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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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28th Jan 2015, 9:50 pm | #1 |
Heptode
Join Date: Mar 2003
Location: Waiheke Island, New Zealand
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Yaesu FT757GX No AM/FM rx
HI I have just obtained one of these and disappointingly, It won't receive on AM or FM tuned to the MW broadcast band, or any other I can hear stations with SSB or CW wide or narrow mode, but on AM, there is just static as if no aerial is plugged in. FM hisses loudly, but of course, no FM stations on that band. I have check the protection bulb is intact.
There appears to be a fault on transmit too, (SSB works but no carrier am/fm) which I will save for another day but mention it in case it is related. Any ideas where to start? Thanks Peter G0PWH |
28th Jan 2015, 10:15 pm | #2 |
Pentode
Join Date: Mar 2011
Location: Sevenoaks, Kent, UK.
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Re: Yaesu FT757GX No AM/FM rx
There are some nice diagrams in this document showing signal paths, well worth a look at in your case: http://www.iw2nmx.it/Yaesu/FT-757GX/...e%20Manual.pdf
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28th Jan 2015, 11:29 pm | #3 |
Heptode
Join Date: Mar 2003
Location: Waiheke Island, New Zealand
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Re: Yaesu FT757GX No AM/FM rx
Thank you. I have this booklet, but unfortunately as per the online version the signal path drawings are nearly unreadable (to me anyway)
I have an RF sig gen, perhaps there are some test points where I can inject a signal?? |
29th Jan 2015, 5:36 am | #4 |
Pentode
Join Date: Jul 2014
Location: Bangalore, India
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Re: Yaesu FT757GX No AM/FM rx
Hi OM Peter,
The problem appears to be related to switching of the filters since both receive and transmit do not work on AM and FM modes. You need to focus on the filter switching diodes. Economy versions of FT-747GX were plagued with similar problems due to faulty plated-through-holes on the double-sided filter board. These were solved by wire interconnections at faulty PTH points. The same problem on PA boards resulted in 'no drive' to the final transistors. Good luck! 73, Nandu.
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29th Jan 2015, 10:02 am | #5 |
Dekatron
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,932
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Re: Yaesu FT757GX No AM/FM rx
Wonder if it is some thing like dirty relays? Certainly common to rx/tx.
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29th Jan 2015, 10:36 am | #6 |
Rest in Peace
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Re: Yaesu FT757GX No AM/FM rx
I assume that you have the Technical Supplement for the FT757GX? There are some alignment/level checks for FM in there both on RX and TX. Also check that the Drive control on the front panel is set fully clockwise, although that would only affect TX. Rigs of this age can suffer from Xtal failure. Ideally you will need an RF millivoltmeter and an AF generator. You should then be able to isolate the fault.GL.
Ian |
29th Jan 2015, 10:59 am | #7 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
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Re: Yaesu FT757GX No AM/FM rx
Just a few thoughts:
Might be worth making sure that all the controls are correctly set (attenuator etc) before assuming that the rig is malfunctioning, but I assume that you’ve done that anyway. I believe that the 757 has more than 450 semiconductors, many of which are diodes, which anecdotally, are said to be a known weakness (given the age of the rig). Obviously you need to narrow it down - you can't crawl all over the rig checking every diode just in case. If you are lucky, it could just be tarnished switch contacts from lack of use if the rig has been stored for some time. You might wish to post the symptoms at the ‘Fox Tango’ group here: http://www.foxtango.org/phpBB2/viewtopic.php?t=481 There’s quite a long list of topics at the MODS DK site (which you need to join to read/download): http://www.mods.dk/index.php?ModelId=56 And here: http://vss.pl/mods.dk/mods.php3-radi...lectid=all.htm Also some ‘mods and fixes’ here: http://www.normsweb.com/tektips/ft757.shtml Best of luck with it Peter.
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29th Jan 2015, 6:23 pm | #8 |
Heptode
Join Date: Mar 2003
Location: Waiheke Island, New Zealand
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Re: Yaesu FT757GX No AM/FM rx
Thanks for all the above info which I shall go through. The filter switching diodes sound promising. I would add that I have since noticed the internal "marker" tone can be heard on SSB but not AM/FM.
Yes I do have the technical supplement also linked above. Unfortunately I am not technical enough to understand it fully. |
26th Feb 2015, 3:32 am | #9 |
Heptode
Join Date: Mar 2003
Location: Waiheke Island, New Zealand
Posts: 503
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Re: Yaesu FT757GX No AM/FM rx
Possibly. the xtal has failed. Is there any way to test it??
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2nd Mar 2015, 2:43 am | #10 |
Dekatron
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Location: Newcastle, Tyne and Wear, UK.
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Re: Yaesu FT757GX No AM/FM rx
If you have a scanner receiver - ideally one with an S-meter, which is able to tune to the marked frequency on the crystal, try doing that.
When the crystal is running, you should see a strong indication if you take the tip of the scanner's aerial close to it. When you switch the set off or select a mode or setting which turns the crystal off, the signal should vanish. The advantage of using a receiver to check a crystal is that it does not require a direct connection to the oscillator: That might detune or even stop the oscillator altogether. |
6th Mar 2015, 2:31 am | #11 |
Heptode
Join Date: Mar 2003
Location: Waiheke Island, New Zealand
Posts: 503
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Re: Yaesu FT757GX No AM/FM rx
Thank you SiriusHardware, I'll give that a go.
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18th Jul 2015, 7:03 am | #12 |
Pentode
Join Date: Jul 2014
Location: Bangalore, India
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Re: Yaesu FT757GX No AM/FM rx
Hi OM Peter,
Problem solved?! 73, Nandu. |