Richard,
U3B should be easy to test. The voltage at pin 7 should always be the
same as that at pin 5.
Set the P7 jumper to the EXT position and use the internal voltmeter
probe while checking.
Connect the probe to the 5 volt regulator (U5) output pin and monitor
the voltage at U3 pin 5 with your DMM - the DMM should read very close
to 1 volt if the voltage divider (R9 and R10) resistors are correct.
Whatever voltage you have at U3 pin 5 should also be found at U3 pin 7
if U3 is good.
Check again connecting the probe to the 8 volt regulator (U4) output pin
- this time the voltages at U3 pins 5 and 7 should be close to 1.5 volts.
If the voltages at U3 pins 5 and 7 do not match, U3 is bad, but if they
do match, look for the problem somewhere else - like R9 or R10 - check
the soldering of these resistors carefully as well as the soldering of U3.
73,
Don W3FPR
Dianne & Richard Gething wrote:
When I first switched the unit on to test it the Voltmeter and Current display were working fine. However after I built & tested the Voltmeter Probe, the Voltmeter display read 23.0, and no matter what voltage I applied, internally or through the probe, the reading did not change. The current readings were fine and I was able to complete the all the tests in Part 1.
I have carried out resistance tests, and all the components and the circuit
board look fine, so I feel that the Voltage Sensor IC U3 may be the problem. I
am hoping the fault would not have got to the microprocessor U6, as all else
appears OK.
Before I remove U3 and request a replacement, has any one any thoughts?
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