[ICOM] IC-746 tx power problem

[Frank A. Ellis]

Hi All,

The following may be interesting reading:

http://www.rudius.net/oz2m/ic746pro/index.htm

http://www.qsl.net/k5lxp/projects/746Pro/746ProNoTX.html


73, Frank W3UHF




--- 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
> Icom FAQ: http://www.qsl.net/icom/
> 
> ----
> 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/
> 


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