Hi Tom,
You've probably done all of this, but just to run over some of it.
Have you tried putting a dead short to ground on the + pin (3) of U4?
Check the output, it should follow.
Best to confirm this with R6 out of the circuit.
The old LM234 op amps were a bi-polar technology and they would
sometimes crow-bar.
I didn't dig that far into the LM358 datasheet, but its a slight
possibility even with a new chip.
Did you check C52 to see if the residual voltage is present also there?
Check pins 5 & 6 of U4, verify ~.1 ohms to ground.
Pin 7 of U4 should ~ 0 volts.
Can you unplug J3 Aux-bus?
If so, check to see if the voltage is still present at R6 with it
disconnected.
If not, the residual voltage is coming from the aux bus.
Just for giggles, check pins 2 & 5 on J3 Aux bus, verify ground.
Have you been using the KAT2 successfully until now, or is this a
installation from a build?
If so, is there any residual flux on the PCB in the pin areas of the
circuit?
Hope those suggestions help :)
Duane
N1BBR
To the list - I have yet to solve my problem with my KAT2. So far I
have
replaced T1 which yielded the correct 3.4v on TX on the input to U4 and
0.74v on RX. Unfortunately U4 was not buffering that through to R6 so I
replaced U4. I now see the 3.4v on R6 in TX BUT 2.4v in RX. I have yet
to
find where this 2.4v is coming from. NOTE: this is present even when R6
is
not connected to the board. Obviously when R6 is in line, it blanks the
receiver due to the high voltage on it.
I'm already in deep correspondence with Don and Gary but if you have any
ideas please do not hesitate to drop one to me as well...
Thanks and wish us luck!
Tom W0EA
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