Hi,
I don't know about your make of diathermy but some units that I've worked on have a thermal sensor on the main heatsink for the power transistors. This causes the diathermy to shut down if it's used too long on load. Are you testing on load and is the unit completely shutting down? If so, then I agree that this could be a problem with the mains circuit, leads, plug, fuses or supply. If not, as Geoff says go around the ciruit with the old freezer spray. Also, I suggest if the system comes back after cooling use a hairdryert or heatgun (with a small diffuser) on the suspect component. This is a very useful approach with components like power transistors etc as it definately proves by cooling and heating that the suspected component has a thermal problem. You can also use your
soldering iron to heat the component after cooling but you need to be careful as the bit is usually "earthed" and can cause shorting problems if you touch the wrong part of the circuit. By the way, be very careful if you use a heatgun as they have very high air flow temperatures (up to 400 degC) that's why you need to use one with a diffuser fitted.
Best of luck, you've probably found the fault by now.
Ian
