[ICOM] IC-746 tx power problem

Thu Jun 29 15:16:02 EDT 2006

Icom CERTAINLY mentions this fix at their site.  Unless I am misunderstanding your comments.

Sean
W8OKN

Here is what they say...

There are rumors on the Internet that the circuit around IC-151 is incorrectly designed causing overheating of this part and that the protection diode fix is an attempt to mislead customers by ICOM. Nothing could be further from the truth. Measurements show that the surface temperature of IC-151 is low, even after 4 hours of continuous operation. Our service technicians do not see any discoloration of the circuit board or the deformation of parts that are the symptoms of overheating. As stated above, protection diodes have been shown to resolve the issue and no heating calculations are needed.  ICOM America service records show that the failure rate of this part has been reduced to 1/10th of the previous value.

Since February of 2003, this circuit change has been installed on every IC-746PRO that has been repaired at ICOM America. Our Authorized Independent Service Centers have also been asked to install this modification.

If your IC-746PRO is SN 3351 or higher, you already have the factory installed protection diodes. Or if your ‘PRO’ has been serviced by ICOM America in the last year, the protection diodes have been installed and you should enjoy years of trouble free service from your transceiver. (If it has been serviced by an independent service center, please contact them to determine if the protection diodes have been installed.)

If your radio does not fall into any of these three categories, do not worry. ICOM America’s standard repair policy is to offer all technical updates free-of-charge to any ICOM America radio still in production. Please complete the form below and a Technical Service Representative will contact you with more information on how to send your radio in for the Technical Update.

Please keep in mind that external forces may cause failure to RF equipment. Some precautions to take during the installation and operation of the radio are as follows:

Recommendations made by the ARRL and PolyPhaser include the addition of lightning protection. This includes disconnecting and grounding the antenna feed line when the radio is not in use. 

It is also necessary to ensure that the antenna has a low VSWR. 
Use a transistor buffer circuit to key a non Icom linear amplifier. This will prevent damgage to or sticking of the send relay contacts in the radio. 
Do not use the radios internal antenna tuner when using an external antenna tuner. With a linear amplifier use an external tuner after the amp and leave the internal tuner off. 

If running multiple transmitters or antennas be sure to keep antennas spaced far enough apart to prevent RF overload to the receiver front end. 
Keep all accessory cables as short as possible especially when using a linear amplifier. Shield cables that are too long and ground the shield at both ends to prevent RF Feedback. A ferrite bead around the cable may help if shielding the cable is not an option.

ICOM is committed to your satisfaction. 

---- Bert Almemo <balmemo at sympatico.ca> wrote: 
> Hi Paul,
> 
> Thanks for your reply and suggestions. I don't know the difference in the
> circuitry between the 746 and 756pro2 and if the problem you're describing
> relates to the 746. 
> 
> I know that the 746 had a lot of problem with the IC151 in the beginning.
> ICOM modified the circuitry in the later ser.nos to fix the problem. What
> I'm really after is what ICOM did to fix the problem permanently. I've
> surfed the ICOM site but can't find anything relating to this problem. If
> anybody can shed some light on this I would really appreciate it.
> 
> I'll check the mods.dk fix regarding the relay as I haven't seen or heard of
> that one before. When my friend's 746 is fixed I'll post what was done to
> bring it back to normal again.
> 
> 73 Bert, VE3OBU
> 
> 
>  
> 
> -----Original Message-----
> From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
> Behalf Of Paul Playford
> Sent: Thursday, June 29, 2006 12:59 PM
> To: ICOM Reflector
> Subject: Re: [ICOM] IC-746 tx power problem
> 
> When IC151 failed in my IC-756PROII, I also found a shorted PIN diode and .1
> Uf coupling capacitor in the adjacent circuitry shorted.  I concluded the .1
> Uf capacitor went first and took everything else with it.
> 
> Then later a problem with my radio surfaced on 10m.  It would go to full
> output as soon as the transmitter was keyed, regardless of the RF POWER
> setting.
> 
> My fix was to just stay off of 10m as the sun spots were going away.
> 
> Then I acquired a 6m antenna and I was back to full output.
> 
> So the radio went on the bench.
> 
> All was well (into a dummy load).
> 
> Back in service, no worky.
> 
> I found that by paralleling the dummy load and the antenna on 6m the radio
> worked fine.  Cost me 3dB on receive but the preamp overcame that.
> 
> Then the mods.dk fix was suggested to me.  I installed the relay and now all
> is well on all bands.  Three years and no further problems.
> 
> As the modification suggests: there is just too much coupling between the NO
> and NC reed relay pins and during transmit (and perhaps with a little swr)
> enough rf feeds back into the input of IC151 to cause the oscillation.  I am
> convinced that that oscillation shorted the capacitor and PIN diode and
> killed IC151 the first time around.
> 
> de Paul, W8AEF
> 
> ----- Original Message -----
> From: "Bert Almemo" <balmemo at sympatico.ca>
> To: "'ICOM Reflector'" <icom at mailman.qth.net>
> Sent: Thursday, June 29, 2006 6:20 AM
> Subject: RE: [ICOM] IC-746 tx power problem
> 
> 
> > Hi Paul,
> >
> > Thanks for your reply. I have been to mods.dk and I found a couple of
> > articles about the tx problem. I'm just wondering if there is more to it 
> > or
> > if this is all you have to do to get rid of the problem?
> >
> > The only answer I got was from you so I hope someone else is reading this
> > and might have some more comments. Thanks.
> >
> > 73 Bert, VE3OBU
> > 
> ----
> Your Moderator: Dick Flanagan K7VC, icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
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> 
> ----
> 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/