[Radio Wrangler]

very Very VERY VERY bad idea to put a Sony 21' monitor on a variac.

I found out that they have a thick film microcircuit which switches their main rectifier (on the mains input) between bridge and voltage doubler modes to handle mains in the 120v countries and 240v.

The circuit spots mains coming up, decides on where in the world it must be, and then locks in that rectifier mode. It does not change its mind even if the voltage keeps on going higher. Eventually big breakdown diodes do their job and pull the fuse. However, the diodes also die in doing so. If someone just replaces the fuse and tries again, the main electrolytics will let go as well as the SMPS transistors etc.

Some neds threw a lump of chain or wire rope they found in a field across an 11kV feeder a couple of miles away from HP's plant. The mains went down in the area and the network made re-start attempts before the affected branch was isolated. We saw all the lights go off, then half-voltage start attempts, then all came back up. We lost monitors totalling 3/4 of a million quid. As they were HP branded ones, the warranty was from HP not Sony. HP did not warrant stuff sold from one division to another.

The accountants set about planning the purchase of replacements, managers started planning priorities to have minimum impact on income and product intros.

This mere engineer took one apart and found two protection diodes and a fuse would get mine going. I ordered up a job lot of fuses, diodes, and I decided to do bridges and reservoirs just to play safe. Two happily glowing monitors on my desk got noticed... I fixed a number for various people. HP had a wee word with Sony, and Sony sent a bunch of people to repair all the monitors in the place. They remained vulnerable to an action replay of the event, but we now knew the recipe.

As design whoopsies go, chalk this one up as a biggie.

So, now you know the pitfall, don't risk falling in!

David