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  1. #1
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    micro-tel 1200 service

    Hi, I am trying to figure out if a micro-tel 1200 wide range receiver needs service or not http://bama.edebris.com/download/mic...0_receiver.pdf
    The receiver seems to work ok but:
    If I connect a ham radio receiver at it's 30MHz IF output, I can notice the demodulated AM signals but there is lots of hum also there, so much that only the very strong AM stations can be heard along with much noise.
    I also notice that exactly the same demodulated signal (and hum) is produced not only on the IF port but also on the video detector and the bolometer port of the receiver. These three ports connect to the same PCB inside, that of the IF amplifier.

    So I wonder, should I open the receiver and try to see if there is something wrong with the IF PCB or the PSU (a faulty capacitor?) or does this behaviour seem normal in that receiver (maybe the RX is not intended for AM demodulation)?
    Last edited by neazoi; 7th February 2019 at 09:56.
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  2. #2
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    Re: micro-tel 1200 service

    I would replace C4 and C6 in the power supply and see if it makes a difference.
    Bear in mind this is not a communications receiver, more like a manually controlled spectrum analyzer but nevertheless it should not have any significant hum.

    Brian.
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  3. #3
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    Re: micro-tel 1200 service

    Quote Originally Posted by betwixt View Post
    I would replace C4 and C6 in the power supply and see if it makes a difference.
    Bear in mind this is not a communications receiver, more like a manually controlled spectrum analyzer but nevertheless it should not have any significant hum.

    Brian.
    Thanks Brian, I will do so and let you know. I may also place the scope to the 20v line to see if I will notice any 50Hz hum or not before replacing these capacitors.

    A thing that worries me is that the IF the VID out and the bolometer outputs should not present the same information. (page 93) IF should be RF, vid should be AF and I do not know about the bolometer. However when I connect each of them to a nearby 30MHz RX, I get the same demodulated signal out of them. To my view the RX should not be able to demodulate already demodulated audio in it's RX input.

    But anyway I will try the PSU thing first.
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    Re: micro-tel 1200 service

    Inevitably there will be some RF breakthrough so connecting to the input of a sensitive receiver is perhaps being a little unfair.

    The 30MHz IF out should be at quite a high level, certainly enough to overload the input of an external receiver. It is RF carrier.
    The Video output is the only AM outlet but in this context "video" refers to AM with a bandwidth extending to several MHz rather than in the TV sense.
    The Bolometer is the 'signal strength' output although technically it is 'power output', it should not be demodulated, more like an 'S' meter reading.

    Bolometers work on the principle of feeding the signal to a very sensitive (and usually low power rated) thermistor. They are used to measure power by noting the temperature rise when signal is fed across them, and hence the change in their resistance. The signal is then removed and the temperature allowed to settle again then a DC current is passed through it and increased until the same change in resistance is noted. The original signal power is then V*I of the DC source, the DC power that gives the same resistance shift. They have an advantage over measuring voltage across a known load in that they are non-inductive, work over very wide frequencies and do not care what the waveform shape is.

    Whatever you do, never connect a power source directly across the bolometer output, it will fry the thermistor and at best make it inaccurate, at worst kill it completely.

    Brian.
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    Re: micro-tel 1200 service

    Quote Originally Posted by betwixt View Post
    I would replace C4 and C6 in the power supply and see if it makes a difference.
    Bear in mind this is not a communications receiver, more like a manually controlled spectrum analyzer but nevertheless it should not have any significant hum.

    Brian.
    I have been notified by a colleague that the 30MHZ out may be a video signal representing the scanning function, hence the buzz as it sweeps. Could it be true so that this buzz is normal?
    To try to figure it out note that as I change the sweep control, making the "span" wider and wider, the buzz tone changes.
    Also I have noticed that at "Zero" span (the sweep setting recommended for hearing the signals instead of just seeing them on the screen) the buzz volume depends on the signal strength. A higher signal gives a higher buzz. On pulsed signals the buzz is not much heard, probably it happens too sudden to hear loudly. This makes me suspicious that the buzz is not because of PSU hum, as it should not change on different signal strengths if it was so.
    What do you think?
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    Re: micro-tel 1200 service

    It could be from the scanning, basically you would be converting the sweep range to an audio signal and hearing the shape of the spectrum amplitude. As I said before, this is not a receiver for listening to, it is designed as a selective signal strength meter. The 30MHz out is just a high level IF signal, not demodulated so it isn't 'video', the real video output is the demodulated one.

    Brian.
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    Re: micro-tel 1200 service

    Quote Originally Posted by betwixt View Post
    It could be from the scanning, basically you would be converting the sweep range to an audio signal and hearing the shape of the spectrum amplitude. As I said before, this is not a receiver for listening to, it is designed as a selective signal strength meter. The 30MHz out is just a high level IF signal, not demodulated so it isn't 'video', the real video output is the demodulated one.

    Brian.
    I wonder how is this scanning buzz generated? I mean what generates it, the different actual signal levels or some kind of leakage from the sweep oscillator output? It is weird that the IF is 5MHz whereas in comms receivers this is narrow. I mean what would be the use of such a wide IF is a mystery!
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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  8. #8
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    Re: micro-tel 1200 service

    I doubt it comes directly from the sweep generator, if it does it points to power supply problems.

    The easiest way to visualize it is to think of a normal S-meter signal fed to an audio amplifier. On a steady carrier level you would hear nothing because regardless of actual voltage, it is only changes that are audible. Now imagine you rapidly sweep across a signal so the S-meter goes from low to high and back to low. If you did it fast enough, the voltage across the meter would start to change fast enough to fall within hearing range. It follows that any change in received signal, if swept through fast enough would be audible. The change in voltage would make it quieter or louder and the rate of tuning through the signal(s) would change the pitch of the tone.

    A wide IF can be very useful if the signal you are looking at is also wide! Narrow band signals are used where space is limited but when you go to GHz frequencies, transmissions 30MHz or more wide are quite common. Even on low frequencies, some of the random pops and crackles you hear on short waves are just a fragment of a wider band signal falling inside your receiver passband.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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