Arc voltage oscillating

This is my first post so go easy on me please!

I have built a plasma table and finally up to the part of configuring Plasmac.  This is my first time using LinuxCNC so still getting used to how it all ties together.  So far so good though, the machine moves nicely and I have got as far as completing Plasmac configuration.

I am stuck on a little problem where the arc voltage is unstable.  In fact, it is oscillating like crazy in the HAL scope.  My configuration is:
 - Mesa 7i76e
 - THCAD 10 wired F0+/- pins to ENCA+/- pins.  Analog side connected to PowerMax pins 5/6. Freq/32, unipolar mode

What I tried:
 - Bridging the analog input terminals to ensure it was not oscillating on the input.  No change.
 - Changed the frequency divider jumper.  The amplitude of the oscillation in hal scope changes but not the frequency, so is sort of working!
 - Reading lots of form posts!

I did read tonight that there is a special firmware for the 7i76e for using dual frequency inputs (I have two THCAD, one for ohmic).  I need to work out if I have the right firmware in it.  I think I did install it a few years ago when I got it, but honestly don't remember (cant access the machine tonight).  Is special firmware even required to get the one frequency input going for a start? 

I have attached some pictures and files, please let me know if anything else is needed.  Thankyou in advance.
 

Try removing the green earth wire from the THCAD. I have seen an AC ripple with it connected.
Also, if you go through halshow, and look at your mesa pins, if you have more than one encoder you have the right bit file installed (file name ends in *pl.bit if you need to install it.)

Thanks.

I did try with and without the earth wire but no difference. The top THCAD is the arc voltage one, lower is for ohmic.

I cant get to the machine right now to try hal show (after I work out what that is!), but in the hal scope list, there was only one encoder 00 listed.

I do have the *pl.bit files, downloaded them tonight. So will install tomorrow (after I work out how!).

Hi, some times using a ferrite bead can help, you can make 4-5 turns around a ferrite core, (most times it help with high frequency noise)

 

Putting the *Dpl.bit file in the Mesa board was all that was required. Worked perfectly with correct scaling after that.

I thought I had updated the board in the past but obviously not!