[GreenKeys] Stupid pet tricks...aka selenium rectifiers

[Jeffrey D Angus]

Ok, the Teletype machine has a small DC power supply mounted towards
the rear on the right side to supply 90 volts DC (or so) to run the 
various control relays in the machine. Such as start /stop the tape
reader and the tape punch.

The machine was built in 1962, so it's already 51 years old. Selenium
rectifiers that were a "big thing" back then all die of old age.
Eventually.

So the last time I had the machine apart, I rebuilt the bridge
rectifier.

<http://thumbs2.picclick.com/d/w500/pict/251069526269_/Vintage-SIEMENS-West-Germany-60V-08A-SELENIUM-RECTIFIER.jpg>

Note the markings on the face of the package, two ~ symbols and a + and
a - symbol. The two ~ symbols mean "Connect the wires from the trans-
former here" (AC, hence the "sort of" sine wave shaped ~.) And the +
and - are the positive and negative DC output, respectively.

The aluminum cover snaps on and off. When I rebuilt the internal part of
the bridge I made SURE I had it properly connected and orientated. I
even marked the + terminal with a red felt pen and the - with a black
one.

Then, being asleep at the wheel, I put the cover back on facing the
wrong way and hooked it back together. And proceeded to reconnect it
per the original note's I'd taken "blue wire to here, yellow here...and
so forth."

Put it all back together. turn the TTY machine on, NONE of the push
buttons work. Harumph.

The other day, I pulled the fuse. Hummm, blown. I replaced it with a
slightly bigger rating. Plugged the machine back in. Still, none of the
buttons work, BUT, "We have smoke now!" Aw crap. Pull the plug.

Ok, this after noon, pull the machine back out of the cabinet, put it
on the bench, pull the cover off of the side of the power supply and
shine a flash light in there so I can see what I'm doing.

As Homer Simpson would say, "Do-oh!" I see that I put the aluminum cover
back on backwards, the lead next the "-" mark on the case is clearly RED
from the felt pen.

Well, that's easy enough to fix. Kind of tedious though, I have to
remove the supply from the machine, then completely disassemble it to
get to the part.

But, at least it's OBVIOUS what I did wrong.

On the bright side, the RS-232 to Teletype loop converter I built is
working perfectly.

Jeff-1.0
wa6fwi