Electronic Circuits and Projects Forum



Relacement of mosfet in Radio Shack TV preamp

  1. 3rd September 2009 04:05 PM #1
    Sid Emmons Sid Emmons is offline

    Relacement of mosfet in Radio Shack TV preamp

    I've several of their 15-1109 preamps with blown mosfets. The only replacement ones I have are not capable of standing the source voltage (12vdc+) for very long. Can anyone suggest which mosfet (surface mount or conventional) I should use?
    While I realize this amp's noise figure could be better, the one I'm using allows HD reception from 100 miles out using an old Channelmaster parabolic UHF antenna at 60 ft being fed by about 90 ft of RG-6. I'm located some 60 miles south of Buffalo, NY and about 100 miles from Rochester. The fine educational station (WPSU) at State College, PA also rolls in nicely
    Tnx, Sid
  2. 5th September 2009 08:44 AM #2
    mneary mneary is offline
    If the parts list I found on the net is accurate, then I see your problem. The transistor isn't a MOSFET. The part that I see in the parts list is BFG520X, which comes back as a 9 GHz bipolar transistor from NXP.

    It's available at Mouser for 47 cents.

    If you have a schematic, it would be nice to see. I couldn't find one.

    [edit] If there is no protection at the mast, maybe a 12V zener or surge protector is needed. [/edit]
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    Last edited by mneary; 5th September 2009 at 08:53 AM. Reason: gramer n splining
  3. 5th September 2009 04:20 PM #3
    Sid Emmons Sid Emmons is offline

    Bipolar versus Mosfet

    Well that makes good sense! Thank you very much for your trouble. I've never seen a schematic for the preamp either and likely never will as it normally wouldn't be worthwhile repairing same. They work well on my home brew dual rhomboid UHF receiving antenna.
    Regards, sid K8ZES/2
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  4. 5th September 2009 07:06 PM #4
    bountyhunter bountyhunter is offline
    There are some really good UHF and HI-VHF/UHF combo antennas available that make the need for an amp go away. For a little $$ now, you could make your life simpler..... those amps blew up for a reason.

    http://www.solidsignal.com/prod_display.asp?PROD=HBU44

    http://www.solidsignal.com/prod_display.asp?PROD=MXU59

    http://www.solidsignal.com/prod_disp...cid=2455434644
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    Last edited by bountyhunter; 5th September 2009 at 07:08 PM.
  5. 6th September 2009 03:06 AM #5
    Sid Emmons Sid Emmons is offline

    Better Antennas

    Your point is well taken. However I've been TV dxing for some 50 years so the challenge is the key, not just the reception. The antenna I'm using is the highest gain 'consumer' unit ever sold so the purpose of the preamp becomes that of overcoming transmission line loss as well as overcoming the relatively poor sensitivity of the TV itself. Indeed, in spite of using a new Sony LCD set; I discovered the government funded analog to digital converter was significantly more sensitive than the Sony.
    I've also built a dual rhomboid UHF wire antenna which is even more powerful than any commercial unit but suffers from being a bit flimsy in high winds.
    Those of us who tinker with reception will kill for a few more db of signal to noise ratio!
    Tnx again for your reply.

    Sid E
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  6. 6th September 2009 03:53 AM #6
    RCinFLA RCinFLA is offline
    TV Dx'g ain't what it used to be.

    There are so many stations on the air now that preamp overload is the norm.

    With digital TV it is not easy to identify crossmod like it was with 'herringbone' on analog signals.

    Things should be better with all the analog 5 MW UHF 'flame throwers' off the air.

    ATSC digital is a lot more picky about multipath. Again, no more analog ghosts to look for.
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  7. 6th September 2009 06:18 AM #7
    bountyhunter bountyhunter is offline
    Quote Originally Posted by Sid Emmons View Post
    Your point is well taken. However I've been TV dxing for some 50 years so the challenge is the key, not just the reception. The antenna I'm using is the highest gain 'consumer' unit ever sold
    Got a spec sheet? I might be looking for a better antenna. My favorite CBS station is now 110 miles away and I can usually not get it even though I have the highest gain UHF antenna for sale back in 1988, dedicated RG-6 downlead for the antenna and an adjustable booster.
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  8. 6th September 2009 06:24 AM #8
    bountyhunter bountyhunter is offline
    Quote Originally Posted by RCinFLA View Post
    TV Dx'g ain't what it used to be.

    There are so many stations on the air now that preamp overload is the norm.

    With digital TV it is not easy to identify crossmod like it was with 'herringbone' on analog signals.

    Things should be better with all the analog 5 MW UHF 'flame throwers' off the air.

    ATSC digital is a lot more picky about multipath. Again, no more analog ghosts to look for.
    Problem we have is our "local" channels come from two different towers that are about 60 degrees offline from each other. The high gain UHF antennas are too directional to deal with that very well.
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