MTR3B – Kit Version

The MTR3B helps reducing the weight of my SOTA gear significantly. It weighs only about 15% of the KX3 in its Go- Box but has still acceptable output power and performance.

Ordered a kit from the latest production batch of LNR Precision and received it within a fortnight. A plug for DC power supply is part of the kit.

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Assembly was done in about two hours. Before I actually started, the RCA- jack had been replaced with a BNC- jack. Therefore the mounting hole needed to be increased by 2mm in diameter.

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Assembly and soldering went smoothly. However, two screws would not fit into the nuts on the PCB. They seemed to be somewhat different in size. Luckily spare screws from the KX3 were available and fit nicely.

Only after the assembly had been completed I discovered that one of the 3.5mm stereo jacks was defective. As soon as a key was plugged in, the radio started sending dits. This was a bit annoying as I couldn’t find a similar 3.5mm stereo jack in my stock and had to order some for replacement.

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According to the manual, the supply voltage must not exceed 12 volts. However, as I’d like to use a 3S Li-Ion or LiPo battery for my activations, I did measurements up to 12.6 volts. As long as the VSWR of the load is low, the MTR3B will work at higher supply voltages without any damage.

Note: Above 10 volts supply voltage, the MTR3B will reduce the output power if set to „tune“ mode. For my measurements I set the MTR to „straight key“ mode (hold both paddles while power up).

Note: I consider a „low“ VSWR being in the range below 2:1. It is a kind of personal limit.
For my measurements I used a 15 W dummy load from Weinschel Corp. Only after a while it turned out that the dummy load was defective – probably from the beginning!
In other words: I was running the MTR3B at 6 watts output power into an open. Luckily it survived without any visible damage.

Output power and current consumption vs supply voltage:

Band/ Voltage    6V                     7.2V                        8V                         9V                       10V

40m             1.6W/440mA     2.3W/ 510mA     2.9W/ 560mA      3.5W/ 620mA      4.5W/ 700mA

30m             1.3W/ 370mA    2.0W/ 450mA     2.5W/ 500mA      3.0W/ 550mA      4.0W/ 630mA

20m             1.6W/ 450mA    2.4W/ 540mA     2.9W/ 590mA      3.5W/ 660mA      4.5W/ 730mA

Band/ Voltage     11V                     12V                        12.3V                12.6V

40m             5.5W/ 750mA      6.0W/ 800mA     6.5W/ 830mA     6.7W/ 850mA

30m             5.0W/ 680mA     5.5W/ 750mA     5.7W/ 770mA     6.0W/ 800mA

20m              5.3W/ 790mA     6.0W/ 850mA     6.2W/ 860mA     6.5W/ 870mA

RXcurrent consumption is about 40mA.

The built- in keyer works in iambic- mode B. Storing CW messages is a bit tricky for someone who is used to iambic mode A. It took me some time to get an error- free message in.

All in all the MTR3B is an awesome little radio and I’m looking forward to using it in the fields.

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