[ICOM] Mosfet Final Board 756 Pro3 Damage

[Larry Young]

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 <jkolin at optonline.net> 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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