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QMX S9+ noise on receive
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So I am making a second post. I recently assembled a QMX. Fired up at 7v limited to 200ma. Loaded the firmware. The transmit on all band is great with 4 to 5w. This is a 60 to 15m model.
The problem I have is it is mostly deaf. Only if a station is like S9+20 will it exceed the noise level and be able to be heard. I am posting various sweeps here. First the swr sweep for 40m looks good to me. Then the RF 40m sweep looks good to me also. Then the 40m lpf sweep, again I see no issues. Now the image sweep. This compares favorably to the manual so I believe this is good as well. Now the AF image sweep. This is similar to the manual, but my lsb is only down -40 and not -60 like the manual shows. I am not sure what would cause this. Lastly the ADC sweep that Hans just created. I see the I and Q 90 degrees out of phase so I believe that proves my tri filar torroid is fine. The sweep however is not clean like the example. There is significant noise, at least it appears that way to me. Please if anyone can help. I have my suspicions it is the PCM1804, but I am not an engineer. Thanks everyone. |
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Bruce, The PCM1804 is likely bad. That chip has been exhibiting a very wide range of failure symptoms in QMX, from subtle and noise related to complete meltdown. Yours appears to be a case of damage to the I channel but not so bad as to leave the device completely dead. If you are not confident about your ability to change it, consider help from Jeff Moore W1NC, repair wizard and all-around good fellow. JZ KJ4A? On Sat, Jul 13, 2024, 1:23?AM Bruce N9JCV via <n9jcv=[email protected]> wrote: So I am making a second post. I recently assembled a QMX. Fired up at 7v limited to 200ma. Loaded the firmware. The transmit on all band is great with 4 to 5w. This is a 60 to 15m model. |
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Hi Bruce. I'm listening to 15M CW right now and the band is full of CQ TEST. SKCC contest, I think. So, AFAIK, my mid-band QMX is working just fine. I've had QSOs on every band where my antenna is resonant...
John's assessment is probably spot on. But a little noise on the ADC I/Q diagnostic doesn't seem to have impaired my QMX, for whatever reason. .rr |
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Bruce, Another way you might be suffering high noise while having acceptable sweeps is if there is a problem with IC401, the stereo DAC that drives the headphone amplifiers. Finally, sitting between the PCM1804 and the CS4334 stereo DAC is the MCU. All the digital numbers that represent audio signals pass through the MCU, communicating in and out using the I2S serial data bus. It is possible, though less likely, that something is wrong along that path. All of the above requires an oscilloscope at a minimum, and perhaps even a logic analyzer to resolve. On this forum I have seen virtually no reports of CS4334 problems or of I2S data corruption. There have been some MCU failures, but usually they are not subtle ;-( The PCM1804 has been something of a problem child in QMX and several of us are trying to suss out why that is. My bet is on the PCM. JZ KJ4A? On Sat, Jul 13, 2024, 10:11?AM va3rr via <va3rr=[email protected]> wrote: Hi Bruce.? I'm listening to 15M CW right now and the band is full of CQ TEST. SKCC contest, I think.? So, AFAIK, my mid-band QMX is working just fine.? I've had QSOs on every band where my antenna is resonant... |
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James, Check that you have properly removed the enamel insulation and have a good solder joint at the 60 m end connection to L401. JZ On Sat, Jul 13, 2024, 10:59?AM ND6P via <jpkellyburbank=[email protected]> wrote: My 60-15M QMX hears 1 microvolt easily on 40 through 15 but needs an S9 signal or better on 60 meters. I'm not going to operate on 60 meters so I wont be fixing it but wanted to post this in case it helps. |
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My I/Q diagnosis looks much worse. I can see the 90 deg phase shift clearly but the signals are extremely noisy. My receive sensitivity seems to be fine though. I compared it to my K3 with an antenna switch and it’s very close with the K3 having a slight edge. That said, the receive audio is pretty nasty compared to my silk smooth K3. Listening to CW on the QMX for half an hour gives my a huge headache.
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John Z and others. Thank you for all the insight and suggestions. I do have a scope but I have limited knowledge. When I get time I will try to do some probing and see if I can find anything.
I have been practicing removal and replacement of some smd chips, but not sure my skills are up to snuff. It seems I am just one of the unlucky ones as others do have working QMX rigs. Thanks again for everything! |
va3rr
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