to fix this problem.

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latigid on
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Re: Question for Amos: fried phaser fixes??

Post by latigid on » Sat Jan 01, 2011 2:15 am

I'm no tech, but I wonder if a "high" voltage transient may have blown one of the capacitors on the 103? The ones near the power input are 15 V, some others are 25 or 50. If any of these are involved in filtering CV, a damaged cap could reek a bit of havoc.

You can test caps in-circuit with a multimeter set to resistance. Short circuit isn't good. A good cap should lose charge slowly i.e. resistance should increase. If the resistance is always "high" i.e. open circuit, that isn't good either.

I think that's right, but anyone feel free to correct me. ;)

Worth a little prodding with a meter anyhow.


Oh, did you email Moog at all?

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