[ICOM] 756 pro III noise

Fri Oct 15 13:47:52 EDT 2010

Thanks to all who answered my request giving me their hints. I found 2 coax
cables routed different than shown in the bottom view in the service manual
and found one contact of the connecting tape between the top and bottom
boards, the one that goes on the back left side, with signs of corrosion. I
cleaned it while I can get a replacement tape.
Now the radio is working OK, no more annoying noise....
73,

Jim, HK1N

On Thu, Oct 14, 2010 at 8:36 PM, Jim Hargrave <w5ifp at gvtc.com> wrote:

> Jim,
>
> Sounds like perhaps the spectrum sweep generator as the source of the
> noise.
> I would suspect an open ground on one of the inter-connecting cables /
> coax,
> causing it to radiate.
>
>     73's Jim
>      W5IFP
>
> > -----Original Message-----
> > From: icom-bounces at mailman.qth.net
> > [mailto:icom-bounces at mailman.qth.net]On Behalf Of Jaime Gomez
>  > Sent: Thursday, October 14, 2010 6:36 PM
> > To: ICOM Reflector
> > Subject: Re: [ICOM] 756 pro III noise
> >
> >
> > I have made some more checkings and found these 2 issues:
> >
> > 1- The noise is produced by the Spectrum Scope. If I turn it off the
> noise
> > desappears and also the noise changes pitch by changing the spectrum
> span.
> > The S3LO signal is clean as I checked with my scope.
> >
> > 2- When I turn the radio on, I can hear signals and noticed that the
> radio
> > is scanning, eventhough the scan is turned off. If I turn the
> > main dial the
> > scanning stops and the receiver dies, I cannot hear any signal at all, no
> > matter which antenna port is selected or which VFO a or B is
> > selected, seems
> > like the antenna is disconnected from the radio.
> >
> > Will keep going tomorrow, I think this is not an easy one...
> >
> > 73, Jim, HK1N
> >
> > On Thu, Oct 14, 2010 at 3:11 PM, Frank A. Ellis
> > <w3uhf.frank at gmail.com>wrote:
> >
> > > Jim,
> > >
> > > While this result may seem intuitive, don't forget that when
> > the LO power
> > > is removed from a mixer, very little to no signal will appear
> > at the output
> > > (IF port) of the mixer. The little power that will appear there
> > will consist
> > > of the RF signal applied to the RF port, attenuated by the mixer's
> RF-IF
> > > port isolation, which could be in the 30-40dB range. Whatever power is
> > > passed through the mixer, in this fashion, severely attenuated, will
> > > certainly be well out of the frequency range that the
> > subsequent IF stages
> > > are designed to process.
> > >
> > > So - don't rule out the possibility that the signal appearing at the RF
> > > port of the mixer is actually the "dirty" signal, and you are
> > disabling it
> > > from being processed in the mixer by removing the LO power. A spectrum
> > > analyzer would be pretty valuable in verifying the cleanliness of all
> > > signals being presented to the mixer.
> > >
> > > Just one more thought, without me looking it up, is J211 an "LO only"
> > > connector, or is the LO signal just one of the signals carried
> > through this
> > > connector? If this connector is carrying multiple lines, you
> > could also be
> > > disabling something else by disconnecting it, leading to a possible red
> > > herring.
> > >
> > > 73, good luck, and keep us posted on your findings!
> > >
> > > Frank, W3UHF
> > >
> > >
> > >
> > >
> > > On Oct 14, 2010, at 11:10 AM, Jaime Gomez wrote:
> > >
> > > > Thanks to all who have answered my request for help. I opened
> > the radio
> > > and
> > > > so far what I have found is that if I disconnect the cable at
> > J211 in the
> > > > main board, which carries the signal from 3LO the noise dissapears.
> It
> > > looks
> > > > like something in the VCO card is causing the noise.
> > > > Any other hints?
> > > > Thank you,
> > > >
> > > > Jim, HK1N
> > > >
> > > > On Wed, Oct 13, 2010 at 2:47 PM, Alex <alex at kr1st.com> wrote:
> > > >
> > > >> Hi Jim,
> > > >>
> > > >> This sounds suspiciously like some cable became disconnected
> > from a PCB,
> > > >> and
> > > >> is perhaps now close to the CPU board or even touching it. Possibly
> a
> > > cable
> > > >> that connects to the PA board. Very wild guess without looking at
> the
> > > >> transceiver. :)
> > > >>
> > > >> I don't have any experience with this, but I found a similar problem
> > > with a
> > > >> FT-897. The problem was that I could hear a noise on 70cm
> > every time I
> > > >> turned the dial. What I ended up doing was rerouting the small coax
> > > cable
> > > >> that came from the RX/TX board and connected to the PA board
> > away from
> > > the
> > > >> front panel. The noise was then substantially less.
