Ascom SE550
Published on 26 May 2011 by Ross G6GVI

A very popular transceiver on Four, thanks to the thousands of ex-Water Board units which have appeared on the surplus market in recent years. The conversion to the amateur band is by means of a fimware upgrade, effected by replacing the EPROM inside the set. Several versions of amateur firmware have circulated, the most popular of which is the "RATS mk3" or "PA4DEN", which allows control of many functions from the front panel.

Andy G0BEQ has compiled lots of information about this set, which is accessible from his website. This includes a manual for the PA4DEN firmware, and instructions on how to fit the memory back-up battery. This is indispensible when using the mk3 firmware, since Without it, the channel memories and user set-up will not be retained when the set is disconnected from its DC supply. The memories are useful, since the radio can then be set to scan all your favourite 4m channels, so you won't miss any activity.

Denis G4KWT has produced a very useful single-page instruction on how to align the Ascom, including setting the transmit deviation and squelch threshold. Some users have found that the transmit deviation is very low, but please take care when increasing this, so as not to spread into adjacent 12.5kHz channels.
Beware that when aligning the SE550 it is necessary to re-assemble the set outside its case, and without the PCB guide slots on the inside of the case, it is all too easy to mis-align either of the two sets of edge-connectors when connecting the two boards together. Mis-connection of these can produce all sorts of weird errors, and may even cause permanent damage to the set.

The mk3 firmware allows access to an "AUDIO" sub-menu, on which may be found "RX MODE" and "TX MODE". These appear to switch the pre-emphasis and de-emphasis filters between "FM" and "PM" settings. All users seem to prefer the received audio of the "PM" setting on RX, but the prefered TX setting depends on the microphone insert in use: try both settings with a few different stations, and get their reports. Note that if the back-up battery is not fitted, this setting will have to be made each time the set is powered up.

Many users have found that as the set is turned on, the receiver audio is too loud for use in a quiet room. On some sets, reducing the volume to zero, and then back up one step, produces a comfortable level. Others have fitted a volume control or attenuator in the loudspeaker lead.

If upgrading the firmware from mk2 to mk3, in addition to changing the EPROM, the RX VCO will have to be re-set. The G4KWT page referenced above gives details on how to do this - all you need is a voltmeter!

Apart from the Ascom fist-microphone, other boom or desk microphones work well with the set. The microphone connector is a standard 0.1" pitch header, and the connections are given in G0BEQ's manual, see above. There is a DC voltage on one pin, which can be used to bias electret inserts. Note however that the RX audio out from the mic connector is independent of the volume control, so it may not be suitable for a headset application.

Summary compiled by Ross G6GVI