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In order to be able to trace an actual signal through the first few stages of the receiver, I first bypass R43 by temporarily
soldering a 330-ohm resistor in parallel. This is easy to do on the bottom side of the PCB. Be careful to find R43, at the
correct place on the PCB, using the layout diagram - it is right next to the BNC connector. Now the attenuation factor is
only about 1/6th. It doesn't have to be 330-ohms. I wouldn't go below 220-ohms which will load IC3 too much and
probably make the signal a bit too big; but 220-ohms, 270-ohms, 330-ohms, 470-ohms, 560-ohms... anything like this
should be good enough. The later stages of the receiver will be massively overloaded. But we don't care - for now we just
want to be able to see the signal as we trace it through the front end of the receiver.
The signal strength will be a bit more or less depending on whether or not you have a dummy load connected. It doesn't
matter. We are only interested at this stage in seeing a signal which is strong enough that we can trace it through the first
few stages. Here's the 0.8V peak-peak signal that I see with the 'scope probe right at the BNC connector.
Low Pass Filter fault
Check with the oscilloscope for a good strong signal at the drains of the BS170 MOSFET's Q1, Q2, Q3. Or alternatively at
the final pad of the LPF inductor L3 (the pad closest to the microprocessor). What you are checking here is that the signal
is coming through the Low Pass Filter (LPF) correctly. If it is not, we have to investigate why.
The most common fault is failure to properly solder to the wire of the toroids in the LPF, which are L1, L2, L3. If the
enamel insulation is not properly burnt or scraped off, the solder will not make an electrical connection between the wire
