Weller 6966 hot air gun repair - ‘interesting’ design !

[Fraser]

I recently purchased a ‘customer return’ Weller 6966 hot air gun to see if I could resurrect it from the grave. I was expecting an open circuit thermal fuse but found something more unusual. Upon testing the unit the fault was no motor operation but the heater still worked. An unusual situation and thankfully the heater did not burn-out.

I had a spare few minutes today to investigate the fault and got a bit of a surprise when I opened the heat gun to examine its internals.

I reverse engineered the wiring and components of the heat gun and it is simple but of somewhat unusual design. No PCB’s, just point to point wiring with a mix of crimped connections. Solder ‘lands’ were created using brass eyelets in the fans ABS plastic casing ! It is all a bit Heath Robinson and, whilst it is likely safe, it triggers my ‘good design and engineering practices’ OCD somewhat.

The fault turned out to be the resistor that is mounted inside the fan snail shell housing. One leg was dry jointed and had disconnected from the brass eyelet. It is clever to use the fan to cool the resistor in this way, but the whole design makes me think no one has reviewed it since it was likely created in the 1970’s ! These Weller 6966 heat guns can still be bought new and are not cheap at over £200.

I will happily use this heat gun as it is safe, but she sure is an ugly beast inside that casing.

I attach some photos for interest and will also include the schematic that I found after reverse engineering my particular model. Sorry about the picture quality, I was not intending them to be used on this forum but thought they might be of interest.

Fraser

[Fraser]

The schematic and case layout diagram.

The resistor that had effectively become open circuit was the 850 Ohm ceramic that is mounted inside the fan housing and connects to the Neutral (Blue) side of the mains lead via a thermal fuse. One leg of the resistor was a dry solder joint and it would appear that the heat gun was dropped, dislodging the large resistor from the brass eyelet connection point. The user manual warns that soldering to the brass eyelets must be done with care as the ABS plastic around them melts !

Fraser