Note: This is a guest post by David, a friend of the blog owners
Today I got in a Metabo E150 for repair. This machine is a 150A STICK/TIG welder and stopped operating for some unknown reason. On power up it didn't show any reaction at all and it is not possible to esteblish a welding arc neither in STICK nor in TIG mode. Looked like a blown fuse at first glance but I couldn't find any fuse in the mains input line. However I found a 16A relay in the mains circuit which could cut off some part of the main power line but it looked and worked fine. The next to check was if the unit draws any current at all and it did, so it looked like some control logic on the mainboard was operating.
After further inspection I saw that the fault LED on the front was flashing in a very strange dim way. The manual states that the LED is lit on line over/under volatge situation or if there is an over temperature error. Could be a possible bad temperature sensor because the line voltage should be fine. The temperature sensor was easy to locate on the heatsink. After dismounting I checked its value. It was 10kOhm on ambient temperature which matched the printed value on the device. After heating it up with a lighter it was 8.5kOhm. Seems to be working fine despite it being reported by the unit.
There are only some misc parts on the power board which include a optopcoupler (LOC111), basic low power operational amplifiers (MC33171P) and some ultra fast switching high voltage diodes (STTA3006PI) which are not likely the cause of the problem. The more interesting parts are the fast recovery epitaxial diodes (DSEK60-02A) and the low ESR capacitors. Inspecting the diodes showed no suspicious blast, also measuring showed a reasonable voltage drop. Next up were the capacitors which also showed no suspicious bulges nor a high ESR value.
The main control board has only some basic parts on it. It does some basic voltage regulation with a 78L05 and a MC7815C, has some basic comperators (LM339) and a current mode PWM controller (UC2844) on it. The main micro controller is a PIC16F676. Rather interesting is a three way resistor selector for their different models. So it may be possible to hack the unit to a different amperage model, although they might also switch out the main transformer aswell.
Speaking about the transformer, I have not checked that one. Luckily I did after all. One of the windings connection came loose because of a bad cold solder joint. The cold solder joint itself was invisible to spot but the wire going up to the transformer had some dark coloring on it which probably accrued from arcing over the gap. Looks like the unit was right on the under voltage fault. It was quite tedious to resolder the wire because the extremly large traces suck away the applied heat so quickly. Maybe the solder joint was bad since production and finally came loose due to some shock. However resoldering fixed the whole problem. The fault LED stayed off and the green LED came on.
The welding performance of the unit is pretty good for a 150A machine. The machine gives you a 60% duty cycle at 110A in STICK mode or 130A in TIG mode. Continuous power output is 90A in STICK mode and 110A in TIG mode which isn't bad for such a compact welder. The compact size makes it also very esay to carry around. The power cable is a bit annoying for storage that's why I mounted a small hook on the side of the machine to wind up the power cable for storage. Finally it also got a new blue spray paint. Looks and works like a bought one.