My solder station, a WESD51 had become increasingly flaky over the last year – at times, it would fail to turn on. Flip the switch up and down a few times, and the digits would light up and the iron would heat; flip a few more and it would remain inert. Sometimes it would work immediately, sometimes it would just not turn on at all. Every time I took it apart, it would trick me by eventually working again, only to fail when reassembled. To compress months of annoyance into one sentence: I seem to have fixed it by soldering some capacitors onto the oscillator crystal. Note added after the fact: Nope this didn’t fix it. See next post for grousing.
Generally, I like Weller irons. I had used the analog version of this model (i.e., no LED digits, just a knob to set temperature) for a couple years. During that time, I had sunk some funds into buying a variety of tips from teeny screwdriver up to the broad chisel that I use for soldering coax plugs. So, when it came time to move overseas, I found a 230V version online and ordered it. I had considered just putting the US model on a transformer, but thought it better to have a model directly grounded through its plug.
At first, the solder station worked great, but after a few months it was difficult to activate. My first thought was the plug – was the adapter loose? No. How about the iron? Online advice often blames the iron and sends users to check continuity of the heating element. Nope, this phenomena happened whether the iron’s cord was plugged into the base unit or not – it looked like some kind of “boot up” issue.
Because of the transient nature of the problem, I thought it might be mechanical. I tightened the power cord connections to the transformer and replaced a wire nut on the one of the transformer leads (probably there to make it easier to replace the transformer or to use different ones for different voltage models). Again, I fooled myself because it worked before reassembling, but once screwed together, failed.
I went back to the main board and poked around with an insulated stick. Nope, nothing loose. Moving now into the real realm of troubleshoot, out came the voltmeter. Some initial considerations: was the resettable fuse intermittently failing? Had some component into the power rectification circuit fried due to Madagascar’s poorly regulated electrical grid? Of mechanical components on the board, could the magnetic reed switch be sticky or making partial contact?
Slowly, I worked through these items. The transformer secondary puts out 28V, which feeds to the board through red wires, seen at the base on the rear.
Power flows through a 4A fuse, through a diode, is buffered by a capacitor, and finally regulated by an MC78M05C regulator. After closing the power switch, whether the digits lit or not, power was making through this path, yielding a stable 5V to supply the MPU and other components. So, not an issue with the power supply. Darn.
With power at pin one of the PIC16F872 I/SO microprocessor, I wondered why it didn’t seem to be doing anything – no LED digits, no heat on the iron. I probed the external oscillator crystal to see if there was a “heartbeat”, and had a surprise: when I touched the “CLKOUT” side of the crystal, the display came to life and everything was working. When I touched the probe to the “CLKIN” side, the display went dark. Same effect with a DVM probe (with the other probe not connected to anything). Head scratching ensued. Grounding the case of the oscillator was not the solution.
My oscilloscope showed an oscillator frequency of 4.032Mhz. When I looked up the oscillator in the PIC data sheet, this would correspond to operating in “XT” mode. Of note, that sheet suggests that each side of the oscillator be loaded with a capacitor, nominally 15pF. In my inspection of the board, I didn’t see any capacitor at those junctions. Perhaps there is some capacitance designed into the board layout itself, but maybe not enough. According to the datasheet, more capacitance means more oscillator stability; maybe my oscillator wasn’t always stable enough to satisfy the chip’s power-up sequence. The trade-off is startup time, but between slow start-up and no start-up, I’ll take slow.
I had on hand some 22 pF caps, a bit higher than recommended, but worth a shot. I tack soldered them to the crystal terminals, with the ground end brought down to the grounded terminal of the potentiometer that sets temperature.
Since making this modification, the solder station has reliably turned on every time, so I think I found the problem, which is good, because I was just on the edge of scrapping it in favor of the Hakko equivalent. I see a good number of posts about failed Weller soldering irons and wonder if this issue has affected other owners of the digital model.