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Possible Front End Overload

TheRealPorkchop

TheRealPorkchop

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Aug 25, 2015
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Hopefully this is the right spot for this...

I've got a Magnum S9, love this radio, had it a very long time. The receive is very low on it now, don't know what happened. I've heard of front end overload and "blowing out" the front end but I'm not really sure what that is.

Searching for a cure, I came across the D15 & D16 being possible culprits to my problem but I replaced them and that didn't fix it. So my question is what is the front end, which parts? Where and/or how do I go about finding out if that's what's wrong with my radio? I bought this brand new and right out the box, it's never been touched for anything except a real alignment a few years ago and it didn't really need that. Can anyone please help point me in the right direction so I can get this back up and going again?
 

look at the left top of the schematic for the parts. D! and D2 are a couple that can get blown out if you get hit hard.
 
Well it's not D1 or D2, nor is it D15 or D16. Considering I'm not a true tech, this might be a good door stop or paperweight. Looking at the schematic, doesn't tell me much, I know what all the symbols and stuff is but still...

Looking at the voltages on a few of the transistors, like TR8 and going by what CBT has listed for a galaxy, the readings are a little high. If I knew how to hunt the problem, I would.
 
d15/16 are the front end diodes ...
d1/2 are the NB diodes and are way to deep in the 10.695mhz I.F chain to short i would think

tr017 (2sc1674) often goes even thou d15/16 are "supposed to provide "some protection " but rare to see that happen the collector to emitter opens up on tr017 before they conduct hard enough ..
worse case i have seen the coil open up internally on the primary of L6 which means a replacement of that coil
..a simple signal injection from the hot side of C34 and then C45 then primary of L7(via a small cap and probe to not load down circuit will confirm which pat of the front end loses signal

if the AGC chip is shorted internally that can pull the receive down too..another common issue with the 3600/14 hybirds

of course checking all supply rail electro caps to see if any are shorted is another way receive can be pulled down ..there is 7x 10v electros on the s9 supply rails ..being a 12 year old design one could look at them also
 
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I've checked and I still haven't figured it out so I'm laying it to the side for the time being. Gonna try to look it over again this weekend and I'll report back with anything new I come up with.

Thanks guys!
 
The "overload" problem you hear discussed is a frequent hazard to truck drivers. Pull up with your antenna three feet from the one on the tractor next to you and that guy's Band Smasher 5000 could pump enough RF power into the receiver's antenna circuits to damage the diodes in question.

Maybe.

Lightning hitting a tree across the street can put this sort of surge into a mobile radio parked in your driveway, if the antenna is connected to it.

This is not the only kind of failure that can make the receiver become wimpy.

But it's a common one.

Other stuff in the receiver can break down just because it was time.

73
 
I can pull C45 and inject a signal from my generator directly to L6 and I get nothing. I can inject on the collector of TR17 and I get receive but there is no gain control.

At C333, I get nothing, which leads directly to the RF gain control. I removed L6 and injected directly to R66, still nothing. Removed C333 to be sure it wasn't shorted, nothing still.

And yes, I have an RF generator, a Marconi 2018. I also have various other "tools" that I bought with the intentions of learning more than what I know already, which is little it seems. If I just send it out, I'll never learn and when I have this issue again, I'll have to send it off too. So I'd like to fix it myself and learn something new...
 
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You can learn it. My curiosity started when I was 9 years old and it lead down a long career path that I do not regret. Go for it! I fixed my first radio when I was 9.
 
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Yea I've been tinkering with stuff since I was a kid too but I've not made as much progress as some of you guys, obviously.
 
TheRealPorkchop said:
I can pull C45 and inject a signal from my generator directly to L6 and I get nothing. I can inject on the collector of TR17 and I get receive but there is no gain control.

At C333, I get nothing, which leads directly to the RF gain control. I removed L6 and injected directly to R66, still nothing. Removed C333 to be sure it wasn't shorted, nothing still.

And yes, I have an RF generator, a Marconi 2018. I also have various other "tools" that I bought with the intentions of learning more than what I know already, which is little it seems. If I just send it out, I'll never learn and when I have this issue again, I'll have to send it off too. So I'd like to fix it myself and learn something new...
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Try replacing L6. TR17 is known to go bad . Signal generator is money in situations like this , inject and listen for the tone.
 

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