> > > >>
> > > >> So it may not be something terribly complicated what is
> > wrong with your
> > > >> radio.
> > > >>
> > > >> I hope this helps somewhat.
> > > >>
> > > >> 73,
> > > >> --Alex KR1ST
> > > >> http://www.kr1st.com
> > > >> http://www.airlinkexpress.org
> > > >>
> > > >>
> > > >> -----Original Message-----
> > > >> From: icom-bounces at mailman.qth.net
> > [mailto:icom-bounces at mailman.qth.net
> > > ]
> > > >> On
> > > >> Behalf Of Jaime Gomez
> > > >> Sent: Wednesday, October 13, 2010 2:36 PM
> > > >> To: ICOM Reflector
> > > >> Subject: Re: [ICOM] 756 pro III noise
> > > >>
> > > >> Well, I did the reset, as per Walt's instructions. Same thing, no
> Tx,
> > > >> popping noise on all frequencies, not affected by the noise blanker,
> > > AGC,
> > > >> filters, etc....and now I noticed something else, when I
> > turn the dial,
> > > the
> > > >> popping increases and depends of the dial turning
> > speed....Obviously the
> > > >> auto tuner is not working because there is no rf to excite it....
> > > >>
> > > >> What can I look at, if there is anything?
> > > >>
> > > >> 73, Jim, HK1N
> > > >>
> > > >> On Wed, Oct 13, 2010 at 12:36 PM, Walt Heilsnis - WB2VSJ <
> > > >> wb2vsj at earthlink.net> wrote:
> > > >>
> > > >>> This is beginning to sound a little bit familiar - I
> > vaguely remember
> > > >>> something somewhere about a transmit issue like this and I
> > think it was
> > > >>> Auto-Tuner related.
> > > >>>
> > > >>> Anyway, according to the PRO III Manual the reset procedure is:
> > > >>>
> > > >>> - Make sure the transceiver power is OFF.
> > > >>> - While pushing [M-CL] and [(F-INP)ENT], push
> > > >>> [POWER] to turn power ON.
> > > >>> - The internal CPU is reset.
> > > >>> - A/D convertor calibration of the DSP unit starts and it
> > > >>> takes 10 sec.
> > > >>> - The transceiver displays its initial VFO frequencies when
> > > >>> resetting is complete.
> > > >>> - Correct the set mode settings after resetting, if desired.
> > > >>>
> > > >>> I have a PRO II and have not had any issues like this (yet) :)
> > > >>>
> > > >>> Walt
> > > >>>
> > > >>>
> > > >>> -----Original Message-----
> > > >>>> From: Jaime Gomez <cwdx.hk1n at gmail.com>
> > > >>>> Sent: Oct 13, 2010 1:07 PM
> > > >>>> To: ICOM Reflector <icom at mailman.qth.net>
> > > >>>> Subject: Re: [ICOM] 756 pro III noise
> > > >>>>
> > > >>>> Hi Walt, I have connected a dummy load to the antenna terminal, no
> > > >> change,
> > > >>>> noise is present in all bands, all modes, not affected by
> > attenuators
> > > or
> > > >>>> preamps, all other radios in the station (7800, proII, proIII,
> 781)
> > > work
> > > >>>> fine. I just noticed also that it is not transmitting, the Tx red
> > > light
> > > >>> just
> > > >>>> blinks and turns off and no power output.
> > > >>>>
> > > >>>> How do I hard reset the rig to factory fresh as Rich sugested?
> > > >>>>
> > > >>>> Thanks, 73
> > > >>>>
> > > >>>
> > > >>> ----
> > > >>> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > > >>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > > >>> Icom FAQ: http://www.qsl.net/icom/
> > > >>> To support QSL/QTH.net: http://www.qsl.net/donate.html
> > > >>>
> > > >>
> > > >>
> > > >>
> > > >> --
> > > >> Jim, HK1N
> > > >> ----
> > > >> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > > >> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > > >> Icom FAQ: http://www.qsl.net/icom/
> > > >> To support QSL/QTH.net: http://www.qsl.net/donate.html
> > > >>
> > > >> ----
> > > >> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > > >> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > > >> Icom FAQ: http://www.qsl.net/icom/
> > > >> To support QSL/QTH.net: http://www.qsl.net/donate.html
> > > >>
> > > >
> > > >
> > > >
> > > > --
> > > > Jim, HK1N
> > > > ----
> > > > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > > > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > > > Icom FAQ: http://www.qsl.net/icom/
> > > > To support QSL/QTH.net: http://www.qsl.net/donate.html
> > >
> > > 73, Frank A. Ellis, W3UHF
> > >
> > >
> > >
> > > ----
> > > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > > Icom FAQ: http://www.qsl.net/icom/
> > > To support QSL/QTH.net: http://www.qsl.net/donate.html
> > >
> >
> >
> >
> > --
> > Jim, HK1N
> > ----
> > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > Icom FAQ: http://www.qsl.net/icom/
> > To support QSL/QTH.net: http://www.qsl.net/donate.html
>
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
> To support QSL/QTH.net: http://www.qsl.net/donate.html
>

-- 
Jim, HK1N