[ICOM] IC-746 tx power problem

[Bert Almemo]

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/