[ICOM] 756ProIII PA Failure

[Jim Vohland]

Just curious if there is any correlation between this and the rash of recent PA failures with the 746 Pro. Icom sure has had bad luck! Part suppliers perhaps???


> From: farson at shaw.ca
> To: icom at mailman.qth.net
> Subject: RE: [ICOM] 756ProIII PA Failure
> Date: Mon, 25 Aug 2008 09:48:57 -0700
>
> Hi Frank,
>
> Icom America Parts in Bellevue WA should be able to supply you with a
> matched pair of 2SC1972's.
>
> It is standard design practice to leave the PA unit live even when the radio
> is powered down. Manufacturers do not wish to include a 30A contactor in
> the radio to switch the primary DC supply rail to the PA.
>
> As other list members have suggested, all radio equipment should be isolated
> from antennas, power sources and peripheral circuits (such as phone lines)
> when not in use - especially in a lightning-prone location such as Florida.
>
> The new MOSFET PA Unit may not be compatible with the older BJT version of
> the Pro3.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
>
> -----Original Message-----
> From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
> Behalf Of Frank Haas KB4T
> Sent: 25-Aug-08 05:20
> To: icom at mailman.qth.net
> Subject: [ICOM] 756ProIII PA Failure
>
>
> This past weekend the output power on my 756ProIII dropped to 3 watts while
> I was calling CQ on CW. Upon investigation I found that both of the 2SC1972
> driver transistors were shorted base to collector. One of the two drivers
> had gotten so hot it charred the circuit board holding
> D1 in place over it. I don't see any other problems.
>
> There is no obvious cause for this failure. No power line surge or lightning
> at the time of the failure. The PS-125 power supply seems to be delivering a
> stable 14V to the radio. Nearly one year ago to the day my home took a
> lightning strike which damaged other gear but didn't seem to affect the
> ProIII. I suppose it's possible that this is a long delayed stress failure
> from that incident. The ProIII was running 100 watts output to a tuner and a
> ladder line fed 80M full wave loop that has no obvious problems.
>
> With the ProIII down, I moved my backup ProII into the primary operating
> position and I'm still on the air without any problems now.
>
> It appears that 2SC1972 transistors are hard to find now. I read on Adam's
> web site that there is a later version of the PA available using newer style
> components.
>
> My question is: Should I try to locate a pair of 2SC1972s and complete the
> repair for about $150 or replace the PA?
>
> I was surprised to find that power is applied to the PA all the time even
> with the rig turned off. I wonder if this is a good practice?
>
> 73,
>
> Frank N. Haas KB4T
> Florida
>
> ----
> 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/

_________________________________________________________________
See what people are saying about Windows Live.  Check out featured posts.
http://www.windowslive.com/connect?ocid=TXT_TAGLM_WL_connect2_082008