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Signal-tracing through the rest of the audio chain
I didn't take any more screenshots of the process. You get the idea, hopefully. Just keep going, from left to right across
the circuit diagram, checking the output of each op-amp - which is either pin 1 or pin 7 of the dual-op-amp chips IC5, 6, 7,
8, 9 and 10. Use a 330-ohm (or similar) resistor temporarily soldered across R43 in the first few stages, where the signal
level is low. Change it to 10K to check IC5, IC6, IC7 outputs (since 36dB of gain has been applied by the pre-amps made
from IC5 op-amps). Then remove the 10K resistor completely for checking IC8, IC9 and IC10 outputs. If the volume
control is too high (too far clockwise) then you will see the output of IC10b and IC10a is a distorted sinewave or clipped
(reaching near the voltage rails). This is normal, it just means that you are over-driving the amplifiers. Even with a 120K
signal, the 2mV input injection is a MASSIVE signal compared to the weak signals well below 1uV that the radio can
detect. Just reduce the volume.
As you move from left to right, if there is a fault in the Receiver, then at some point you will find the error. The signal will
disappear, or the amplitude will be wrong, or the signal distorted, or some other problem. In these cases, check the
relevant components near that op-amp carefully, for soldering errors: dry joints, solder bridges, solder blobs, etc. Some
thin solder whiskers can cause a short, and be nearly invisible - so use a bright light and optical magnification if possible.
You can use a DVM to check for shorts between nearby components, or shorts to ground, shorts to the supply voltage.
Using these techniques, I have a 100% success rate so far in finding faults in the receiver chain and getting them
working.
