Available at http://www.4sqrp.com/n-75.php
This is a 75 meter AM Transciever (1 khz steps in amateur band, 5 khz outside) with 3.0 to 6.0 mhz general coverage receiver. The transmitter is rated at 5 watts carrier, 20 watts PEP. The transmitter is locked to only transmit from 3.605 to 3.995 mhz. It has a 4 digit LED display readout with adjustable brightness.
It looked interesting so I figured it would be a nice project for an evening. The kit is about 80% completed with all SMD parts pre-soldered. The most demanding requirement is wind 4 toroids. That is easily done, as they are a single winding on each one. The remainder of board assembly is wiring the display (4 wires) then inserting and soldering the remainder of the through hole parts. The case is unique and made of PC material. Follow the instructions using rubber bands when assembling it. The method works really well.
I modified the instructions for the display and speaker leads. The assembly instructions tells you to solder wires to the display and speaker. Then to points on the printed circuit board. I don’t like doing that, as wires seem to get broken off at the solder joints when I am tinkering with things. The bag with the display comes with an angled header. I had some 30 cm bread board wires with plugs on each end and decided to use the included angle header to make the wires removable. This worked out well and I used a header on the PC board end also. I went one step farther and used single pins to make the speaker leads removable.
After assembly, came the all important smoke test. No magic smoke was released upon power up. I connected my IFR 500 service monitor and after determining the RX was off frequency, I adjusted the RX calibration per the instructions. RX audio was weak. I had to turn the audio gain up over half way to hear comfortably. Power output on 3885 was 5 watts with just over 20 watts PEP after adjusting the mic gain for almost no clipping. But the radio was deaf! It took about 15 uv of signal to be detectable. Not good!
I did some testing and the results are below:
Frequency error on TX: -200 hz
Power output and RX sensivity: 3605 – 7 watts output and 10 uv, 3885 – 5 watts output and 15 uv, 3995- 4 watts output and 18 uv
Above 4 mhz, the higher you go in frequency, the less sensitive the receiver becomes.
A quick look at N75 on groups.io turned up 2 reasons for reduced sensitivity. Q2 ZVN3310 MOS-FET oriented backwards and bad solder on the NJM2113 SMD audio amp. The NJM 2113 device caught my attention in one post because the author stated it had to be seated against the copper of the PC board in order to dissapate heat. It had been confirmed the device ran hot at high volume unless the voltage that fed it was reduced or the speaker load impedance was increased by using headphone jack or inserting another speaker in series with the existing one.
I discovered a friend of mine (who I will not identify) had purchased one of the kits several months ago. He is very competent when it comes to electronics and he said his RX sensitivity was bad also. Other reports in the posts I read seemed to go no where after checking Q2 and re-soldering the audio output SMD IC. There were also complaints that short wave sensitivity was really bad the closer you got to 6 mhz which can be expected.
I’m starting to look at a few things to try to “fix” the bugs in the design. The bandpass filtering needs closer inspection. The IRF 510 MOS-FET power device is used as the RX mixer and it may not be a good choice. An extra stage of amplification may be required in the RX RF/IF chain somewhere, but it looks like the easiest way (but maybe not the best way) to do that is after the mixer, unless RF switching via PIN diodes or small relays is used.
After studying the data sheets for the NJM2113 it looks like the SMD mounted device used was not a good choice. This is an extremely small part and the power dissipation is 240 mw with a note it must be mounted on the board. A better choice would have the through hole PDIP NJM2113 as it has 400 mw of output and no requirement of being mounted. I’m looking at removing the SMD part and glue the PDIP to the main board dead bug style to allow turning up the radio to a higher volume.
I’ll contribute more when I get back to this project. To sum it all up:
Pros – Makes the rated output power with good audio. The overall transmitter design seems very good.
Cons – The RX sensitivity is not good. The audio output IC selection needs to be reconsidered. Needs more research to fix issues.
My opinion: The concept of this kit is really good. An experienced kit builder should be able to complete this kit in 3-5 hours. The instructions are pretty clear and easily followed. However, until the RX sensitivity and audio output IC issues are resolved, I would not recommend buying it.