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MXT400 Bad 239 Connection & Audio Circuit Failure


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#1 marcspaz

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Posted 14 June 2019 - 01:12 AM

Today, the audio circuit in my MXT400 took a dump. Turned out to be a free fix. A bias resistor on the audio amp transistor base had a cold solder joint. I looked around the board and found 4 more, so I touched them up too.

Worst thing I have ever seen... the center lead of the so239 was NOT soldered to the RF output circuit. The connector was just screwed to the chassis and the tip was simply pushing against a pin that was sticking up from the main board. No solder... just spring pressure. Needless to say, I fixed that, too.

Not too happy I spent 2+ hours fixing a brand new $250 radio due to poor workmanship. At least none of the parts fried, which is a plus.

I am becoming less and less of a fan.
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#2 axorlov

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Posted 14 June 2019 - 01:08 PM

The SO239 not soldered, could be intentional. Some Icom transceivers use the spring loaded center contact against the board, to prevent PCB traces cracking from repeated thermal expansion/contraction. I'm not saying it is the case with your MXT400, I can't say anything, never looked inside, do not own one.


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#3 marcspaz

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Posted 14 June 2019 - 01:18 PM

If there were actual springs or spring steel, I could see that. This was a solid pin sticking up from the board and someone or something clearly tinned both parts... they just didn't drop the last bead to make the connection. If it wasn't for the fact that they were touching by coincidence, the radio would have smoked.

#4 n4gix

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Posted 16 June 2019 - 06:57 PM

Was this a surface mount transistor? I didn't think anyone was using those these days... ;)
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#5 marcspaz

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Posted 16 June 2019 - 11:13 PM

Was this a surface mount transistor? I didn't think anyone was using those these days... ;)

 

 

LOL  Too funny.  3 leg through-hole.  Actually, all of the cold joints were on through-hole connections.  Guess the wave pan wasn't hot enough or the board flexed to much when it was heated and cooled.



#6 n4gix

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Posted 17 June 2019 - 01:00 PM

LOL  Too funny.  3 leg through-hole.  Actually, all of the cold joints were on through-hole connections.  Guess the wave pan wasn't hot enough or the board flexed to much when it was heated and cooled.

Good grief! How "1990's of them. Many folks are astonished to learn that most of the CCRs being imported have the complete transceiver on a single surface mount chip. All the rest of the components are there to support that single chip... laugh.png


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