Yaesu FT757GX No AM/FM rx - UK Vintage Radio Repair and Restoration Discussion Forum

PWH · 28th Jan 2015, 9:50 pm

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

_Clint_ · 28th Jan 2015, 10:15 pm

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

PWH · 28th Jan 2015, 11:29 pm

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??

vu2nan-nandu · 29th Jan 2015, 5:36 am

Quote:

Originally Posted by PWH

Any ideas where to start? Thanks

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.

HamishBoxer · 29th Jan 2015, 10:02 am

Wonder if it is some thing like dirty relays? Certainly common to rx/tx.

Hermit6345 · 29th Jan 2015, 10:36 am

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

David G4EBT · 29th Jan 2015, 10:59 am

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.

PWH · 29th Jan 2015, 6:23 pm

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.

PWH · 26th Feb 2015, 3:32 am

Possibly. the xtal has failed. Is there any way to test it??

SiriusHardware · 2nd Mar 2015, 2:43 am

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.

PWH · 6th Mar 2015, 2:31 am

Thank you SiriusHardware, I'll give that a go.

vu2nan-nandu · 18th Jul 2015, 7:03 am

Hi OM Peter,

Problem solved?!

73,

Nandu.

28th Jan 2015, 9:50 pm	#1
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	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
_Clint_ Pentode Join Date: Mar 2011 Location: Sevenoaks, Kent, UK. Posts: 101	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 __________________ If it sparks, it was either meant to spark, or something is wrong, very very wrong.

28th Jan 2015, 11:29 pm	#3
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	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, 10:02 am	#5
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,932	Re: Yaesu FT757GX No AM/FM rx Wonder if it is some thing like dirty relays? Certainly common to rx/tx. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

29th Jan 2015, 10:36 am	#6
Hermit6345 Rest in Peace Join Date: Oct 2006 Location: North Wiltshire, UK. Posts: 638	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
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,761	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. __________________ David. BVWS Member. G-QRP Club member 1339.

29th Jan 2015, 6:23 pm	#8
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	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
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	Re: Yaesu FT757GX No AM/FM rx Possibly. the xtal has failed. Is there any way to test it??

2nd Mar 2015, 2:43 am	#10
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	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
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	Re: Yaesu FT757GX No AM/FM rx Thank you SiriusHardware, I'll give that a go.

18th Jul 2015, 7:03 am	#12
vu2nan-nandu Pentode Join Date: Jul 2014 Location: Bangalore, India Posts: 132	Re: Yaesu FT757GX No AM/FM rx Hi OM Peter, Problem solved?! 73, Nandu.