Weller WESD51 Repair

wesd51My 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?

Front of board: Resettable fuse above the digital display; LMC6061 op amp to the right of the display. The big chip is the PIC16F872. The bottom right side of the board is the power section, with diode above the electrolytic cap, and the voltage regulator to the left of the cap. The hole accommodates the shaft of the temperature control potentiometer.
Front of board: Resettable fuse above the digital display; LMC6061 op amp to the right of the display. The big chip is the PIC16F872. The bottom right side of the board is the power section, with diode above the electrolytic cap, and the voltage regulator to the left of the cap. The hole accommodates the shaft of the temperature control potentiometer.

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 enters the board below the potentiometer. A fuse is just above the wires. To their right is the BTA12-600 Triac that modulates the soldering iron temperature.
Power enters the board below the potentiometer. A fuse is just above the wires. To their right is the BTA12-600 Triac that modulates the soldering iron temperature.

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.

The power section. Disconcertingly functional.
The power section. Disconcertingly functional.

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.

Pin out of the PIC
Pin out of the PIC

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.

Datasheet comments about the oscillator.
Datasheet comments about the oscillator.

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.

Bodgy but functional.
Bodgy but functional.

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.

7 thoughts on “Weller WESD51 Repair”

  1. I am researching for soldering stations and happen to find your blog. According to this site http://www.whichsolderingstation.com/ , Weller WESD51 seems to be a decent soldering station. But, looks like you have a slightly different experience. What your opinion on it? I am just getting into electronics space. Sorry about the newbie question.Thank you very much.

  2. Hi Gary – It is hard for me to generalize from my experience. My original 110V station worked fine for a number of years and I collected a few different tips. It worked well for light soldering, but could also be set up for delivering a lot of heat to solder to lugs, coax connectors, and heavy ground connections. The current unit also worked great until it got flaky. It is possible that environmental conditions here had something to do with its failure, particularly poorly regulated mains power. My strongest reason for getting a Weller this time around was that I already had tips. If I were starting out, it seems like many people prefer the Hakko series of soldering stations. They also offer a range of tips. Either way, I don’t think the digital display really matters, and might just be one more thing to go wrong; I’d say there’s nothing wrong with getting the model in either brand that just has a big knob and markings for temperature. Hope you find a good iron! – Jack

  3. Hi Jack, appreciate your input very much. Actually, I heard a lot of good things about Hakko series too. Plus, it is less expensive than Weller. I might give Hakko a try. Thank you for your input.

  4. hi guys, i need help for this kind of soldering (WESD51), i have 2 defective units for repair, where i can find the list of components and their specs?
    I need to know the specs of the reed switch used in the ckt brd (B615CH). thanks for the help. regards, Sam

  5. Hi Sam,

    The schematic is available in this thread:

    However, it doesn’t provide specs of the reed switch. I’d suggest trying whatever reed switch you have on hand. I don’t think it handles any substantial amount of current and while sensitivity may vary a little, it’s probably not a big deal.

    Good luck,


  6. Jack,
    I have used the WESD51 now for over 10 years and all things considered it has been a reliable station. Which brings me to the how and why I found your page. Well, it seems like the iron temperature has a wider variation between the set point and the drop when you began the solder process. I have used this same unit with several of the PES51 over the years to build a piece of equipment, same connector gets soldered with same wire which is TFE 18AWG. The last year it seems to get worse and now it is causing a problem with the drop in temperature affecting the final solder joint. This used to not be a problem, ever but it seems to be getting worse. I have noticed that the available tips seem to not fit inside the iron as well as the original ones did. Maybe my imagination but something has changed. The station still works fine for most work but the importance of doing this particular job is what bought the station to begin with. I have tried different size conical and screw tips to try to isolate something that would indicate the cause. Even using my favorite 60/40 solder I tried cranking the heat up to nearly 700 degrees as the set point the temp can be sucked down over a hundred degrees making the wire and pin begin get sticky together becoming a wrestling match between me and it. I will be honest that I don’t know what the drop was in the past, it was not a problem so I guess I never looked. I apologize for this nontechnical “cry on your shoulder” but I was amazed to Google the model and find you.

  7. Hi George. Thanks for your comment. I’m actually pretty happy with this soldering station — I have one 120V unit for when I’m in the US and a 220V for overseas. My issue turned out to be related to faulty AC wiring, so I really can’t blame the Weller unit. At this point, I’ve got a nice collection of tips, so I’m inclined to stick with them. I haven’t noticed any change in tip dimensions, but maybe it’s because I haven’t bought any recently.

    I did see a video, I think it was on the EEVBLOG, comparing similar Weller and Hakko units in terms of ability to maintain temperature against jobs requiring high thermal flow. In that test, the Hakko came up to temperature and stayed on temperature much better than the Weller, so if I were outfitting from Scratch, I might go the Hakko route.

    Anyhow, glad to provide a shoulder and hope things work out for you!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.