[ICOM] Mosfet Final Board 756 Pro3 Damage

Mon Nov 30 17:27:06 EST 2015

    Before I bought my 576PROII I'd heard about the RX antenna not being 
disconnected from the RX during TX and several radios were like that 
including the very popular and older FT-1000MP. I'm not an engineer but 
it didn't seem like a very good idea to me? I'd had a FT-1000MP before 
the 756 PROII and never used it with a RX antenna and no problem with 
that issue. I did loose the PROII on a nearby lightening strike that 
also took out my main TV and telephone answering machines some 
indicators on a few rotors and my desktop computer used in the shack for 
contesting and everything else.. The PROII was well out of warranty and 
I sent it to a repair guy I've had good luck with before. He deemed it 
not worth fixing as the damage was so extensive as lightening had come 
thru every port of the radio with anything connected to it like the Icom 
PS and my main computer for logging and contesting, It seemed to him 
that lightening had come in via the neutral side of the power line. He 
felt that with this much damage it was not worth fixing as there was no 
telling how long it would be before other things would fail. Interesting 
at the time this happened it was connected to the matching Icom PS that 
came with the PROII. The PS was off but plugged into the wall outlet as 
was the computer. The PROII showed damage where the power leads came 
into the rig and where I'd had some connections for logging with WL. 
Amazingly the PS is still in used today and powers my replacement 
PROIII. There was no antenna connected as it was summer and CO is 2nd to 
FL in the number of lighten strikes in a year and it is mostly a summer 
thing here and I disconnect the antennas any time I leave the house when 
their is storm potential plus unplug everything from the mains. The 
Homeowner's ins tech guy convinced them that the PROII was not worth 
fixing but the insurance CO wanted me to get a TV repair guy to find 
that the TV was also due to lightening. There are only 2 people in the 
yellow pages that advertise they work on TV s and one only works on 
large home entertainment centers and the other one doesn't answer phone 
calls and may not even have a shop. Also who repairs TVs these days, you 
just get a new one. I did order "Meter Treaters" from the power company 
for the meters on both the house meter and the water well meter about 
100 yds down the Mesa for about $75 each installed and have had no 
further issues but did loose the contactor for the well motor control 
that was about $50 and took a few minutes to install myself on my side 
of the meter. I might add that my well is 1/4 mile from the house and I 
run 230VAC that far to the well buired in the pipe ditch but not very 
deep., With both the II and III I use one of KD9SV's DX-pedition II 
boxes with the "rig saver" feature that disconnects the RX antennas 
during TX.
    I'd highly recommend the Meter treaters as since then I've seen 
lightening hit the power pole beside the house twice and there was zero 
delay between the flash, the bang and the smell. The meters treaters 
cover damage of up to a few hundred thousand dollars. All my antennas 
are higher than the power pole except for the 2m EME array and I'm 
almost on top of a Mesa. I've never noticed any damage on any of the 
towers or antennas so far and it has been about 8 years since the first 
event.
    Icom does not seem to think this is an issue with the PROII and III 
design!
    Phil N0KE

Gary Fiber wrote:
> 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.
>>
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>> Any ideas? Anyone?
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>>
>> Thanks....
>>
>>
>>
>>
>> Jay   NE2Q
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>>
>>     
>>> 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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>>> Icom FAQ: http://www.qsl.net/icom/
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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/
>> To support QSL/QTH.net: http://www.qsl.net/donate.html
>>     
> ----
> 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/
> To support QSL/QTH.net: http://www.qsl.net/donate.html
>
>