Fixing a Systech Flanger?

[PrettySuite]

I'm not sure I'm posting to the right forum so my apologies if I'm not.

I recently purchased a Systech Flanger from a dishonest individual on eBay.  The main issue at the moment is a "clicking" that goes in time with the modulation rate.  Does anyone have one of these?  Is that normal?  If not, how would I go about finding the source of the problem.  It looks awesome (though I've discovered the knobs aren't orig...going to throw some red ones on there instead) but I want to have it in fighting shape for recording.

Thanks for any help,

Brandon

(FYI...my tech knowledge is fledgling...my friend will be helping me through it)

[MikeH]

Are there even schematics floating around for these pedals?  I've never seen one.  I'd assume it uses an LFO like other flangers(?), so what causes an LFO to make ticking noises?  Anyone know?

[analogguru]

guts, guts, guts... we want guts 

analogguru

[DanMcCullough]

I have both Systech 4000A and 7012 flangers.  I have repaired both.  I have schematics for both.  These are IMO the best rack-mount analog flangers available for multi-purpose studio use.  I would be happy to repair your's for a modest fee.

Best regards,

Dan

[Mark Hammer]

That 7012 looks pretty sweet!  I can see why you commend it.

LFO ticking is a fairly common problem with a great many modulation effects - tremolo, flanger, chorus, phase shifter, you name it - and you can find no end of threads here complaining about it and discussing how to ameliorate it.

The ticking is most often due to the sudden current draw from an LFO that begins with a square wave and then transforms it into something less square (more triangle-like or sine-like).  The instantaneous current draw for a sharp rising edge of the square wave puts a spike on the power line shared with the audio path, which we hear as a tick.

There are essentially 3 things that people and companies have done to minimize audible tick:

1) Use an op-amp type for the LFO that draws very little current to do what it has to do.  That's why you'll often see an LM358 or TL022 used in many LFOs for modulation effects.

2) Decouple the LFO op-amp/s from the main power line.  This can be something simple like a 10R resistor between V+ and the V+ pin of the op-amp chip, and a small-to-medium value electrolytic cap (2u2 to 10uf) from that v+ pin to ground to provide a sort of "reserve supply" for the LFO to draw from without affecting the rest of the circuit too much.

3) Modify the LFO circuit design, such that it produces something more trapezoidal at the outset, rather than purely square.  By creating a non-instantaneous rise time, that distributes the current draw over a brief interval, such that no sudden spike is generated.  That strategy is explained here: http://www.moosapotamus.net/IDEAS/stompboxology/Vol4No4/Page_02.gif, and is illustrated nicely in the Boss BF-2 flanger here: http://www.freeinfosociety.com/media/images/677.gif

[DanMcCullough]

Excellent suggestions Mark...  I have an original 18V Electro Harmonix Electric Mistress with the LFO "Tick" problem.  I will try the decoupling method you described.  I knew what was causing it but not quite how to solve it.  Much appreciated.

[StephenGiles]

I also found that increasing the 33uf decoupling electrolytic to 220uf helped considerably to remove the ticking.