giz
Ok, this sounds similar to a problem I had recently. I also found an old Lagunamatic VMX500 drawing I had and the 120VAC circuit was not tied to chassis ground on one side. Disconnect both the input power and output wiring leaving the chassis ground connected on both power supplies. Then using an ohm meter check the resistance between each input/output terminal and chassis ground. There should be no low ohms connections. On the input side you may get a high ohms reading because of the input filter. Also check between the input and output terminals, again there should be no low ohms connection. One of the power supplies may have the output 0V or return terminal connected to chassis ground, this is done to help with noise reduction. If this is the case then you will need to either get a new power supply with isolated outputs or find a way to break the 0V/chassis ground connection.
While you have the power supply connections removed it is a good time to check all the DC/AC control circuits for faults to chassis ground or open circuits. Using an ohm meter trace each control circuit and verify it is good. Find all the problems and fix them. If the 120VAC circuit is wired the way the drawing I have shows there should be no connection to chassis ground on either leg. Make sure all DC relays have working (correct polarity) flyback diodes across the coils to protect the Mesa card.
John
