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I have recently finished a QMX 60m-15m which appears to work, but not quite as it should.

It transmits OK in all bands, although with much reduced power on 14MHz and up. What?

bothers me most is that the hardware tests do not seem to match expected behaviour.

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1.? In Audio-sweep-40m.png (enclosed) the red race is a mere 20db-30db below the yellow

???? one, not the 60db that operating manual says it should be. Enclosed only for 40m, behaviour

???? in all other bands is similar (graphs not included).

2. LPF sweeps seem right. Perhaps in the 15m band the drop starts too soon?

3. RF sweeps do not nearly look like those presented in the manual (I enclose the full set).

4. IM-sweep-40m.pnq? (all other bands show smimilar graphs) are not quite the example

???? presented in the manual. I am unable to decide if this is a problem.

5. ADC-IQ-40m.png does not nearly look like the nice sinusoidal shapes shown in the

??? manual. I have included only the graph for 40m, they look similar for other bands.

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All hardware tests performed with .25 firmware and 11V.

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Where should I start looking?

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On Wed, Aug 7, 2024 at 01:20 PM, Fernando TUSELL wrote:

All hardware tests performed with .25 firmware and 11V.

Fernando,

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did you connect a dummy load when testing?

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The document QMX Terminal Hardware Sweeps provides some informaton.

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All your sweeps using RX mode showing a much to high level (30 dB or so). The test signal is injected via R518 (100 k). Could you check this region of the PCB and the value of R518? Of course there cuold be an other problem related to the RX section.

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How is the audio signal coming from the QMX?

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73, Ludwig

On Wed, Aug 7, 2024 at 03:14 PM, @Ludwig_DH8WN wrote:

did you connect a dummy load when testing?

And at first do a factory reset.

If no problem with the dummy load, you may check if there is a problem related to the ADC or in the RX part before. Please read QMX QMX_plus Audio PCM1804 from the QRP Labs Wiki. (Afaik in most cases broken PCM1804 may bring to low levels.)

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73 Ludwig

Ludwig,

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Thank you very much for coming to my help. Yes, I mounted a 50 ohm dummy load and? did a factory reset

prior to performing the sweeps.

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There are no apparent faults in the solder unions around R518. I measure about 29K across, but as I understand this

is? the Thevenin resistance of the circuit rather than the value of the resistor. I am reluctant to unmount the tiny thing to measure

it in isolation, but will do if necessary.

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The sound from the QMX is very noisy, nothing like the cristal-clear sound I get in my QCX. When I tune 7074, 14074, 21074

in digi mode I can hear the characteristic FT8 (again, with noise).

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I will read the document you mention? about the PCM1804, although I am afraid replacing is beyond my level of expertise.

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Tnx again, Ludwig.? 73 de EA2BEE.

I went for the simple method? to test the output of the PCM1804 outlined by KJ4A and quoted in

/g/QRPLabs/wiki/37111.

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I put the QMX in IQ mode and can hear and visualize in Audacity the output from the QMX USB

port. First thing I notice is that there IS some output (does it mean PCM1804 is operative?).

Second thing I notice (see screen capture attached) is that the two channels are not similar

as I would have expected (again, does this tell something about PCM1804 health?).

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If what I have obtained shows the PCM1804 is not the point of failure, I will continue searching

upstream, as suggested by W1NC in the wiki page linked above.

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Tnx for any insight you might care to offer, I am rather lost.

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ft. EA2BEE

The second channel in Audacity looks bad. The waveform seems to be not symmetrically to 0. It would be nice to see a zoom in to a short time period, to see the waveform.

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Maybe the amplifier for this channel is not ok (IC405 or IC406? or output of IC403 to IC405 and IC406). Of course one input of PCM1804 could also be affected.

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73 Ludwig

After a few days out of town and a detached retina I come back to this. I have not tried to use my scope

to trace signals in the pins indicated, I will if my vision improves.? But I have done the following, inspired by the

document ?/g/QRPLabs/wiki/37111.

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This seems to confirm that the receiver path is uninterrupted: am I right with this?? But when I connect an antenna I hear the usual FT8 tone in 14.074MHz, but no CW at all anywhere from 14.000 to 14.040.?