Refitted module 6 and the (now identified) Schottky diode, attached a handset and battery. Checked 24v at cathode of the diode. All looking fine... turn on...
...sound and stench of another component somewhere in the radio expiring!
At least the dial light works now!
Pulling the modules, and performing a classic 'sniff' test indicated the 12v regulator module 4. A check of the voltages on its connections showed 25v, 1.2v, 0v. Compared with the other regulators these are correct, except that the 0v should be 12v! (that 1.2v is on the connection that goes to the regulators 'ref bypass' pin, whatever the heck that does!)
The actual electronics in the regulator modules, at least the 12, 6 and 3v units, is actually the same! Its a 1-40v adjustable regulator. Sadly, a quick metering doesnt show a short this time. So this one is going to take some testing!
Edit -
Well, it didnt take much testing! The ferrite bead inductor in the supply rail, before the 22uF capacitor, was open circuit! It had burnt out. Why? Because the bloody 22uF tantalum leaks like a sieve! So thats two of these caps dead! It didnt test short because A) it only shows short one way, and B) the leg had melted off!
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