[ICOM] Mosfet Final Board 756 Pro3 Damage

[Billy Cox]

Yes, that is what I mentioned before. Here is a simple test
to see if this might be part of the challenges there ...

Hook up a watt meter and a dummy load to your Beverage coax.
In other words, it is NOT connected to any rig at this time.

(The watt meter should be connected in-line so that it reads 
the "forward" power to the dummy load)

Now, key the rig and measure the amount of RF on the RX line,
start with low power, and use the watt meter to measure the RF.

Check all the bands you use the Beverage on ... 

Now, do the same test again (CAREFULLY!) with high power.

It may surprise you ...

73 de Billy, AA4NU

----- Original Message -----
From: "Bruce" <bruce at sultantronics.com>

You may have created spurious Tx drive from coupling of the beverage antenna port input and the antenna you were putting that
1,500 watts into. The RX port is sensitive to RF overload and I have heard of others causing damage to their 756's when the beverage
antenna is too close to the Tx antenna. Even though your antennas may not be close that 1,500 watts could couple significant RF
into the beverage antenna.
That coupled RF could have caused damage to some of the switching diodes on the RF pcb and that in turn created spurious Tx output
which caused damage to the PA in the 756. I would check the RF unit for damage along with repairing the PA damage.

-----Original Message-----
From: Icom [mailto:icom-bounces at mailman.qth.net] On Behalf Of JK
Sent: Monday, November 30, 2015 8:50 AM
To: ICOM Users <icom at mailman.qth.net>
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
>
>