RM Italy RL-203
RM Italy produces nice RF amplifiers. Some of them are for the CB “Export” market and produce a lot of watts for a few bucks. The official ham radio amplifiers are much more expensive. They cover the common ham HF frequencies between 3 and 30 MHz. The CB ones focus on around 27 Mhz of course.
For RL-203, I read that it is possible to modify such a cheap amp to use it on complete HF range. Only one or two capacitors need to be replaced.
Because I got such a device cheap, I tried that mod.
The scheme is part of the manual. There is capacitor C14, with 180pF. This needs to be extended in parallel by a second capacitor with 1-2nF, as I read from internet sources. I used a 1nF one. The cap needs to be capable of handling 500 Volts or more. I used an 1KV type.
In the image above, C14 is at bottom left, above C1 and right from C18.
In image below, I added the blue capacitor with 1nF/1KV.
Tests show that it was possible to get out good power down to 7 Mhz. At 3.5 Mhz, I got nothing, but could hear that the relais in the little amp was flipping.
This relais detects incoming TX via a capacitor C1 (not in the scheme, but can be found on the PCB). C1 has 8p2, which seems to be too low for low frequencies. So I added another cap in parallel, with 10pF and the relais issue was gone.
In the image below, I have added a small 10pF capacitor parallel to C1.
To use the amp with the mod, you also need to add band filters to prevent electrical interference with the harmonics.
I made two tables of output power versus input power, and compared it with and without the changes. I “measured” output power with the ATU-100 antenna tuner (so this is not very exact I assume).
First table is without mod. The amp does output well down to 20 meter band. Below that, I got no output at all. Signal is LSB/USB, just speaking into mic (so it is not too reproducible).
Input [W] -> | 1 | 2 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
---|---|---|---|---|---|---|---|---|---|
Frequency | |||||||||
3.6 (80m) | |||||||||
7.1 (40m) | |||||||||
14.27 (20m) | 40 | 60 | 69 | 70 | 71 | 73 | 74 | ||
21.0 (15m) | 20 | 39 | 44 | 56 | 66 | 72 | 69 | ||
28.0 (10m) | 14 | 22 | 30 | 39 | 42 | 47 | 59 | 60 |
Second table is with the mod (both C14 and C1). Output goes down to 80 meter band.
Input [W] -> | 1 | 2 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
---|---|---|---|---|---|---|---|---|---|
Frequency | |||||||||
3.6 (80m) | 46 | 72 | 85 | 87 | 89 | 89 | 89 | ||
7.1 (40m) | 25 | 37 | 60 | 63 | 62 | 60 | 67 | ||
14.27 (20m) | 30 | 67 | 85 | 90 | 95 | 101 | 100 | ||
21.0 (15m) | 29 | 34 | 65 | 55 | 74 | 95 | 77 | ||
28.0 (10m) | 16 | 27 | 44 | 55 | 66 | 68 | 80 | 84 |
References
Scheme - http://www.radiomanual.info/schemi/ACC_PA/RM_Italy_KL-203_user.pdf
Italian page with the C14 mod - http://www.in3eci.it/index.php?fl=4&op=mcs&id_cont=170&eng=POWER&idm=214&moi=214
Mod for the CTE747, which has almost same scheme as the KL-203 - http://www.radioamatoripeligni.it/i6ibe/pdf/cte747.pdf