Treat,
I assume your scope was AC-coupled when you measured U3-4; some scopes
misbehave when DC-coupled.
I looked at the PC layout and wondered if a solder bridge to a nearby
pad might cause the symptoms, but it doesn't look like it. The trace
running from U3-4 goes around a bit, but it doesn't pass by any node
whose impedance drops when the circuit is energized.
You would do well to verify the DC voltages on U3 given in the DC
voltage chart. Perhaps that will help you determine if U3 is defective.
That's all I can think of.
Marshall Jose, WA3VPZ
Treat Hull wrote:
....First, I used my scope and verified that I had a good BFO signal to U5
pin 6.
Then I set up my audio signal generator for about 50 mV peak-to-peak at
about 1,000 Hz and fed it into the mike connector pins 1 and 7 (which
I had
wired for a Radio Shack mike with AF to Pin 1, PTT to Pin 2, and 7 and
8 to
ground).
When I used my scope to trace the AF signal, I started with the K2 powered
down. I found that I had a strong AF signal all the way to Pin 4 of U3.
However, when I powered up the K2, I lost all AF at Pin 4 of U3. Working
backwards, I found that I had AF as far as the junction of C32 and
C33, but
no further. It's as though Pin 4 of U3 is going to ground when the unit
powers up. It behaves the same way with PTT open or closed.
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