[ICOM] 756 pro III noise

Thu Oct 14 16:11:06 EDT 2010

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