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Thread: Yaesu 757gx 757at 757hd trio

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    Default Yaesu 757gx 757at 757hd trio

    I recently got this wonderful trio as a gift and have some questions about it. Looking at the instruction manual for the 757hd it very plainly explains (common sense tells you as well) how to hook the unit up to 120v ac. My question comes in with hooking it up to a vehicles battery system. there are red and black posts on the rear (see attached picture), showing 13.5 volts. I assume that this is an input port for battery leads, but do not want to damage my radio or power unit by under-powering the system. I have attached the schematics and a rear view of the power source, from the manual. Maybe I am being over cautious with this, I just don't want to damage anything and intend to use this for years to come. 73s KD9KVS

    http://www.radiomanual.info/schemi/Y...757HD_user.pdf

    I have a brain injury. I am not a brain injury.
    This does not define me.
    Last edited by KD9KVS; Tue 9th Apr 2019 at 15:22. Reason: adding manual with images

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    K7KBN's Avatar
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    Measure the output from the AC power supply. It'll probably be pretty close to 14 or 15 volts dc. (Hint: most cars' electrical systems provide a nominal 13.5 volts dc.) If you measure the AC power supply output without connecting it to anything, 14-15 vdc is pretty normal, depending on the supply. Now when you connect the supply to the radio and turn everything on, you can expect the voltage to drop down closer to the "13.5" nominal voltage.
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    Pat K7KBN
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    Default How is appears

    Looking at it and asking advice from others it appears the legs are output for any other radio that does not use the four pin system. the four pin line feeds directly off this. Almost makes me wonder if I can use it as a feed-line by installing a switch to bypass the capacitors. All thoughts positive and negative are welcome on this.

    KD9KVS



    I have a brain injury,
    I am not my brain injury,
    It does not define me.

  4. #4
    Super Moderator 5B4AJB's Avatar
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    Quote Originally Posted by KD9KVS View Post
    under-powering the system
    There are Voltage regulators (or similar) inside the radio to deal with this, for example, when powering my TS-440S from a battery and the output Voltage sags below about 11V, the frequency display goes out before other LEDs, then the radio completely stops working at around 9V.

    What you should consider is the current required at full output power. For a 100W radio, with a bad SWR, you'll be pulling around 15A at 12V.

    Run thick (as possible) wires directly from the car battery to the radio, fuse both the red and black leads at 15A and you should be good to go...

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    Quote Originally Posted by 5B4AJB View Post
    There are Voltage regulators (or similar) inside the radio to deal with this, for example, when powering my TS-440S from a battery and the output Voltage sags below about 11V, the frequency display goes out before other LEDs, then the radio completely stops working at around 9V.

    What you should consider is the current required at full output power. For a 100W radio, with a bad SWR, you'll be pulling around 15A at 12V.

    Run thick (as possible) wires directly from the car battery to the radio, fuse both the red and black leads at 15A and you should be good to go...


    Is this for running directly to the radio, or feeding through the power supply? I have considered running off the battery, to an inverter, power conditioner then to the power supply to the radio, to ensure clean power. This would be in a 77 impala, so running a secondary alternator and battery would be no issue. Am I overthinking this, or thinking enough outside the box to be onto something?

    KD9KVS


    I have a brain injury,
    I am not my brain injury,
    It does not define me.

  6. #6
    Super Moderator 5B4AJB's Avatar
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    Quote Originally Posted by KD9KVS View Post
    battery, to an inverter, power conditioner then to the power supply
    That would be too inefficient, you'll only get a few percent of what you put in, no need to "recondition" DC.

    A second alternator & battery would be overkill, although it would prevent you flattening your starting battery (park on a hill?).

    If you do intend to flatten the battery, go for a deep-cycle (marine) or SLA type and use a charge controller, again, a bit overkill, but probably the best solution.

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    Quote Originally Posted by 5B4AJB View Post
    That would be too inefficient, you'll only get a few percent of what you put in, no need to "recondition" DC.

    A second alternator & battery would be overkill, although it would prevent you flattening your starting battery (park on a hill?).

    If you do intend to flatten the battery, go for a deep-cycle (marine) or SLA type and use a charge controller, again, a bit overkill, but probably the best solution.
    I would love if it was a standard but it is automatic (off subject turbo 350 trans with inline 6 250 engine, pure torque, and love it). so running directly from the battery is best it sounds like. I don't think anyone ever really "intends" to flatten a battery, I think it just kinda happens hahaha. Okay this has been extremely helpful. Thank you so much. I realize some of this may seem fundamental, with my brain injury retaining things is rather difficult. After I learn something I have to study it continually, or lose it. the Bible is about the only thing that seems to stick. so lots and lots of notes and things like this to help. I really appreciate your time.


    KD9KVS



    I have a brain injury.
    I am not a brain injury.
    This does not define me

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