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Thread: NLE8912A Amplifier Issue

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    Default NLE8912A Amplifier Issue

    Okay so I wasn't sure where else this should be posted so, I figured Legacy was the best place.

    Just some background: I have a vehicle where I have 3 radios installed; a UHF CDM (~40W), a VHF ICOM (~40W) and a XTVA. The XTVA is then connected to a NLE8912A-3 (450-470) amplifier.

    I'm a bit stumped on an issue with the setup, and although my personal feeling on it is, that there is some sort of RF feedback due to the proximity of nearby high power transmitters (i.e. UHF CDM), I'm unsure. The problem is, when the UHF CDM is keyed, it seems to cause the transmit relay inside the NLE8912A to very very rapidly switch on and off (which is bad for the relay I know).

    If it helps I've attached a photo of the antenna configuration.
    2020-06-23.jpeg

    Yes, I'm aware the antenna installation may not be perfectly optimal due to the offset positions of the UHF antennas. Any thoughts?
    - Bryan
    "So the best solution is to paint them to blue color and throw them to the sea?" in reference to MTS2000 F01/93.


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    I see now that after further searching this should probably be in "Mobile Radio Installations"; any mod feel free to move it there.
    - Bryan
    "So the best solution is to paint them to blue color and throw them to the sea?" in reference to MTS2000 F01/93.

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    I think what is happening is this. The NLE8912A amplifier has a circuit inside to detect RF from the convertacom. That circuit is tied to the input port of the amplifier and controls the switchover relays. When the amplifier is in the receive mode, that same circuit is tied to the antenna by the same relays. Transmitting on your CDM radio conducts enough power into that circuit to activate it.

    Your solutions are:

    1) See if the amplifier has a wire to inhibit that circuit that connects to the CVC and is not connected. I don't have schematics of either the CVC or amp so cannot help in that regard. It would be a simple modification to add a transistor circuit that is controlled by the CVC mike and holds off the detector until someone has mashed the PTT. Or

    2) you could add a resistor to swamp the detector bias so that at least a couple watts of energy are present before it switches.

    It would be interesting to put a wattmeter and load on the CVC/amp antenna to see how much power is being conducted.

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    Quote Originally Posted by RFI-EMI-GUY View Post
    I think what is happening is this. The NLE8912A amplifier has a circuit inside to detect RF from the convertacom. That circuit is tied to the input port of the amplifier and controls the switchover relays. When the amplifier is in the receive mode, that same circuit is tied to the antenna by the same relays. Transmitting on your CDM radio conducts enough power into that circuit to activate it.

    Your solutions are:

    1) See if the amplifier has a wire to inhibit that circuit that connects to the CVC and is not connected. I don't have schematics of either the CVC or amp so cannot help in that regard. It would be a simple modification to add a transistor circuit that is controlled by the CVC mike and holds off the detector until someone has mashed the PTT. Or

    2) you could add a resistor to swamp the detector bias so that at least a couple watts of energy are present before it switches.

    It would be interesting to put a wattmeter and load on the CVC/amp antenna to see how much power is being conducted.
    I have the schematics and as you mentioned I see the circuit in question, tapped off the RF IN signal appears to be a cap, a pair of diodes (probably reverse protection), and a resistor.
    Image 10.png

    I would imagine increasing the value of R2 would, then increase the required signal to trip the relay?
    - Bryan
    "So the best solution is to paint them to blue color and throw them to the sea?" in reference to MTS2000 F01/93.

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    So I experimented with R2, and replaced it with a 220 ohm then a 330 ohm -- and it seem to have an impact the power level required to trip the relay; but the issue remains so I'm not really sure.
    - Bryan
    "So the best solution is to paint them to blue color and throw them to the sea?" in reference to MTS2000 F01/93.

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    Ground your amplifier and use low loss short run cable between xtva and the amplifier see if that helps.

    Check also the antenna grounding and vswr of the cdm antenna.

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    Those diodes rectify and double the RF voltage to bias Q3 on. Changing R2 might require a bigger value. But, at this point I would change it back and focus on what is happening at JU2.

    Where is the rest of the schematic showing K2 and JU2? Those parts if placed, look like the solution to getting external TX inhibit from the CVC. Can you post up the manual with schematic, because the schematic is unavailable on the interwebs.

    I think the way it can work is that if K2 is normally closed and shorts Q3 base to ground via JU2, the amplifier will be unable to be keyed with RF. Then if K2 is energized via the CVC from its mike PTT, the amplifier can key with RF. Read closely the CVC and amplifier schematics and theory. Post those manuals here and you will get more help.

    There is nothing in your antenna installation that is out of the ordinary for a patrol car, s the solution has to be in the wiring options between CVC and the amplifier.

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    Quote Originally Posted by RFI-EMI-GUY View Post
    Those diodes rectify and double the RF voltage to bias Q3 on. Changing R2 might require a bigger value. But, at this point I would change it back and focus on what is happening at JU2.

    Where is the rest of the schematic showing K2 and JU2? Those parts if placed, look like the solution to getting external TX inhibit from the CVC. Can you post up the manual with schematic, because the schematic is unavailable on the interwebs.

    I think the way it can work is that if K2 is normally closed and shorts Q3 base to ground via JU2, the amplifier will be unable to be keyed with RF. Then if K2 is energized via the CVC from its mike PTT, the amplifier can key with RF. Read closely the CVC and amplifier schematics and theory. Post those manuals here and you will get more help.

    There is nothing in your antenna installation that is out of the ordinary for a patrol car, s the solution has to be in the wiring options between CVC and the amplifier.
    Sorry about the delay. I only have the supplement pages of the manual with the schematics, not the full service manual with operations theory (if anyone has this, I'd be greateful!). I will attach the supplement PDF I have here.
    Attached Files Attached Files
    - Bryan
    "So the best solution is to paint them to blue color and throw them to the sea?" in reference to MTS2000 F01/93.

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    OK so K2 is not a relay? It is a thermal protector. Weird they would label it that way. No problem. that point at JU2 you want to go to ground when the PTT on the CVC is NOT pressed. You may do this with an NPN transistor across JU2 to ground that is biased ON normally, working as an invertor, and then wire to the PTT in the CVC. I am oversimplifying this as you have to have a diode and another resistor at the PTT line to be sure you isolate CVC PTT from the base of the new NPN transistor. No doubt a bypass cap and choke to keep RF from fighting the fix.