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My FT 747GX has a problem tripping the high SWR shutoff whenever I tune up the radio on high frequency bands (15M-10M) using certain types of antennas.

Does anyone know how to defeat the high SWR shutoff circuit? Thank you in advance for your help.

AB7JK

Please note: I turn down my power output to a few watts whenever I tune up so my finals will not be in danger if I defeat this circuit. Thanks.

Kim:

Your question raises some other questions.

Apparently you are not having the problem on all antennas. This suggests the automatic protection circuit is working. What is different about the antennas giving the problem? Grounding, perhaps?

Have you used a known-to-be-good VSWR bridge to determine the actual SWR of the "problem" antennas? If so, are the SWR reading within the designed tolerance of the transceiver as provided in the rig operator manual? (If not, you need to fix the antennas or the feed line, not the rig.)

Have you checked the quality of your feed line? To do this place a 50 ohm dummy load at the ANTENNA end of the line and see if the rig works OK. Check the SWR. If the problem exists, the feed line can not handle higher frequency transmissions, is causing the excessive SWR and needs to be replaced.

Good luck on getting it working like it should.

AL

K4ICL

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Hello Bill,

I've found that most Power Supplies used for Amateur Radio Service do not provide enough DC Filtering such as .01 uFd Chassis Mounted Feed Through Capacitors and RF Chokes on the DC Lines. I'd almost bet if you had the opportunity to connect the transceiver to a commercial Motorola MICOR Power Supply, you wouldn't experience the problem.

The Marine Deep Cycle Battery is providing a certain amount of internal capacitive value to the DC Voltage and the only draw back to this source is it can't regulate the DC Voltage as the transceiver draws the +20 Amperes on Transmit.

73,

Dan
WA9WVX

As a post script to this thread my 747GX finally bit the dust. I was transmitting PSK-31 when I suddenly got a high swr then turned off the rig to save the finals. When I turned it back on the cpu was fried. No TX not RX - caput. Served me right for not using isolated cables on a radio with RF feedback problems. I'm looking for another 747GX.
AB7JK