Why +12V max?

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    I just ordered a MTR3B and was looking at the schematic in the online manual. Unless I’m missing something, I can’t see why the power supply input voltage should have a maximum of 12V.

    The MCP1703-5002 has a max voltage of 16V. The NDT2955 has a max Vgs of 20V. The input to pin 19 (P2.3) is divided down to 12% of V+. The 2N7002 can handle 20V. BS170s can take 20V.

    The only thing I can think is the derating voltage of the capacitors or maybe it’s the SA612AD that is limiting.

    LNR, can you provide any insight? Is there a mod that would allow a higher voltage? I’m mainly asking because I’d like to be able to use my various 13.8V supplies and not have to worry about letting out the magic blue smoke.


    I am interested in this answer as well. Is the spec “nominal 12v” or “really really 12v”? It seems odd that the rig would not tolerate typical DC supply voltage.
    The answer to this is holding up my buy decision…

    Thanks & 73


    i believe they set the “ideal” voltage at 9.6v not sure who decided that this was the ideal voltage, i personally think it was a little retarded.


    Does LNR monitor these posts???
    I have a Tri-Bander, a dual-bander and a PFR-3B, all by Steve Weber and they all accept 13.8V no problem. I don’t think the MTRs are much different.


    I tried my MTR3B with a 13.8 VDC battery, and on powering up the rig it immediately blew the fuse I put between the battery and the rig. I’m sure glad I used the 1A fuse as suggested in the 3B manual.

    I’m no engineer, but I know the rig will suffer damage if powered by more than 12 VDC.



    My guess is that it is more about heat dissipation than actual operating voltage, 4 W out is pretty good without heat sink in such a confined space.
    But I can’t speak of experience, my MTR3B is on it’s way to Belgium.
    72 Yves ON7BAS

    • This reply was modified 6 years, 8 months ago by on7bas.

    Why doesn’t LNR respond to this thread? I also sent several emails asking the same questions and haven’t received any response. I wish LNR would take product support a bit more serious.

    LNR admin

    Hi Mike,

    We have some staff that occasionally monitor the forums, but they are truly meant as a user group forums. We do have a contact us form, email and phone that where we answer support calls. Unfortunately, being a small company, that is the only way we can keep up.

    Regarding your question, the 12VDC max is the voltage you need to stay at or under where we can guarantee that you will not damage the finials. We routinely get emails from customers running 12.5 VDC or in the 13 VDC range with no issue, but they are pushing the limits beyond what is rated. Hope this helps.



    I’ve always associated QRP operations with overcoming challenges. Because of this, I don’t really consider the nominal 12V maximum voltage limit to be insurmountable. Of course, being able to plug into a 13.8V DC battery or other power source would be handy in some circumstances, but I also take into account the typical size and weight of such power sources. What I’ve done is purchase two AA 8-cell battery holders with 9V snap connectors. These are inexpensive and they mate well with the 9V optional power connector of my MountainTopper rig. I use 8 AA NiMh rechargeable cells in series, which produces a fully charged voltage of 11.2V that tapers down to a long and steady 9.6V until replacement with a fresh set of batteries is needed. At 2600 mAh, this is a very good and comparatively light weight power source for the MTR-3B that keeps voltage under the 12V recommended maximum.



    I have a MTR-4B with similar power supply limitations and I also use a battery holder for (8) AA NiMH batteries.

    One advantage of this arrangement is that you can use (1) dummy cell and (7) AA alkaline batteries if you have to and have 10.5v, safely under 12v.

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