[ICOM] Mosfet Final Board 756 Pro3 Damage

[Rich Dailey]

A ground loop?

Last time I was QRO was back in the early eighties, and not active since
2006, so caveat my advice. ~ N8UX

On Mon, Nov 30, 2015 at 11:49 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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>>
> ----
> 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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