Views on the design of this tv aerial with amplifier I bought

Discussion in 'General Electronics Chat' started by BrainFog, Aug 22, 2012.

  1. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    Hello everybody

    Recently while randomly looking online for possible parts for the audio amplifier I am designing I came across a TV aerial with a built in amplifier that seemed perfect to solve my poor TV reception problem so I put a bid of £3.33 and won it at £3.31.

    It arrived today and I tested it out and it works well. After testing it I realised that as I was holding it that it was vibrating slightly and I noticed a hum. I suspected a poorly wound transformer and used it as an excuse to open it up. I am very interested in seeing what makes things work and sometimes I will buy something that is very cheap just to take it apart.

    When I opened it up I was surprised at what I found:
    [​IMG]
    Seeing as I am doing lots of research into amplifiers and filters and so on I expected to find something like an amplifier. Probably a few ICs and transistors. Instead I found lots of resistors and inductors sporadically placed and only a single transistor. I must say that I am impressed that someone managed to design something like this which works. It was clearly built to a budget (saying it nicely). They even seem to have forgone the complexity of a bridge rectifier to save on diodes and gone for a half bridge with 1 diode. Interestingly enough it has an optional DC input despite having a plug and transformer built in. I would have preferred it if they had at least splashed out on a fuse.

    I am wondering what you all think of the design? I must know, how on earth can this hodge podge of resistors, capacitors and inductors function as an amplifier?

    Thanks
     
  2. cork_ie

    Member

    Oct 8, 2011
    348
    58
    TV signal amps are generally fairly simple affairs.The amp is normally mounted close to the aerial and I suspect what you have is actually the power supply.

    As there are two potentiometers on your board it possibly has secondary amplification built into the power supply with variable gain.

    Most of the circuitry is band selection filters that allows DC pass up the cable to the masthead amp & block it on the TV outlet. In most cases there is also VHF pass through.
     
  3. #12

    Expert

    Nov 30, 2010
    16,355
    6,853
    An RF amplifier uses more parts getting the impedance matching, bandwidth, and gain right than it uses for amplification. No surprise to see 8 or 10 parts for tuning a single transistor stage. Best to keep your fingers away from the coils! After all, FM is the acronym for F***ing Magic! :D
     
    Last edited: Aug 23, 2012
  4. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    Thanks for the replies.

    I assumed that they would be highly complex as they must process many frequencies individually. Clearly this is another case where my assumptions were wrong. It is what I love about physics, it always makes me rethink almost everything I thought I knew. I will have to do some research into TV and radio signals. It does make me wonder why they are so expensive to buy. This one here is only £25 http://www.maplin.co.uk/digital-tv-signal-booster-36576

    I wish I had made one instead of buying now. Maybe a possible project for the future. It looks like it would only cost a few quid in parts, most of which I already have.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Many years ago when TV transmissions were still analog, I bought a cheap TV "RF amplifier". It also used a simple circuit with only one transistor.

    It was severely overloaded by all the strong local stations (about 20 of them) so it was completely useless.
    I replaced its Oriental ordinary transistor with an American microwave transistor and biased it so it had much more current then it worked very well.
     
  6. #12

    Expert

    Nov 30, 2010
    16,355
    6,853
    The parts are cheap enough. It's the ability to measure and tune circuits at 900 MHz that is expensive.
     
  7. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    I'm surprised you did not use something Canadian to replace the Oriental part.... :rolleyes: Since you are so into "not using oriental" parts, how much of your home electronics have you reworked to remove all the oriental made parts?
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Please don't be rude and sarcastic.
    Everybody knows that most Chinese electronic products have poor quality.
    The horrible Chinese TV RF amplifier was sold by RadioShack when they were still in Canada.

    Silicon Valley is in California, not in Canada and not in China.

    I worked in Canada for an excellent Japanese PRO Audio manufacturer for a few years.
    They were Oriental but I never made anything with their parts. Their high power amplifier guarantee said, "No questions asked" and none ever failed.
     
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