[ICOM] 756ProIII PA Failure

[Adam Farson]

No - none at all. The IC-746Pro failure affected the exciter output stage,
not the PA. 

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 Jim Vohland
Sent: 25-Aug-08 14:14
To: ICOM Reflector
Subject: RE: [ICOM] 756ProIII PA Failure


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/

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