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QMX Receive suddenly stop working

 

My QMX was working fine and I made couple of QSO via FT8. However I noticed the receiver stop working:
1. I can still hear some strong signals.
2. When I plugin my attena, the S-meter in WSJT-x is not changed. keep in less than 10db. when it was working, it should be around 30-40db.
3. I use rs232 console to test RF filter, it show very large att.
I verified all bands are like this. Compare to operation guide, the Y-axis is significant small (-30db).

Any suggestions what I should check?


-Howard?

 

Howard ,

There is some additional info that you can provide to help steer the suggestions in the right direction.

What band model QMX? (HB or LB) are you using? What version level? Was it factory made or did you assemble it? Does it still transmit and produce proper power levels?

There are several possibilities that could result in a deaf RX. Loss of an intermittent solder joint especially at the band switch inductor is one. Failure of the band switch mux chip is also a possibility. Failure of the T/R switch transistor will also deny signal passage, but TX power would likely be affected, too.

JZ KJ4A?



On Wed, Feb 14, 2024, 3:05?AM Howard Su <howard0su@...> wrote:
My QMX was working fine and I made couple of QSO via FT8. However I noticed the receiver stop working:
1. I can still hear some strong signals.
2. When I plugin my attena, the S-meter in WSJT-x is not changed. keep in less than 10db. when it was working, it should be around 30-40db.
3. I use rs232 console to test RF filter, it show very large att.
I verified all bands are like this. Compare to operation guide, the Y-axis is significant small (-30db).

Any suggestions what I should check?


-Howard?

 

I am using 20m-80m model of QMX. It is self assembly. It is still transmitting and produce right power level.

Seems like I should check 3253 from you description. However I am not sure how to do so?
 

Howard,

Checking the bias voltage to the mux usually provides a good test of 3253 health. It can be measured at a number of easily accessible points, such as any terminal of T401, or on C406. It should be approximately 2.5V relative to ground. Any reading that deviates significantly from that, up or down, is a strong indication of mux failure.

Good luck ! JZ


On Wed, Feb 14, 2024, 6:36?AM Howard Su <howard0su@...> wrote:
I am using 20m-80m model of QMX. It is self assembly. It is still transmitting and produce right power level.

Seems like I should check 3253 from you description. However I am not sure how to do so?

 

Thank you for the suggestions. I finally figured out. The mos Q508 is shorten, which switches RX/TX. I already did the modification of adding a diode IN4148 as suggested by the official document. I suspect that I lifted SWR protection to 5 and was tuning the antenna. Any other thoughts why this happen?

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