Need Help With Curious Problem On Original MXR Flanger M-117

[Paul Marossy]

Someone wants me to repair an original MXR M-117 Flanger, the one that looks like this:



It apparently developed a rather strange problem: it has a whining sound around middle C. Has anyone heard of this problem, and if so, what the heck was it?! It sounds like some sort of low frequency oscillation, but I'm wondering where that might be coming from.

Schematic: http://www.diyguitarist.com/Schematics/MXR-Flanger-Schem.jpg
Picture of PCB: http://www.diyguitarist.com/Schematics/MXR-Flanger-PCB.jpg

[Mark Hammer]

Does the Resonance/Regen control work properly?

[Fender3D]

Check the 500k "trim clock" trimmer.
If the whining changes its pitch when turning the trim, it may be an issue in the clock area

[Paul Marossy]

Does the Resonance/Regen control work properly?

Don't know, I don't have the pedal in hand yet.

Check the 500k "trim clock" trimmer.
If the whining changes its pitch when turning the trim, it may be an issue in the clock area

Hmm... that sounds like a possiblity. But why would this problem come on suddenly? Component failure maybe?

[oldschoolanalog]

The pic is of the reissue.
The gut shot/schematic is the original.
When you get the pedal let us know which one. I have a bunch of those (117's & 117R) for comparison.

[Fender3D]

Hmm... that sounds like a possiblity. But why would this problem come on suddenly? Component failure maybe?

maybe dust or moisture into trim, or any transistor in the clock path... C21 maybe... is C13 tantalum? Those da** caps always short out when growing old....

[Paul Marossy]

The pic is of the reissue.
The gut shot/schematic is the original.
When you get the pedal let us know which one. I have a bunch of those (117's & 117R) for comparison.

Thanks!

Hmm... that sounds like a possiblity. But why would this problem come on suddenly? Component failure maybe?

maybe dust or moisture into trim, or any transistor in the clock path... C21 maybe... is C13 tantalum? Those da** caps always short out when growing old....

More things to check...

[Paul Marossy]

I have the pedal on my workbench. I fed it a signal with my audio oscillator and it's doing a nasty buzzing, like the DC power is not being filtered at all. The first suspect at this point is the power filter cap being bad. The controls all seem to work fine.

EDIT: Problem fixed! It was the power filter cap. When I took the old one off, I could feel the internals rattling inside. It was quite literally dried up inside.