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Unknown component...

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russ57:
Yep, and to be precise it (was) a selenium rectifier...
I removed and opened it, to the accompanment of falling pieces.
So maybe it had failed, or maybe not, as measuring a stack of selenium junctions with a multimeter Does Not Chooch...
But from what I read of selenium if it wasn't dead, it was dying.

Replaced with 2 x 1n4007. All functional.

russ57:
(and for interest I traced the circuit and loaded it into an online circuit simulation. It is functioning as a voltage doubler, pretty much straight off the mains. Although I could not perform the necessary topology gyrations in my head to make it look like a doubler. Charges a big cap to 700odd volts which are then dropped across a coil.

BaronJ:
Hi Russ,

The only problem with using silicon diodes to replace a selenium rectifier is that the voltage drop will only be 0.7 volts per diode compared with maybe three or four with the selenium one, which will also vary with current draw. Depending upon application and load a few ohms series resistance might be required to get back to the original working conditions.

russ57:
The circuit is only supposed to be powered up for a couple of seconds. Turn on, dirty big cap fills up with 'angry pixies', to quote a certain Canadian, then said pixies are escorted out of the cap into a coil when power is removed. The collapsing magnetic field disrupting any magnetic alignment in a tool located in the target zone.

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