2018-09-11 Vectronics Aircraft Radio Kit

I got the Vectronics VEC-131K aircraft band radio kit in the mail yesterday and warmed up the soldering iron to assemble it. It came at a decent price, so some of my complaints seem to be cost cutting measures. It comes with a decent assembly instruction manual, but some of the pages including the schematic were printed from a low resolution graphic, which makes them difficult to read. The Parts Placement Diagram page was not readable and useless. The part numbers are marked on the PC board, so I had to use that.

The assembly time was about 3 hours, but would have been less if I hadn’t hit a few snags – I’ll talk about those later. I would estimate this to be a medium difficulty kit, because it has a lot of parts and there other reasons I’ll get to next.

The instructions get to the point where the two glass diodes are installed. It says to pick the larger glass diode, the 1N270, and I’ll need a magnifying glass to read the markings. But both diodes are the same size, and neither one said 1N270. One said F 5235 so I assumed that was the 1N5235 zener diode. The other diode said 41 BAT, which was not 1N270, so I assumed it was a substitute for the 1N270. I installed it and the zener in their proper places and hoped I got it right.

Another snag was the winding of the three coils. The instructions said to use the length of 24 gauge enameled wire, but I found none in the bags. There were three short lengths of purple hookup wire in the bag, but they didn’t look like it was possible for them to do the job. The instructions say to wind the coils on a 6-32 screw, but the heavy hookup wire would not fit into the threads like they said. So I found some 24 AWG enameled wire that I had and used it instead.

There were seven “103” .01 uF ceramic disk capacitors to be installed into the board. The bag had seven big blue .01 uF capacitors that were rated at 1000 volts and their leads were very wide, making it difficult to install them close to the board. So I got seven smaller .01 uF capacitors from my parts box and used them instead. They could still be a lot smaller, so I may replace them later.

With the rest of the parts properly installed, I finished up the assembly. then the instructions take me through the testing and alignment. I used a short lengths if wire for the antenna, plugged in a speaker, powered it up and turned up the volume but I heard nothing. I heard some clicks and buzzes but that was my cellphone, so I moved it away. I did the procedure “10.7 MHz IF Stage Alignment By Ear” and suddenly the receiver started to put out noise, which was a good sign. There is no squelch control on this radio.

The instructions said to “Set the Local Oscillator Tuning Range Using the Signal Generator” but I don’t have a signal generator that can go to 120 MHz. So instead I used the “Setting the LO Tuning Range Using Off-air Signals” procedure. I tuned the frequency control, but it was super sensitive and a very small change covered many channels. I think this 10k potentiometer needs to be replaced with a multiturn pot that will give finer control of the frequency setting. Also the volume control seems to be linear, not logarithmic. When I turn it to minimum the speaker is silent. But if I turn the pot just slightly above minimum the volume gets very loud very quickly. If it was logarithmic, the volume would be normal over a much wider range of the pot.

I have been listening to the local airport radio traffic with just a few feet of wire connected to the RCA antenna jack. But I don’t know what frequency I’m listening to. I need to get some way of calibrating the dial so I can choose 121.8 MHz, the frequency for the big jets.

Update 12 Sep.

I made a modification to the volume control. I soldered a 680 ohm resistor between the center wiper pin and the pin that’s common negative. This makes the volume control less sensitive so it doesn’t get so loud when it’s barely turned up. The resistor could be lower, maybe 560 or 470 ohms, to make it even less sensitive.

The second modification was to add a 1k pot in series with R7. I unsoldered the negative lead of the 2.2k resistor R7 and soldered a short piece of wire to the lead and to the center wiper pin of a 1k pot. I soldered another short wire to the other ‘cold’ pin of the pot and to the hole where the 2.2k resistor lead was. This is much easier to tune to a channel than the main 10k tuning pot.

Now I need to print a list of the John Wayne Airport frequencies.

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