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Thread: 2-pill MRF492 home made amp problem

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    2-pill MRF492 home made amp problem

    Hello all. I've ran into this situation before with an amp that had these same 2 transistors in them. I get massive back swing in this amp and a very high dead key.

    Right now with the amp on, dead key is just under 100 watts and swings down to 45 watts.
    With that said, the radio I am using with it is a cobra 29. The cobra is aligned and has a dead key of 1 watt and swings to about 4 (5.5 on PEP). I don't know what I'm doing wrong with these transistors. I don't k is whether to raise the dead key, or lower it even more. Maybe too much swing? Help is much appreciated.


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    *I don't know whether


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    Try some ferrite chokes at the feedpoint of your antenna. 6 of them. Make sure they are the right mix. You can google RF chokes for HF or CB. The mix is important. Like mix 31, 43 and so one. Also place one on your mic on the wire just under the mic itself. And wrap one around your speaker wire as many times as you can. Make sure the wraps are tight.
    Also you can try a longer jumper as well and see if that works. Something like a 6,9,12, or 15. Or somewhere in between. A 4ft one might work. Or any length that you have laying around that will work. Not 18ft, but you get what I mean. Try the jumper thing first, moving the amp away from the radio as well. And make sure your antenna is properly grounded. If this is your base station, add a ground to the amp and take it to the equipment grounds. Everything you have setup in your base should be grounded. Antenna mast, all equipment and any other things that are in line with the system. Use as many ground rods as needed and make sure they are all tied together to form a single point ground system. If this is confusing, look up what a single point ground system is. Hope all this helps and have a good one!!

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    Thanks Sean. It's my mobile. Not my base. I will try what you have recommended and let you know what I find.


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    Also if you can lower the dead key of the radio to .5-.75 watts. Yes that little! It might help as well. There is talk about some similar transistors on a different forum and what I have seen is drive levels are very low. Like I said, .25-.75 watts. That's 1/4 watt to 3/4's of a watt dead key. Also try to maintain a 1:4 ratio of dead key versus pep output from the radio. Or near that. So 1 watt dead key would be 4 watts or near that. If you do end up with a lower dead key needed, try to keep this 1:4 to a 1:5 ratio of power out of the radio to the amp. And also you can try the longer jumper thing I spoke of. It may help as well. Hope you get it resolved. Also if this is your mobile, look into bonding all body panels to frame as well as hood, doors, trunk, engine, and exhaust all to the frame using flat strap ground cable. You can buy the exhaust kit at most auto parts stores. Order the rest off Amazon or eBay. And get at least 1/2" or larger. Some spots can take a 1" flat braid ground strap. Just some food for thought. Again, hope you get it resolved. Good day.

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    Thanks Sean! I'm going to work on it this weekend and I will post my results.


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    Yep. No problem Matt. Hope it helps. Also check all your coax jumpers as well as your connector at the end of the antenna coax. It might not show with stock power, but add some power and things start to show that otherwise might not show up. Just some food for thought. Hope you get it all resolved and working man. And Happy New Year!!

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    And also try lowering the dead key some more if possible Matt. Not sure if I already said this or not. But it might only take 1/4-3/4 of a watt to make it work. That or like I said, You have some RF coming back into the amp causing it to go into oscillation. I am not an amp tech or not do I claim to have much knowledge about them, just throwing some things out that might be the issue is all. Again, I hope you get the issue resolved!!

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