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
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
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


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