Repair of a Duratool ZD-552 desoldering gun

 

 I've had a Duratool D00674 Solder Station for many years now. Like George Washington's axe which had both new handles and new heads it's had new soldering irons and new desolder guns and even a new box.
In the past I've successfully fitted new heating elements into both tools and recently the desolder gun failed to heat up. New elements were out of stock with my usual supplier and ridiculously expensive elsewhere so I put a couple on back order and ordered a new gun.
After many weeks the new elements arrived and I fitted one into the old gun. Plugging it into the box resulted in the element heating up but the display read only 25C not the target temperature. The 25C display flickered a few times but, when the gun was clearly overheating, I unplugged it and checked the wiring.

The element has five wires. One goes to the metal casing, two slightly thicker wires to the heater and the two thinner to the temperature sensor. Having got a working gun it was easy to confirm the wiring to the repaired gun. The ground wire has a blue sleeve and the other four have white sleeves. Checking the resistances I found the heater was around 1.5 ohms and the sensor wires about half an ohm but also measured around the same value to the metal casing. Both the new gun and the repaired gun were about equal in terms of resistance readings.

Next, I took the second new element and connected it to the gun cable using croc clips. The gun behaved in exactly the same manner so I assumed both new elements were faulty and arranged replacements.

The replacements arrived and I connected the first using croc clips. Much to my surprise it again failed just like the first two. Something was obviously wrong. I'd confirmed the connecting lead was OK and I'd made resistance measurements of the working gun and the new elements but there were no anomalies.

As I'd used croc clips I decided to switch round the two sensor wires. The cable had a blue and a black wire but the element sensor wires were both white. Much to my surprise the gun warmed up normally. The 25C figure gradually rose to the target temperature and then remained steady. Just in case I'd made an error I switched the two croc clips around and got the same fault as before. So, to ensure I connected the wires correctly when crimping them I marked the white sleeve going to the black wire, when working properly, with a black marker pen.

After finishing the job the repaired gun worked perfectly. Presumably the sensor output has a polarised output, perhaps plus and minus some voltage about ground but why on earth didn't the manufacturer use black and blue sleeves to match the cable wires instead of identical white sleeves?

Thinking back to previous gun repairs I must have been lucky and wired the blue and black wires the correct way round?

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