[GreenKeys] Flesher TU-170 followup

[Dave Meier]

Thanks to all the offers for help in obtaining a manual.

I have repaired the TU.  The problem was not the loop fuse, resistor or
transistor.  It was the loop power transformer.  The primary was open.

The transformer is a ST-5-120, manufactured by Signal Transformer.  I was
surprised that this part is still available from places like Digikey.  Only
$5.79 each when you order the minimum quantity of 100 pieces.  Not needing
99 spares, I came up with an alternative.

The original transformer has two 60 volt secondaries.  In the TU-170 they
are used in a full wave center tap circuit.  Any replacement should isolate
the AC line, provide about the same voltage and current and fit in the box.
I got the idea of using back-to-back transformers salvaged from "wall wart"
power supplies.  I cracked a few open and found that the actual transformers
inside were small enough, but not center tapped.  I decided to get around
that by adding two additional diodes to the TU-170 loop supply circuit,
changing it to a full wave rectifier.  For this I would only need a single
60 volt transformer output to match the original supply.

Two identical transformers back-to-back would output 120 volts, too much for
this application.  I fould that a output of a transformer originally
providing 9 volts DC could be connected to another intended for 15 volts AC,
connected to step the voltage up, to provide about 69 volts.  Of course I
removed the rectifier and filter components on the 9 volt side.  I was able
to fit both transformers back in the TU-170 by careful arrangement.

As an aside, in this project I found an easy way to crack open the wall
warts.  Clamp their edges in a vice so that one side glue line is between
the vice jaws.  Crank the vice closed until the side bows and the glue pops.
Repeat for the other sides.  This worked quickly and cleanly on the several
I tried.

Dave Meier N4MW
New Kent, VA
www.n4mw.com