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Magnum Sfx-350 bottom plate

Discussion in 'Amplifiers' started by johnecashflo, Mar 19, 2017.

  1. johnecashflo

    johnecashflo
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    I'm a doit yourself electronics novice. (Like to bring stuff back to life and still learning). I am working on a 350 bottom plate that got smoked. I pulled and checked the finals with transistor tester checked out okay. There was a area of the board that some traces where burned and the 100 ohm resistor that you see in the pics was fried. Fixed all that area. Not sure what the yellow core with the copper wire wraps is and could that be my problem.
    Symptoms- radio works fine with the plates 8 volt ptt off.
    With 8 v. Ptt on the radio/board together only put out about half the power and if you keep talking on it with the board on it will smoke the 100 ohm 2 watt resistor.
    IMG_3357.JPG IMG_3354.JPG IMG_3356.JPG


     

  2. nomadradio

    nomadradio
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    Odds are you need to replace all four of the ERF7530 transistors. Three of the four ERF7530 in the pic appear to have telltale bulges, revealing that the chip inside got hot and swelled the plastic from the heat as it failed. The pic makes it look as if three out of the four transistors are smoked.

    Could be that only one of the two push-pull pairs is bad. The resistor that smoked is a 'balance' component. The only power that it should throw is the difference between the left pair and the right pair. When everything is working and the power from each pair is equal, this is not much power at all.

    When one of the two pairs goes down, that resistor gets about half of the remaining RF power thrown onto it. Smokes it pretty fast.

    Just watch out for damaged foil traces. Any of them that are burned may have to be reconstructed with wire jumpers.

    73
     
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  3. Robb

    Robb
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    Yup

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    Sooo; is there a replacement/substitute for the 7530?
     
  4. johnecashflo

    johnecashflo
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    thanks for reply..as stated before I pulled and checked all four transistors.. they tested ok..as far as I can tell none of the rf is flowing thru the resistor when the linear is on as it puts out less power than with it off..still wondering if the yellow ring with the wire wraps in front of the resistor is bad or do they go bad..
     
  5. Robb

    Robb
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    Yup

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    They can go open circuit. There are a couple of different paths thru it, so you just might take it out and check it for continuity with the resistance setting on your DVM/meter. Eyeball it for a burnt open wire too.
     
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  6. johnecashflo

    johnecashflo
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    I meant to say that it put out less power with the 8 volt ptt on.. if I unhook the 8volt ptt wire the radio will flow full power to the meter/dummy load..with 8 volt ptt wire connected the power actually gets lower say about half the power..25 to 30 watts..and the resistor will get hot enough to burn you...thanks
     
  7. 9C1Driver

    9C1Driver
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    And that is the question of the day. They are very very hard to find, if all 4 are smoked good luck to him even getting that back on the air.
     
  8. johnecashflo

    johnecashflo
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    irfp250n ?????
     
  9. johnecashflo

    johnecashflo
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    found the problem for those interested it was/is the silver mica 10 pf cap on the left of the 100 ohm resistor.. tracked down where the rf stopped with my scope and damn I got lucky..thanks (y)
     
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  10. nomadradio

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    Way lucky. Good to hear!

    'Scope to the rescue! My favorite swiss-army knife for RF trouble.

    The ERF7530 is a re-branded part. Eric Lewis sold them through his "EKL Compnents" operation before his dad Sam died. Eric is now running the whole RF Limited/Magnum/Palomar operation. I don't think selling transistors is at the top of his list right now. I suspect he has a lot on his plate.

    Nobody has admitted to cracking the original identity of the ERF7530. Best kind of "substitute" would be to figure out the original number that they scraped off before printing "ERF7530" on the part. I have no doubt that it is originally a switch-type MOSFET intended to use in switchmode power supplies.

    But which one of thousands of types is it?

    Somebody needs to take some ERF7530 parts, make some measurements and figure out how to reverse-interpret them to match a part with an original-manufacturer's number on it.

    Most every MOSFET in that large package that I have looked up looks too slow to keep up with 27 MHz RF and use as an amplifier.

    Somebody will crack this. Just hope whoever it is will pass it along to the rest of us.

    And if he's smart, he'll just scrape off the number, print a new one and sell them for twenty times the price he pays for it.

    Kinda like Eric was doing.

    73
     
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