[Elecraft] KPA 100 Resistance Checks

[raycollins]

I thought the following observations may be of help to anyone having
difficulty achieving the KPA 100 resistance measurements on page 44 of Rev H
of the KPA 100 manual.  

I initially performed the checks using a DMM with the PCB illuminated by a
40 Watt bench light a few inches away. Resistance values were very variable
and only Q1 and Q2 base measurements were within specification.  The other
measurements were too low in value and would ramp up and down, with the DMM
sometimes indicating a minus resistance value (whatever this meant!).  The
orientation of all relevant diodes and transistors was checked to no avail,
and then I happened to switch off the bench light and hey presto, everything
worked properly with all values easily within specification.  The two PA
transistors appear to be photo sensitive and must produce photo electric
currents similar in magnitude to the DMM test current.  If I perform the
checks with my moving coil multimeter (2000 ohms/volt), the correct
resistance readings are achieved with or without the bench lamp on. DMMs are
very useful but care has to be taken in understanding the results!

Ray



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