MXR Flanger producing chorus-like effect at negative peak

[joelap]

Hey guys,

Got a vintage MXR Flanger, the ones that run off wall voltage.  Its beat up, bought it that way.  Anyways, at the bass-end of the flanging effect, it gives off a weird chorus-like warble.    Almost like when the effect is descending on its way down it descends into a chorus for an instant then kicks back into a flange.  There's three trimpots inside, neither of which I know what they do... two of which i cant move with a screwdriver to adjust either.  Any ideas what could be causing this?  I know nothing about chorus/flange effects.

Thanks,

Joe

[joelap]

bump... any ideas?

[oldschoolanalog]

Does this happen only at slow speed settings, or at any/all speeds?
osa

[oldschoolanalog]

http://i82.photobucket.com/albums/j279/bainzy007/flangerm117_12.jpg
For a better idea of what the trimpots do.
osa

[joelap]

Thanks for the link.

It happens at all speeds, and only when the flange effect is on the bass peak of the effect.   I wish I could record a clip, but I wont be able to do that for another week.

[oskar]

And if you put manual full up/ down what happens then?
I think the chorus effect kicks in when the vco reaches a point where it can go no slower, thus the delayspeed is fixed for a short time.

oskar

[oldschoolanalog]

  Anyways, at the bass-end of the flanging effect, it gives off a weird chorus-like warble.   

By this do you mean a strange "zipper" like thing. Kind of like a quick phase shift within the sweep of the flange? Also, how do the manual & range controls affect this? Is it more pronounced at certain settings? I would rrreally like to hear this. I'm patient. Whenever you can post a soundclip; I'm all ears  . Till then, lets check it out from other angles.
All the Best,
osa

[Mark Hammer]

The three trimpots are:
- ONE 20k triot which one of the schematics I have labels "distortion trim".  This is the bias voltage trimpot that provides a DC voltage the audio needs to ride on.  It is called "distortion trim" because there is a point either side of the correct setting where the delay signal is present but distorted.  IN the middle of that range it will become clean (but still wet).
- ONE 20k trimpot at the output of the SAD1024 that sets the level of the wet signal, essentially providing the wet/dry mix.
- ONE 500k trimpot refered to as "clock trim" which adjusts the overall clock speed range.

The voltage that controls the master clock has essentially two mixing nodes (i.e., points where several sources are combined.  One of them is the wiper of the Width control where the Manual voltage and the LFO voltage are balanced off against each other (more of one equals less of the other), and the other is a point further downstream where the combination of the Manual and LFO voltages is then mixed with the voltage coming from the "clock trim" trimpot.    The clock trim voltage and the combined Manual/LFO voltage, when combined/summed, need to stay within a certain range or esle the clock that steps the BBD chip through its paces will misbehave.  Wy do they need it instead of just building it right?  Because of the number of components involved in producing the clock pulse and the potential consequences of having all those 5% tolerances (and those are the tight ones!) interact.

Seems to me you simply have that trimpot adjusted wrong.

[joelap]

Thanks for the replies.  Upon checking out those schematics and comparing to a layout, the trimpot that I thought was the culprit is the culprit.  I'll give it a go sometime tomorrow and hopefully can get it sounding right.  Or maybe I'll wait till next week and record a sample for inquiring minds.