I already had a cheap (incredibly cheap, actually) Yihua 936 soldering station which I got from Hobby King for €14.
Dave Jones at EEVBlog reviews it in his usual entertaining style here. Its actually not bad, especially for the price, but when you put it side-by-side with the Hakko the differences are apparent.
Anyway, despite its virtues, I resolved that the Yihua was going to make the ultimate sacrifice and become a transformer donor for the FX888D conversion. This is where I got a very pleasant surprise: it turns out that the transformer in the Yihua is a perfect fit for the Hakko. The transformer has the same dimensions and even the mounting holes line up perfectly with the pillars in the FX888D body. This was shaping up to be the easiest hack of all time !
Here is the Hakko with the transformer from the Yihua already fitted (but with the secondary side not wired in yet). The original Hakko transformer is just below for comparison
This is probably a little bit dodgy, if I'm honest: I may yet go back and use the power switch and fuse assembly from the Hakko transformer. Assuming that I'm not electrocuted first, that is.
I had a little debate with myself about the best way to connect the secondary from the new transformer to the controller board. Strip connector? Solder+heatshrink the wires together? In the end, I desoldered the original wires (the red ones, visible in the top right-hand corner), cleaned out the holes (the most difficult part of the entire conversion !) and soldered the outputs from the new transformer directly to the board.
When I first powered up the unit after the conversion, the temperature on the display quickly shot up to 750F (just under 400C). It seemed to me that the iron was running hotter than this and - sure enough - when I checked with a thermocouple the tip was closer to 900F. I could compensate for this by just reducing the setpoint and the controller would hold the temperature steady, but it was a bit of a bummer to have the displayed temperature completely wrong. I suppose I could have a faulty unit, but it was too late to submit a warranty claim now :-). Anyway, I came across this YouTube describing how to reset the controller to its factory defaults. Without much optimism, I followed these instructions and it worked !! The tip temperature measured with a thermocouple now (broadly) matches the temperature shown on the controller's LED display.
I'm used to thinking in degrees Celsius/Centigrade rather than in Fahrenheit which is the out-of-the-box default on the FX888D. Thankfully, changing the display units is easy.
(these were real "wish I had a third hand" photos to take !)
This is a fantastic result: the conversion couldn't be any neater and I couldn't buy a transformer any cheaper than I got the Yihua 936 for. As a bonus, I will reuse the (rather nice) enclosure that the Yihua controller came in for something else.
UPDATE January 2015: I have been using this soldering iron regularly now for a few months and I couldn't be happier with it. It is quick to heat up, holds its temperature rock-steady and it is comfortable to use.