OK - here's the pic.
To get it to this stage, first remove the aluminium panel from the back, the one with the table on it, by slipping a thin pocket-knife blade under it. It's only held on by a few dabs of contact adhesive, but work carefully to avoid bending it.
When you have that off, you'll see the four cross-head screws that hold the case together. Remove them.
Then flip it back over so it's right side up, and carefully lif the top half (with the dial) away from the bottom half. The circuitry stays attached to the bottom half.
You'll see the red wire that leads from the battery compartment forward along the edge and under the circuit board. You can carefully prise this upwards - it is stiff wire (solid, not multi-strand).
The diode itself is only a couple of millimeters long. Snip the red wire and trim a couple of mm of the insulation from the end. On my diode the end with the black stripe goes away from the battery positive. Trim excess wire from the tag on the diode - leave about 5mm.
With your soldering iron and electrical solder, tin the exposed ends of the wire (ie give them a coat of sloder in preparation for joining). Then solder the diode in place, one end at a time. Try not to apply more heat than is absolutely necessary - ie work quickly.
I didn't bother with heat-shrink tube over the diode as the red wire is stiff and won't move around, and you can push it back down into the space along the edge of the case. If you have some small diameter tube that would be better.
To determine which way the diode goes - I asked the man in the shop! Failing that, test with a battery and a multi-meter, if if you don't have that, just try it one one and see if it works - change it if it doesn't work - you can't damage it even if it is the wrong way.
To reassemble, line up the two halves of the case, being careful for the two bits at the front with the incident dome and the sliding panel that adjusts for the two lighting scales - this is the fiddly bit. Screw the case back together, and test with good batteries. When you are happy that all is ok, stick the aluminium panel back in place with a few dabs of contact adhesive.
Good luck! 🙂
