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Help for QMX in Germany
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Harry, Those are curious and interesting BPF plots. It looks like 80m-40m is "sort of OK" but with some unexplained variation. You also get TX power output on those bands. It is as if there is a problem somewhere in the 30m/20m LPF and that problem may also be affecting the other bands somewhat. The first suggestion regarding LPF problems is always to check and make sure that the inductors are properly soldered, and that no residual enamel insulation is blocking connection. That is best done with an ohmmeter, measuring the resistance from one side of the inductor to the other. Probe the solder pads, not the copper wires, as you may get fooled otherwise. Your specific problem may be elsewhere but it is good to do this step just to be sure. Make sure that no solder splashes or stray wire bits are fouling that LPF. Inspect the inductor L509? which feeds the PIN diodes un that LPF. Make sure it has not been damaged and is soldered properly. An ohmmeter should show a low resistance, perhaps an ohm. Inspect the PIN diodes that are part of the 30m/20m LPF. Are they oriented correctly? Are they properly soldered in place? What voltages do you measure on their leads when 30m or 20m is selected? What voltages when 80m-40m is selected? Those steps should get you an answer or at least bring an answer much closer. 73 JZ KJ4A? On Sun, Mar 10, 2024, 4:25?PM Harry via <DK2GZ=[email protected]> wrote: Hello |
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Hello Checking page 31, measuring are o.k. A<-->A connection(piep) On my QMX the blue and organge Cs not on the same place as in the manual page 27. The RX on all bands is a little deaf. |
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Measurement is *not OK*. Page 31 is clear: there should be no connection between A and B. A reading of 1.31 ohms is almost a short circuit. Something is wrong. What is the Rev level of your QMX? The picture on page 27 (Rev 2, 3 manual) is showing the capacitor placement for a? 20m-10m unit. Note the differences in highlighted capacitors, page 23 and 24. JZ On Thu, Mar 21, 2024, 5:56?AM Harry via <DK2GZ=[email protected]> wrote:
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Page 31: if I checked in the "piep-mode" the value was 1310 not 1.310 between A and B. I am still not sure about mine (look to the main board above here in the post). There are only two blue of them on mine. I do not know abot the Rev level, bought it used, sorry. |
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A reading of 1310 Ohms (1.31KOhms) is plausible and makes much more sense. Regarding capacitors, as the manual says on pages 24 and 25, capacitor color may be yellow or blue, depending on availability. If you can see and read the values printed on the caps (difficult!), do so. It is easy to make a placement mistake. Do you know if this was a factory assembled unit? JZ On Thu, Mar 21, 2024 at 8:12?AM Harry via <DK2GZ=[email protected]> wrote:
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Harry, I have seen the following cause a "deaf on all bands" condition: In order of declining likelihood: a) a bad connection at either L401 or T401. This is absolutely the first thing to check. Wire enamel can be difficult to properly remove. b) a failed 3253 mux chip. There was a rash of this early on in Rev 2. It was corrected in Rev 3 with the addition of a diode. I have seen only one failure of a corrected board, and it was due to an open circuit created at L503, voiding the T/R switch function. Checking for the correct bias voltage of 2.5V at C406 is usually a good test for the mux. c) a short at the BNC connector. This is not so easy to determine, as by design an ohmmeter will show a DC short at that point while all is OK for RF. Only close inspection can help. c) a failed T/R switch transistor. Rare, but it has happened. With power off, an ohmmeter check of resistance from each terminal to every other terminal should yield very high resistances for a good transistor. d) a cracked capacitor C526. This will generally be accompanied by zero output power. GL JZ On Thu, Mar 21, 2024 at 8:43?AM Harry via <DK2GZ=[email protected]> wrote:
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Harry, one more thing. If IC407 should become damaged, that could also create a deaf condition. That stereo ADC chip is the most vulnerable to overvoltage of all the chips on the 5V distribution. A failure there is often accompanied by heating of the chip. I note all that here for completeness, though I think your problem is elsewhere. JZ On Thu, Mar 21, 2024 at 9:02?AM John Z via <jdzbrozek=[email protected]> wrote:
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