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SWR is too High!

 

Hi guys, I'm back with my QMX Mid-Band. I was having trouble with no output. I worked on it on and off and finally am getting output. My new problem is the SWR protection keeps engaging. When I do a diagnostic sweep, the stars remain straight across the top of the scale. I was using my power meter and a homebrew dummy load, which measures at 49.8 ohms. It works fine on my G90 without the auto tune engaged. Still, I removed that and attached a commercially bought dummy load to the QMX but still have the same results. While trying to document the power output levels on each band, (they are a little scattered and I'm getting too much power out on one of the bands) The swr auto protections still engages with only pressing the "T" key for a few seconds at a time. It's wired for 9V operation and my bench supply is set at around 7.5-8VDC. I have rewound T507 several times making sure nothing is shorted. It is sitting down far enough to not interfere with any components of the control board and to make sure, I've taped up the bottom of the control board with kapton tape. I've also matched continuity from T507 wires to their respective connection paths. Anyone have any ideas on what to check next???
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Your feedback is much appreciated,
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James
 

Maybe it's a problem with the BNC connector.?
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73, Dan - W2DLC
 

Perhaps there¡¯s a solder whisker or bridge between the centre BNC pin and one of the ground pads on the SMA footprint or vice versa.
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Daniel ZL2DTL/ZL4YOTA?
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I removed the BNC connector, but I didn't measure any short on the connector. I also rewound T507 and rewound T501 for 12v. I ordered a nes BNC just in case.?
 

The SWR is measured by the T507 and the components around it - you could check carefully to make sure that none of the nearby components has been disturbed.
SWR is also affected by the efficiency of T501 and the Low Pass Filters.? Differences in the low pass filters, such as soldering issues, component tolerances, and parasitic resonances can also affect power output level and SWR.? It would probably be good to check that only one of the LPFs is getting switched in as the bands are changed.? Easy to do this by measuring the 'line end' of the diodes: they should have a voltage (5V or more) for un-selected filters, and be 0V on the active filter.? If there is a short that is keeping one filter active all the time, even when another is switched in, could affect SWR on all bands.?? Since the SWR is high on all bands, the main issue is more likely at or around the 'choke' points where all the bands use the same hardware, which is the circuitry around T501 and T507.
In the diagnostic screen, is your PA bias drive at 30mA as it should be?
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Good luck, Stan

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