SDR II Lite Kits.
- Dear Members,
I hope I can get some help here for the SDR II Lite kits. I am a Mentor for
of Kids building a Soft Rock Lite II kit and we have run into a problem that
doesn't make any sense.
On one Kit we are having a problem with the 2.5 VDC bias that is generated
R3 and R4 from the +5VDC Bus. It reads .24VDC. Which indicates that
something is shorted.
At first we suspected a bad OPAMP chip since the 2.5VDC went to pins 3 and 5
to bias the
+ input of the OPAMP. I was able to open the trace of the bias feeding
those two Leads
but the Bias stayed at .24 Volts. So I went the other way towards U3 and
I detected about a .02 voltage drop on R1 and R2 that meant that U3 must be
of about 2ma pm both gates. Since the voltage divider of the 2 1,000 ohm
resistors (R3 and R4)
only has 2.5 ma to make the 2.5 VDC bias... a current through R3 of 4.5 ma
would create a
voltage drop of 4.5 Volts. So, I thought the U3 might have been damaged to
It was causing the extra current in the gates. I removed U3 and measured
the voltage at
the R3 test point. It measured 2.5 volts. I installed another FST3253 from
kit. Measured the voltage at R3 test point and it was back to .24V.
So, To keep from chasing my tail, I thought I would ask the group if anyone
else has seen
this problem and what you did to cure it. I looked at the board real good
under the magnifying
glass to see if there were something there that would cause this problem
like a unetched trace
or a short between pins that didn't belong but nothing was noticed.
Any Hints or cures for this problem gratefully accepted. I am at a loss as
to what to do next.
David R. Hassall WA5DJJ