Weller AG 700 Heißluftstation Repair

Published 2026-04-11

I recently bought a Weller AG 700 Heißluftstation (“hot air station”) second hand (Figure 1). This is a particularly old model of hot air reflow station. So old in fact, that I haven’t been able to find a definitive record of just how old it is. The Weller tools discontinued products list¹ does not include it, and it is mentioned in forum posts^{2 3} from 2005 by a user named “Frank”, as well as an arcade site from 2003⁴.

“Frank” and I share a common experience with our second hand Weller AG 700 purchases, as have other Weller AG 700 users⁵. While heating, the green LED above the temperature control knob flickers irregularly and the “hot air” is closer to “warm air”. In my case, this rendered the hot air station practically unusable unless set to the maximum temperature and air flow setting.

My cursory research into this issue led me to the advice from “gb”³ that this type of issue can usually be traced to the TRIAC or temperature sensor. Tracing back the wires from the heater in the hot air station’s pen led me to the front most PCB (Figure 2). On the parts diagram⁴, this PCB is marked as “Circuit Board-Heating”.

There are two trimpots on this circuit, presumably set in place during a calibration procedure as a part of manufacturing of the hot air station. Small tweaks of either potentiometer change the behaviour of the heater which are evidenced by changes of the temperature of the air exiting the heater and the glow of the heater coil.

I manually adjusted the two trimpots until the behaviour of hot air station matched my expectations. Since there is no documentation of this circuitry available and I am wary of probing an unknown circuit, I did this by trial and error with the hot air station de-engergised whilst I made adjustments. Only small (~10 degree) movements of the trimpots were required to achieve desirable behaviour, larger movements tended the device to driving the heater fully on or fully off regardless of temperature.

Further research I completed after effecting the repair uncovered a blog post by Thomas Reitboeck⁶ in which they repair another Weller AG 700 with a similar guess-and-check repair. Since the complaints of “Frank”³ and “Marco69”⁵ are similar to those of Thomas Reitboeck⁶ and I, it is possible that a similar repair could have been applied to their devices. Presumably this issue is caused by drift of the trimpot setting over time or other degredation of the circuitry around them.

As a next step, I would like to reverse engineer the heating circuitry in the Weller AG 700 to infer the purpose of the two trimpots. This should be achievable due to the 2-layer design, but the unmarked part in the centre of Figure 2 may prove troublesome. Considering the surrounding circuit, the unmarked part could be an op-amp or otherwise involved in driving the TRIAC on the right. Hopefully, the circuit context will tell.

Footnotes