[ICOM] 756 pro III noise

Thu Oct 14 21:36:18 EDT 2010

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