[TowerTalk] HD-73 Rotator issues--digital readout

Richard (Rick) Karlquist richard at karlquist.com
Mon Sep 23 14:43:18 EDT 2013


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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