Thumping Phase 90

[Curt]

I really think I'm in effects hell.  I purchased an MXR Phase 90 from Ebay months ago...knew it was broken.  Turns out it is the newest version with the switch and jacks on the PCB.  Polarity protection diode was open, so I replaced it and it fired up...but would not phase.  I noticed it had 1 TL062 and 1 TL064...being the genius I am, I know these are noisy, plus one was probably popped so I replaced both with TL072/74's.  Now I've got some phasing, but hardly noticeable.  When I adjust the bias trim pot, as I get more phasing it thumps (brrrrrrTHUMP) - LOUD- in time with the LFO.  So, I bought about 10 jfets and matched up 4 and replaced them all.  No change.  If I adjust the bias so that it phases, it thumps.  Tried changing the 5.1 volt zenier to 4.7...no phase...back to the 5.1. -and- the thump.
The thump is loud...the phasing seems, well ... in the background.
Anybody have a clue?  
Thanks

[MartyMart]

Hi, my problem didn't run so deep, but my 1977 "script logo" 90  developed a nasty "thumping" in time with the speed control.
I changed out the 5 or 6 ceramic caps, which were very old and can "dry" out if you like, problem was solved almost completely !!
Perhaps that helps ? though yours is much newer !
Cheers, Marty.