Hello--
I assembled an RXTX Ensemble for a fellow radio amateur who
suffers from impaired vision. At an appropriate stage, I measured the
current drawn from the 12-volt power supply and found it to be
approximately 70 mA instead of approximately 24 mA as specified
in the assembly instructions.
I removed the 5-volt regulator and found it to be okay. Measuring
the unpowered 5-volt line's resistance to ground, I noted approximately
70 ohms resistance.
Finding the problem would be easier if a dead-short circuit existed between
the +5 volt line and ground, but making fractional-ohm relative measurements
at a 70-ohm level isn't too practical.
To find the problem, I attached an ohmmeter between the 5-volt line and ground.
Then I used a can of "freeze spray" to individually chill every component
connected between the 5-volt line and ground.
When I sprayed U5, the 74AC74 clock divider, the resistance increased by several
ohms. Heating the 74AC74's case with the tip of a hot soldering iron decreased
the resistance. Replacing U5 decreased the current drain to the proper range.
73--
Brad AA1IP
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