AT20M
Built by STC the AT20 was designed to a RAAF specification.
The naval designation was 4-SU-6A. The US version was the AMT-120.
Due to its widespread usage, manufacture was also performed by Eclipse, in case STC's facility should be hit.
The modulated version was known as the AT20M, with the US version being the AMT-125.
Post war, the AT20 was used by DCA (Department of Civil Aviation).
The unit is similar in design to AWA's AT13B, but in some ways is superior.
The AT20 consists of two units, the power unit and the RF unit. These may be stacked, or next to one another on a base plate.
The power unit contains a large auto transformer which converts 240V to 110V. All other tranformers in
the set have 110V primaries, so the set may operate from either 110V or 220V AC.
A control on the front of the unit allows for fine adjustment of the output of the auto transformer.
The primary and secondary HT trays are also
contained in the power unit. 866 mercury vapour rectifiers are used. Power control is also performed by the power unit, providing a safety interlock system, overload detection and control of the HT contactor.
The RF unit contains the modulator (where fitted), exciter and PA stages.
The modulator uses two 813 beam pentodes to directly modulate the RF generated by the PA stage.
The exciter provides facilities for crystal locked or
VFO operation. The PA stage consists of four 813 beam pentodes and antenna tuning facilities. Power output is 500W RMS.
This particular unit was used in Rockhampton by DCA as the standby HF transmitter.
The unit shows evidence of significant rebuilding by DCA. The 866s have been replaced with solid state diodes. I will consider returning the unit to 866s, but that is not a high priority at present. The exciter has been rebuilt. From what I can tell it is quite close to theDCA drawings, but not quite. DCA modified the AT20 to facilitate remote control operation. As a part of this, some extra relays and controls have been added.
My sincere thanks to John in Rockhampton who saved this peice of our history from the dump. Also thanks to Steve D for helping me bring it back to Brisbane. We now know to allow plentyof braking distance with 1200lbs in the ute!
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This page will be updated as the restoration project progresses.