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1-8 of 8 messages
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Page 1 of 1
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FT 757GX RX fail
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by VK6NDT on July 17, 2009
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I have a FT757GX and when it is powered up the signal meter shoots right across to the far right. I have checked the Globe that protects the RX side and it seems ok, but it wont Recieve anything.
Im suspecting it as had a spike or something and the radio has spat the dummy. it would be nice to get it going again as i have had it a while and had no problems (appart from this) with it.
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RE: FT 757GX RX fail
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by VK6NDT on July 18, 2009
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No not this time, but i do admit it has caught me out once before, thats one of the first things i checked :)
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RE: FT 757GX RX fail
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by N8NSN on July 18, 2009
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I had a problem similar in the 757GX. Fortunately it turned out to simply be the fuse (lamp).
In my research for fixing my poor RX; I found the following information in a thread. You may want to read this and seek this route for repair if indeed the protection lamp is still doing it's job.
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RF-UNIT BFP filter bank's input-side switching diodes 1SS97 Shottky's (D19, D07, D09, D11, D13, D15, + possibly others) had been damaged and leaked various amounts of RX signal to every BPF filter section.
Tune to 28 MHz and note the sensitivity, or S-meter reading of a carrier (from sig. generator) and then just cut D19's other lead and note the sensitivity & S-meter reading to improve (D20 is the respective diode on the filter bank's other side). I reconnected D19 and just for curiosity cut all the BPF's antenna-side Shottky diode's legs one-by-one and found all more or less RF leaky. If the 28 MHz BPF also has damaged diodes (MA190), the method might not work as assumed. Diodes are cheap and it's better replace many enough, so you get no second thoughts later. I used BA244 and some 1N4148, wich will do, if no switching diodes are at hand.
The damage was most likely a result of static surge from the antenna, but too much RF-power into the receiver from another transmitter (any band) can also cause such damage to any modern HF tranceiver. In fact this fault actually also reduced the transmitter's drive, since TX drive power is piped via the same BPF bank and diodes - the Drive knob had to be turned fully clockwise for 100 W output, but after repair about half-way setting was enough.
Measured sensitivity on FM for 12 dB S/N ratio, RF amp ON, RF amp OFF:
00.7 MHz: -N.A.*********-105.0 dBm (1.25 uV)
01.7 MHz: -120.0 dBm, -108.5 dBm
03.7 MHz: -119.5 dBm, -109.0 dBm
07.2 MHz: -119.5 dBm, -109.0 dBm
14.2 MHz: -121.0 dBm, -111.0 dBm (0.2 uV, 0.63 uV)
21.2 MHz: -118.5 dBm, -108.5 dBm
28.2 MHz: -114.5 dBm, -105.5 dB** (0.41 uV, 1.25 uV)
The receiver LO1 level is +13 dBm, LO2 is -3 dBm from the coax plugs (unplugged, to 50 ohm load).
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I hope you have success in fixing the RX on your 757GX. I still have the one here as one of the HF rigs used.
73,
Jim N8NSN
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RE: FT 757GX RX fail
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by VK6NDT on July 18, 2009
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Thanks for the details, i will go through it and try to bring it back to life.
I just needed a bit of guidence, and what better place when your all out of ideas.
It worked so well when i had used it. i havent been very active but never had a problem getting contacts with it. would be a shame to put it in the "spares" pile.
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RE: FT 757GX RX fail
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by OUXLY40 on September 29, 2009
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I have an area near the center of my drive control which causes a bit of noise in my RX. I carefully cleaned and lubed the control but it made no difference. Would this be due to one or more of those diodes leaking? I have checked all bands and all modes and they are all about the same. There is also a bit more noise when I have the linear button out. Does the linear button turn the bpf on and off as well? It's not really bad and you don't relly notice it unless you are looking for it but if I had an idea of where to start I would probably go in and cure it.
Lee
KJ4JTT
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RE: FT 757GX RX fail
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by KE3WD on October 1, 2009
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>>>by OUXLY40 on September 29, 2009
I have an area near the center of my drive control which causes a bit of noise in my RX. I carefully cleaned and lubed the control but it made no difference. <<<
Most often a sign of DC leaking through to the potentiometer that shouldn't be there.
Look for small blocking capacitor on either side of the potentiometer, one of them is likely either leaking or shorted.
--KE3WD
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RE: FT 757GX RX fail
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by VK6NDT on November 3, 2009
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Just a quick update. i raplaced the signaling diodes on the BPF board but still no luck. it does RX on 10m but very deff.
i dont get much time to look at it as i spend to much time repairing everyone elses commercial vhf and uhf radio's.. hf is my play time :)
i shall keep you guys posted.. thanks again for the help.
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