EBME Forums Forums Maintenance Technical Assistance TAPSI short wave diathermy

Hallo
Any suggestion on what could cause the machine to go off by itself and come ON later by itself,I've tried soldering afresh but still the problem persists

What sort of condition is your building electrical system in? Any interruptions going on? Are they using trailing mains leads or extensions? These are the sort of areas where I would start looking.

African,

As Geoff says, check the hospital power situation, however, any problems here would probably show themselves in other equipment plugged in to mains sockets in the same room or area. If the wall socket you are plugged into has a second socket right next to it, then plug something into this (like a light) at the same time as you (or the operators) are using the diathermy. Kepping both appliances on you can see if you are losing mains power to the socket itself or just to the diathermy. Plug the diathermy into another wall outlet in the same room or better still, another room to test it elsewhere.

Assuming you have already done this, then the next step would be to start at the wall socket and work your way back to the diathermy. Get the maintenance to check the socket outlet itself into which the diathermy is plugged. Bad contacts, loose screws holding the wires or a bad switch could create this fault. (Thes things should be checked on any extension leads used that Geoff mentioned).

Next, if it is a british square-pin mains plug on the diathermy, check the tightness of the fuse inside the plug-top. In any event, check the screws holding the mains wires in the plug-top for tightness. If that is ok then switch on the diathermy and flex the mains cable all the way from the plug to the entrance of the cable into the machine. I have known in my time at least three mains cables that have had what I call a 'natural' fracture in their length (fractures not caused by equipment running over the mains cable to damage them). Tricky to find but by carefully flexing you will find the fracture.

It could easily be that the cable restraining grommet at the entrance to the machine has caused the wires inside the cable to break at this point alone. The intermittency of the fault being due to a high resistances developing over time with internal arcing at the fracture-point.

If the machine stays on all the time you have been flexing the cable then go into the machine and check the mains-switch contacts are clean and working well and also the fuse in it's holder.

If after all this, all looks ok, then it would indicate that the problem lies within the machine itself. Good luck.

… and then it will be time to get out the freezer aerosol, as the fault sounds suspiciously like heat-related (ie, unit fails when it starts to get warm, and comes back on as it cools down again). Have fun!

If you still have one of the older types of short-wave diathermy, you may find that it has an in-the-mains-line temperature sensor somewhere. Try looking in the mains transformer winding area. Usually just under the top layer of insulating paper that divides the layers of wire in the transformer itself. When the temperature rises in a fault condition ie current over-loading somewhere, the system will shut down and when cool reconnect again. Modern machines have thermal fuses that rupture and that is that.

As Geoff says, try "freezer" spray on the electronics. If you have not used this before, find a nearby electronics shop (say Maplin, Radio shack or Radio Spares etc) buy a can of spray and just squirt briefly things like transistors and ICs one at a time to see if the system "wakes up" again.

Bit Alzani,

I would find it nice to know if our comments were of any help and I am sure that others would be interested to know if you have found the answer to your problem, if you could find the time to add a reply it would be appreciated!

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