Neve VR EQ sounds/acts like an oscillator.

O.k. this is a good one.  Ever since we got our vr's at work a couple of years ago they have been a tech nightmare. Requiring all kinds of work. A new thing has come up where the eq when inserted produces a sign wave  and the HF pot for changing frequency causes the frequency to change when you adjust it. Might as well pick the brains around here as to what to look for first before pulling out the schematics

look at how  oscillators and active resonant filters work ;-)

good luck

jensenmann said:

look at how  oscillators and active resonant filters work ;-)

good luck

Click to expand...

yeah. I assume it's part failure or something making a connection that should not make a connection. It will take a while but will get to the bottom of it. FWIW the vr is over engineered and sucks wet  donkey balls

You might want to look at the compensating caps around the opamps.  I've ran across some board rev.'s that tacked small ceramic caps to component legs (even traces) on the bottom of the PCB.  In some cases, I've found broken traces or solder joints around these causing the opamps to oscillate. 

Good luck.

Will do. thanks for that. but I have to say they do not oscillate but rather produce a sine wave that the frequency/pitch gets controlled by the frequency pot on the HF part of the eq. Will check for that though.  Our 2 SSL's are not a problem. SSL's are well laid out and easy to tech. the vr's not so much.

I'm not certain of the topology of the Neve EQ, but I have seen oscillation problems in resonator-based EQs caused by the wiper contacts in frequency pots going high resistance as the pot got older.

The Neve VR is pretty notorious for the pots going bad due to all the heat in there, so I wouldn't be surprised if this was the problem you're seeing.

How old is the desk? Has it ever been re-capped?
Power decoupling caps - there is a row of them, 4 or 5 pairs of 33uF on the EV11491 main EQ card. If they have dried out they could be the problem.

desk is old. between the one in storage and the 2 in use there are 144 channels that have been recapped.  The former AMS/neve tech said only to recap the 10UF and 100UF caps. But been doing those as well.

I bet on caps I had this a while ago; It was dried caps that caused this (100U) if my memory is good..

Nobru said:

I bet on caps I had this a while ago; It was dried caps that caused this (100U) if my memory is good..

Click to expand...

o.k. but this is happening on a module that  just had every cap replaced. What's interesring is that if I hit the module with the screw that holds the 2 pcb's together, it goes away. I am wondering if the ribbon cable between the 2 has a short.

when recapping that era neve, i use 120uf/16v FM or HE caps.

much better ESR than smaller values

i also thought about the wiper loosing contact, but that would cause
quite high gain at the chosen freq...more of a howl.

Even more interesting. If I engage the HF shelf, the oscillator goes away and it behaves like the standard HF shelf. so it's only in  parametric mode that it oscillates.

hmmmmmm.

ooohhh  i saw that in an 8248, that was some years ago, i don't remember the fix.

if it has been recapped already, look for solder schmootz shorting sumthin'.

I mentioned this to a colleague who has spent the last couple of years looking after a Neve VR. He said that he had come across a similar problem that could be solved in the short term by hitting the pot. A more long term fix was replacing the pot...

timshel! said:

looking after a Neve VR... by hitting the pot.

Click to expand...

;D  ;D

yes!  if the bottom of the pot is not loosening, as they do,
        hit the solder pads of 'em with an iron

old thread but the cause of said oscillations was found as well as a solution. Long story short, the new capacitors were slightly larger in size then the old caps. one cap in particular had a lead that was touching up against a resistor.  the moment I saw that and the moment u got those a little seperated. The eq sounded perfect and I will dare say like new as it was a recap job. thanks for all the help and suggestions guys. I luv this place. 8)