MXR EF problem

[soggybag]

I just built an MXR EF. It's up and running with one problem. When I hit a note there is a high pitched whistle that follows the envelope. Imagine someone playing the slide whistle along with every note you play.

The whistle follows the envelope. If I watch on the scope I can see a high frequency signal superimposed on the guitar signal. I measured the oscillator and my multimeter clocked this as 35khz.

With the whistle everything is working. I hear the guitar, the envelope and filter is applied to the signal.

Not sure what the problem is. I'm guessing it could be the oscillator leaking into the signal. Not sure if 35khz is out side of hearing or not. It could also be a high Q in the filter make the filter oscillate.

Anyone have any suggestions?

[NPrescott]

I'm afraid I can't help much except to say that 35khz is well beyond the range of hearing (and into radio). Instead of a guitar have you tried using a tone generator? R.G. had one recently updated for a more 'plucked string' effect. It might help to narrow down the tone you're hearing if you've only got a single frequency source.

I'd guess that you're right in thinking the oscillator is leaking into the signal path - but well below 35khz. Can you lower the Q of your filter easily? Check and see if this affects the whistle?

[soggybag]

Thanks for the reply. I think the oscillation might be due to a high Q. I'm looking at the schematic right now trying to figure out what elements might be involved.

[PRR]

Link to schematic?

[StephenGiles]

http://files.muziq.be/schematics/mxr_envelopefilter_1976.jpg

[soggybag]

Thanks for looking at this. I used the GGG board layout. I modified this a bit so that I could cut to of the corners to fit snug against the crew posts in the corners of the box.

I've been thinking how best to figure out this problem I have. The Q I'm guessing is caused by the gain going into the two filter sections, A2a anf A2f on the GGG schematic. Would I be able to control the gain by adjusting A2c R13, R7 and R18?

I'm getting a really great PMW from the oscillator. I looked a this with the scope. So I know that this section is working correctly.