[ICOM] Mosfet Final Board 756 Pro3 Damage

[Gary Fiber]

So far as I remember in the older Icoms the RX port was connected during transmit so it would be up to the operator to build up some antenna switch to remove the receiving antenna during transmit. I would be almost sure Icom did not change the design in the Pro3 however I have not looked at a schematic of the radio.

Gary P. Fiber K8IZ



> On Nov 30, 2015, at 8:49 AM, JK <jkolin at optonline.net> wrote:
> 
> 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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>>  ----
>> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
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> ----
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