On 9/23/2013 5:58 AM, Dan Evans wrote:
More trouble shooting found two bad diodes where the pot leads come into the
controller. I found a couple of 1N914's in the junk box and swapped these out.
Now the meter works, but is horribly inaccurate.
Yesterday I got a couple more hours to dig into it. I put the circuit board
under the magnifier so I could see it (my vision ain't what it used to be...)
and spotted a resistor that had been pretty hot. I swapped out this 15 ohm
resistor and varified all of the other resistors and diodes checked OK.
Success at last! It now appears to be working normally.
So, if you have a similar problem with your controller, check the diodes on the
pot leads and go over all of the resistors on the circuit board.
And now that I know the workings of this rotator so well, I'm thinking of some
mods. I would love to have a digital display...
73
Dan
AFAIK, there are no diodes across the pot leads; what in the world could
they possibly do? At least they are not shown on any version of the
schematic I have seen. I think what we have here is some poor clown
who got a bunch of RF on his control lines and was attempting to fix
the problem without knowing what he was doing.
Regarding a digital display, I briefly entertained that when I
was working on a control box with a burned out meter (I think a
similar story to yours). If you have a south centered meter,
it is very easy to build a level shifter with one op amp and
a few resistors that will produce zero to 360 millivolts that
can drive a cheap pocket voltmeter (like the $2.95 ones from
Horror Fright) and you're good to go.
Unfortunately, you probably want a north centered meter. In
that case you can add a comparator that will trigger at half
scale and add or subtract 180 millivolts to make it work.
The comparator could be an opamp pressed into service as a
comparator, so you use a dual op amp IC (one 8 pin DIP).
If you are clever, you can do this without a negative power
supply since the meter is floating.
Rick N6RK
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