Hi,
New QMX here, Rev3 2024 received in Late February.? It powered up and everything has gone reasonably well.? However, I can't receive anything under S9.? The noise on the input is a constant solid noise.? I've gone through the operation manual, and done the diagnostics for RF Filter, image, etc, and the results are... bad.? 20db, no image rejection between the two sides, the rf filter sweeps don't start at 0 and go negative, they go between 0 and go +20.? The SWR and LPF sweeps turn out just fine as expected, though the 3rd harmonics seem high, I'm guessing due to whatever is going on.
The operation manual says "check the trifiliar core T401" etc, which I've done.? I have replaced both LM4562's, both 3253's, and here's the big diagnosing issue:? When I short pins 3 or 5 of U405, the audio output goes silent, and the bias voltage (from R401/R402) on T401 gets collapsed to .2V or whatever (The 3253's can handle the <1mA current no problem, so no damage here).? This shows that the 3253 is working.? In addition, the bias dissapears on the other side of the capacitor that is selected by IC402.? Also, a scope shows proper I/Q clock signals from the 5351 going into the mixer/switch.?
When I short pins 3 or 5 of U406, however, there is no change in audio.? The bias voltage all the way to T401 collapses as well.? I thought at first maybe I had a bad opamp or something, so I had replaced them both, but no change.? I measured the resistance between 1&2 and 6&7 on both IC405/406, both showing ~530 ohms (as expected) under no power condition.? Bias voltages are all around 2.45-2.5V as expected, getting passed through L401 to IC407.
The only thing I have left is to maybe swap the PCM1804 codec IC, as this is the only thing I can guess might be failing, but I don't have any of those spare until I make my next digikey order.?
Any suggestions?
