Thanks Oleh! I'm quite confident my problem is further in the chain, as I see nice I and Q signals after the audio amplifiers = at the inputs of the ADC (attached screenshot, measured over the capacitors after the OpAmps)
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Thanks Jerry and all for the magnification and soldering advice!
I have a desk lamp with magnifying glass to solder under, and I also have an USB microscope. I agree, the USB microscope has some issues. For me mainly that I cannot work well under it, as there is little space. Also there is a bit of lag in the image, making my hands feel strangely heavy when I move a tool under the microscope. It seems it's time to start looking for a binocular microscope. This time, I've used the USB microscope to inspect the soldering.
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To return to the image rejection issue.
I've measured the ADC chip pins with an oscilloscope and found this:
- the reference and common pins all have a good-looking voltage near 2.5V, same between left and right channel
- there is a good audio signal on each Vin
- simultaneously measuring LRCK and data out, there is data in the right channel but only zeros in the left. (attached image, red is LRCK, yellow is data. Don't mind the amplitudes, scales are different and one of the probes was on 10x accidentally)
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Frustratingly, once while testing I got a good result on the image rejection test, then it got back to the old behavior of no image rejection and the left channel being silent.
This feels like a loose pin sometimes making contact. But the measurements above make me think the ADC chip is broken. I did try to reheat the chip pins associated with the left channel, with no change.
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I think I'm stuck here, unless I start replacing the ADC chip. Not expensive but I'm not 100% confident I can solder it... and the radio is decently usable? as it is.
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@Ron the original author: sorry for hijacking the thread. I don't know if you have the same problem as me. I'd suggest one way to find out is to measure the audio signal
after theOp Amps with an oscilloscope, with a signal source attached. If you loose one of the channels earlier in the circuit like Oleh, you would see it there.
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Best,
Fredrik
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Fredrik (OH1HSN)