SMC2546L8

This transceiver has been available for amateur use for some time at various prices, and is used as both a normal transceiver for individual use, as well as an internet / radio link or repeater being programmable between 430, and 440 Mhz. The maximum power output is said to be 25W, but in practice it may be lower, and in any event can be turned down as you will see in the following. The deviation may be set to 2.5Khz, but that is also adjustable. The receiver is dual conversion with its first IF being 54.5Mhz, and second IF 455Khz, and it's sensitivity....wait for it....is said to be better than 1uVolt emf for 20dB SINAD. That may not be the best thing since sliced bread, but in practice the receiver seems reasonable, and has proved to be pretty impervious to spurious responses, or de-sensing. Incidentally the two buttons are used for selective call, but only active if that module is fitted, and as far as I am aware none of the ones available has that installed.
The tone squelch module is named FTS-14, and the 5 tone selcall unit is named F5D-9

It has 8 channels which can be programmed using our programmer for simplex, or duplex use. ie each channel can have both tx and rx frequencies the same, or set up with a split ( say 1.6Mhz ). If the unit is fitted with a CTCSS module ( which the test unit was not ) then you can also program any freq for CTCSS, always assuming that you know the codes used for the various ctcss tones, as that is what this unit expects rather than tone frequencies. The programming mode also allows the user to select a tx time out period if required. A screen view of the programmer with typical frequencies programmed is shown below. Remember DO NOT put anything in the CTCSS boxes unless your radio has the CTCSS unit fitted.

I also now have a VHF version SMC2516L8 which is the same appearance as the UHF version. It has the ctcss module fitted, and in case you find difficulty in getting the CTCSS to work on receive, I will point you to pin 4 of the mic plug which is a "mic hanger", This pin being switched to ground ( pin 7) via the mic hanging unit. When switched to gnd it also inhibits ptt, so you need to short this to ground, and then break that connection as you wish to transmit. In use as an rf gateway this can be done with the ptt relay in most cases. So far I haven't found an easier method of enabling ctcss on rx without the use of pin 4.

The programming software is free for amateur radio use, and common to both the VHF and UHF versions. It is not owned by us, and we only supply it as a free item to purchasers of our programmer. It runs in a dos window from windows, and can be used as is on com1, or run with a batch file selecting com2 etc.

The radio is normally off, and the software run. If you have existing frequencies which you want to use you can read from the radio by hitting the READ button, and then switching the radio on. You will see the percentage read running in the screen window. You need to save that as a PRG file under whatever name you decide. Then you can either edit existing entries, or insert new ones, and then save that file. Once having saved the file you can then program the radio. To program the radio, select PROGRAM, and it will tell you to put the radio into program mode. To do this you switch the radio off, and turning it over you will see a small hole. Insert a very small screwdriver into the hole and press and hold the switch pressed whilst switching the radio on. Then hit OK, and the data you have modified will be written into the radio. Switch the unit off, and remove the programming lead plugged into the mic socket. Switch back on, and you will have all those new frequencies available. You can run the software again if you wish, just to confirm they are actually there :-). By carrying out another READ command.
In the centre of the board, and just in front of the pa section is VR03 which is the power output adjust pre set . The deviation adjustment VR02 is centrally located, but near the front of the pcb, to the right of the metal vco box. This is the same style of layout used in both UHF, and VHF versions. The ctcss module sits in the dual header as seen on the right hand side of the pcb. The screened cable to the left with the yellow sleeve is the audio into the TDA2003 audio output stage from the volume control (centre). The screened cable marked with a blue sleeve is the audio coming out of the audio pre amp board to the volume control (top)

VCO setting.

This really needs to be done carefully if at all, and only by those who are used to calibrating equipment. The supplier of this equipment has said that 4Volts is the optimum setting for the VCO, and set at the highest frequency in the range ie around 439.500 Mhz.

The measuring test point is marked VCV on the pcb, and is right in front of the metal vco box just below the earthing strap. There are three holes in the vco box cover. The two on the right one above the other have small variables which set the vco range. the one nearest the front of the pcb is the receiver, and the furthest away is the transmitter.

CTCSS Codes / Frequencies

Here is a listing of the codes used for various CTCSS tone frequencies when programming.

Adding audio output to the mic socket:

Pin 5 on both my UHF and VHF versions is not used, and in order to make best use of the radio on my Echolink internet rf gateway I have brought the rx audio from the top of the volume control, via a 1mfd capacitor to this pin. I say best use, because I can now listen on my gateways frequencies without having to unplug the external speaker which I was using prior to this mod. I also took off the squelch detect to provide for carrier (cor) detection as vox use on an Echolink gateway is pretty poor in my humble opinion hi.

Make sure, by measuring the resistance of both JO2, and JO3 as you move the volume control, that they are wired as it says in the schematic. Mine were reversed, and my af output was variable until I realised that was the case, so I removed the volume / squelch/ ch switch / on off switch, circuit board and rewired it so that the input to the TDA2003 from JO3 was in fact on the slider of the volume control, and the af output from the audio module via JO2 was the top of the volume control.

To make it useful as an rf gateway transceiver it should also have a carrier detect output or COR signal. As it stands that is not available, and therefore interfacing to the AF output muting signal around Q604 is required to provide a +5V output when the squelch is open.

I should add that unless this radio is suitably modified it makes a pretty poor gateway transceiver with only it's speaker, and mic. Besides how can you monitor the frequency you are using, if the only means of doing so is plugged into your pc soundcard. Seriously, think about it if you are not able to modify this radio to make it truly fit for purpose. This article was written some few years ago, and I no longer use these transceivers for my gateways, but left this here for those who truly want to warm up their soldering irons, and play hi.