[ICOM] 756 pro III noise

Thu Oct 14 19:36:22 EDT 2010

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