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Author Topic: FT757gx 10m problem  (Read 2409 times)
PA3BHF
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Posts: 2




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« on: August 25, 2013, 07:28:47 AM »

I have an FT-757gx.
It developed a strange problem which is getting worse over time
Initially after a while (10-15 minutes) the 10M band would just cut out. No transmission, and reception deaf (only very strong stations are still getting through). But now it cuts out immediately after switching on the rig.
It is only the 10M band (25MHz and up), the rest is just fine.
In actual fact, when it doesn't work, you can hear a relay clicking when you turn past 25.000 MHz (ie from 24.999 to 25.000), and at that point the noise level drops to almost nothing.
When it was working on this band, I could not hear this relay clicking.
Has anyone come across this problem?
Does anyone have a suggestion how to get this solved?
thanks and 73
Michael M0MPM / PA3BHF
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N4ATS
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Posts: 851




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« Reply #1 on: August 25, 2013, 08:05:38 AM »

PLL is going out of lock , download the service manual (its free all over the place) , do an alignment on the PLL section.
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KA4POL
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Posts: 2128




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« Reply #2 on: August 25, 2013, 10:50:09 AM »

Difficult to diagnose. Could be as simple as a grounding problem of a board. At least keep that in mind when tracing things.
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PA3BHF
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Posts: 2




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« Reply #3 on: August 26, 2013, 08:19:31 AM »

thanks for the suggestions,
I have the manual, what I don't have is experience tracing faults ...

but I'll give it a try!
73
Michael M0MPM / PA3BHF
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AJ8MH
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Posts: 140


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« Reply #4 on: August 26, 2013, 11:07:12 AM »

The FT-757 is well known for its switching diode problem, and my 757 exhibits a couple issues including RX RF amp switch causing TX power changes, and intermittent operation on 10.  I've traced it back to the bandpass filter switching diodes, and it appears to be heat related.  Since my radio is a secondary rig, it's still sitting on my workbench with a stack of new diodes.  My plan is to replace all the switching diodes.  I think there are 21 or 22.  Not an easy job since the board has to be completely removed (unless you cut and tack solder to the top of the board).  At a minimum, I'd replace D17, D18, D19, AND D20.

Note, no receive or transmit and the relay clicks at 25 MHz.  I can hear the transmit signal on a nearby receiver.

Of course, it could always be something else.  Good luck.

The diodes I ordered are:
  Mouser part number:  512-BAV21
  Diode:  BAV21
  Specs:  250V Io/200mA  
« Last Edit: August 26, 2013, 11:13:09 AM by AJ8MH » Logged
KA4POL
Member

Posts: 2128




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« Reply #5 on: August 26, 2013, 11:26:51 AM »

A heat related problem can be excluded as it is already immediately after switching the rig on.
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