After another look at the schematic, I see no reason to lift the RX_IN wire of L401
or the IC402-9 wire of T401.? Just shorting those wires to ground should be sufficient.
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RX_IN might see 12vdc through R510 and Q508, but R510 is 470k so the current will be miniscule if RX_IN is shorted.
IC402 and IC403 are just analog switches, they do not have drivers and receivers inside.
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I suggested you put a 50 ohm dummy load on the antenna port to ensure the noise source is not external.
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Jerry, KE7ER
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On Mon, Feb 24, 2025 at 03:40 PM, Jerry Gaffke wrote:
What kind of power supply are you using???
Try running off a 9v transistor radio battery, or something else reasonably quiet.
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Do you have a 50 ohm dummy load at the antenna port when testing?
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I'd try cutting the receive chain in various spots.
For example, try lifting the RX_IN wire of of L401, and tying that wire to the ground plane.
If the noise has gone then the problem is back in the transmitter LPF section somewhere and subject for another post.
Should be able to tie 20 feet of magnet wire to that L401 wire and hear some signals.
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If you still have noise, try lifting the T401 wire that goes to pin 9 of IC402 and tie that wire to ground.
If you still have noise, try shorting across C416,417,419,419 so all four inputs to the LM4562 op-amps are grounded.
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If you still have noise you might short across C426 and also C427 so the left and right differential signals
into the PCM1804 are zeroed, but first check with a voltmeter that there is no significant voltage
across those caps or you may blow the op-amps.
Assuming C416-419 are still grounded, I would expect no voltage across those caps.
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Jerry, KE7ER
