[ICOM] Mosfet Final Board 756 Pro3 Damage

[Billy Cox]

Sounds like RF coming on on the RX ANT line. I had a very similar
problem years ago with my IC765 ... acted the same way at high power.

My solution was to use something similar to a KD9SV protection unit 
to prevent the Beverage from allowing the RF back inside the rig.

Hope this helps!

MC es HNY! 

73 de Billy, AA4NU

----- Original Message -----
From: "JK" <jkolin at optonline.net>
To: "ICOM Users" <icom at mailman.qth.net>
Sent: Monday, November 30, 2015 10:49:58 AM
Subject: Re: [ICOM] Mosfet Final Board 756 Pro3 Damage

The power cord is 2' long and I have not seen any significant pwer drop. 
I run it at 13.8V at the radio chassis. I was using a high power amp. A 
home Brew 8877. I use no more than 32 watts drive for 1500 watts output. 
When this problem occurred, I was using 25 watts out from radio. I had 
just hooked up a new 540 foot  beverage antenna to the receiving socket 
specified by Icom for that use. While tuning the amp I noticed some odd 
oscillations in the tuning and power out. I then smelled smoke from xvr 
and stopped transmitting.


SWR into the antenna I was using, a 1/4 wave sloper off my tower is 
below 1.5:1. SWR into the amp input is below 1.5:1.


I found those three resistors in parallel completely destroyed. I also 
measured a different resistance reading on the gates of the finals to 
ground. Q5 was 2.2K and Q4 was 0.56K. Possibly that is a clue.


Any ideas? Anyone?


Thanks....




Jay   NE2Q


On Sun, Nov 29, 2015 at 12:58 AM, Larry Young wrote:

> Jay; It is unlikely that anyone will be able to provide you with a 
> logical and reasonable answer for your failures w/o additional 
> information , such as what bands and mode of operation? Were you using 
> a high power amplifier, 50 Ohm antenna system etc?As to why surface 
> mount resistors crack and over heat? The answer is obviously due to 
> over dissipation. Why the over dissipation? Did you develop voltage 
> drop in your DC power cord and inline fuses? If this happens, the 
> Transmit ALC system will try to overcome by increasing the RF drive to 
> maintain output power, thus increasing PA current etc. A 1 volt drop 
> in the required nominal 13.8 volt DC at the power connector at the 
> back of your radio can mean a drop in power out. A gradual drop will 
> likely go unnoticed until a more substantial problem occurs.
> Larry K4LXV
>
>
>     On Saturday, November 28, 2015 8:42 AM, JK  wrote:
>
>
> My756 Pro3 recently dropped in output on most bands by about 20%. On 
> 160 pwer out is 22 watts max. The PA board is the later version that 
> now uses MOSFET finals.
>
>
> Upon examining the PA board I found three 4.7 ohm chip resistors R38, 
> R39 & R40 burned, cracked, broken completely. The solder holding these 
> three chips became so hot. the solder melted and the broken resistors 
> floated out of their correct positions.
>
>
> This happened 3 years ago on the same radio. But then, in addition to 
>  R 38, R 39 & R 40, 4 final transistor had also shorted the Gate to 
> Ground. 
>
>
> As I get about 80% of power out on some bands, I think the finals are 
> still ok. I have no idea what could cause R 38, R 39 & R 40 to burn 
> up. They are in parallel with inductor 8 so I would think most current 
> should pass thru 8 not these resistors.
>
>
> Anyone else have a similar situation or any idea why these 3 resistors 
> burn up?
>
>
> Thanks all.
>
>
> Jay...NE2Q
>
>
>
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