Gyraff P-EQ buzz on Lo Cut

[substitute]

I'm getting some pretty bad buzz on the Lo Cut pot on my Gyraff PE-Q, here's the basics...

- It's in a 3U case with a don audio front panel, so all the switches and pots are wired off board.
- Unit sounds good in bypass
- Lo Boost, Hi Boost, Hi Cut all behave normally

When I turn up the Lo Cut pot I buzz (not hum).  The buzz is worse the higher the frequency selected, I'm guessing this just make it more audible as I move from 20hz to 100hz.

So far I've, removed the pot connections from the PCB thinking the off board wiring might be picking up some buzz.  Normally this pot is wired as a 100k variable resistor, when I jumper across the PCB pads creating the effect of the pot being at 0 resistance there is no buzz.  When I place a 100k resistor across the pads, I get the buzz.

What should I be looking at?  How can I further isolate the problem?

Thanks!

http://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif
http://www.gyraf.dk/gy_pd/pultec/gy_pd.pdf

 

[gyraf]

There's a long help thread already - iirc, we've seen this before.

http://groupdiy.com/index.php?topic=37837.0

Meta: http://groupdiy.com/index.php?topic=37850

Jakob E.

 

[substitute]

Thanks Jakob, plenty of reading ahead of me this weekend. 

Right now I'm thinking that the problem is from the second PT.  I noticed it was buzzing, it seems very minor but maybe it's coupling into the audio circuit.  It seems to be buzzing at the same frequency too.

I don't have an appropriate 6:110 transformer on hand so I'll have to order one and report back.