A problem sharing a power supply

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Hello all,

I'm having trouble sharing a power supply between a MCU and a VFD
display driver circuit.

The design uses an AVR and a high voltage Supertex shift register to
manually drive a VFD display. The VFD power requirements of 4v AC for
the filament and 35v DC for segments/grids are met by what I think is a
SMPS (this is an old coleco style tabletop game that ran off 6v DC, I'm
bypassing the onboard MCU and just using the power supplies).

This setup works great if I run the shift register and AVR from one
power supply, and the VFD power board from another power supply (both
wallwarts). But when I try and run all three from the same supply,
something bad happens. The AVR and shift reg just will not work, the
display will light but in a way that looks like it has a ground loop or
something, and I have no control over the display etc. I've burnt out
the HV-in to one of my shift regs trying this.

I've tried several VFD supplies from similar games, checked that I have
sufficient current, made sure everything is getting the right voltage
and tried several different main power supplies all with the same
effect.

Can anyone speculate what is causing this? Can I isolate the VFD
supply in some way? Any suggestions?

Thanks!