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G-5400B output voltage problem.

Some time ago I sent a request for suggestions to this list about problems I
was having with the output levels of the G-5400B suddenly being insufficient
to correctly run the KCT or TRAKBOX. Many good suggestions were provided
of which most had already been tried and a few that were not, were also
attempted. However, these were not fruitful.

Since I am getting the station ready and completely automated, it was
necessary to get to the bottom of this problem and resolve. I completely
went through the circuitry and could not find a problem with the 4556 dual
op amps or the input/ouput protection diodes. The strange problem here was
that the problem affected only the output voltages (both elevation and
but not the meter indications on the front panel. I had to check into what was
common. Both split the circuits of the op amps between the meters and the
output and would indicated that this could not be the problem. In any event,
I removed, socketed and tried new 4558 op amp units. As anticipated, this
did not fix the problem.

The problem had to be common since it was affecting both elevation and
azimuth. The only common point was that they both received their inputs
from the pots in the rotator motors. The meter indicators did as well.
However, the meter input "full scale" 10k pots had a 5.1k resistor in series.
The 10k pots for the output voltage adjustment did not. Further checks
showed that the voltage to the pot on full rotation had 6 volts on the
outter leg, but that full 10k pot rotation had a value considerably less On
the center leg. The azimuth had about 3.4 volts and the elevation had
about 2.4 volts. The center legs should have close to the 6 volts.

Removed and disassembled the 2 10k pots and saw nothing obvious, but
an ohm meter indeed showed a high resistance between the rivet connecting
the solder terminal to the carbon raceway. This value was different on
both units. Even though not obvious, it appears as though lightning played
a roll in opening up this particular lead. The lead that goes to the rotator
pot. The meter sensitivity was not affected due to the 5.1k resistor in series
which obviously protected the meter sensitivity pots.

Final test showed when temporarily installing, with clips, a larger 10k pot,
the output voltages were once again correct. I passed this along with hopes
that others may find this useful. This does not appear to be a common problem,
but with small pots and lightning, could happen again. Now I need to find the
parts and replace both of these units. Some form of protection should
probably be installed on these legs to further prevent this problem.

I hope that this information and experience is useful to others.

Reid Bristor
Bristor Associates
URL:	www.bristor-assoc.com
Phone:	(407) 254-1265
FAX:	(407) 253-1162

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