Well, I resisted the hammer-urge and went looking for info on the EX730, and hopefully a schematic, but alas, it was not to be. What I did find was
**broken link removed** site, which contains lots of info on multimeters, including quite a few schematics. Also linked to is a list of multimeters that don't quite make the grade for various reasons. And no, the EX730 is not included among them, but I believe only because the 'offenders-list' hasn't been updated in a few years. What is included about Extech meters is particularly damning however;
Extech EX330 CATIII/600V CATII/1000V Current ranges have glass fuses rated for 250V with no warning of lower voltage (Submitted by T4P, reviewed by Dave Jones) Additional information and reviews on different Extechs show that Extech EX series are to be avoided in general. The quality control appears to be so bad that parts are coming loose inside some meters from the factory, amongst other problems.
The almost-certainly damaged chip in question in the EX730 is the
**broken link removed** by Cyrustek. As I suspected, the voltage-in is tied (almost) straight to the chip itself, with only a 10M resistor intervening. Looking through the Cyrustek pdf, it appears this is the recommended way of doing things, and given that the pdf talks of voltages up to 4000V being applied, I still cannot fathom how a piddly little 5W 230V/12V transformer, with an 18V square-wave applied to the secondary, managed to kill it.
Nothing on Ebay, but you can buy the ES51962 on Aliexpress for about $20. No chance, not at that price anyway! I'll spring for one if someone drops the price to $10, and then only as a penance for frying it in the first place. Yep, Extech are crap.