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