Soemtron ETR220 Calculator

Reversed order ETR220, ©2011 Henning Kraeft, click image for a larger version
Reversed order ETR220
Photos ©2011 Henning Kraeft
Click an image for a larger version
Reversed order ETR220, ©2011 Henning Kraeft, click image for a larger version

S#23325 - Technical Description

This machine the third Soemtron ETR220 in the collection is a Mk1 ETR220 with a serial number of 23325.  It is one of the very early variant Soemtron ETR220 machines where the electronics card cage is "reversed" to all the later MK2 model production machines, that is - board 1, the memory core store is at the centre of the machine.  The power supply regulator transistor is not located in the base chassis, but mounted internally on a large "L" shaped anodised aluminium panel to the rear of the main electronics card cage.

It is believed that there were possible production or operational problems with this internal layout as the memory for the Soemtron ETR220 series being a magnetic core store, is very close to the transformer, possibly giving problems from stray magnetic fields around the transformer, or from temperature drift of the core store exacerbated by the internal heat sink for the power supply.  Long term stability of the machine is dependent on the core store and its voltage supply settings which are automatically varied with temperature to try and lessen any of the temperature induced variable operation.

Further technical description of this particular machine will follow in the future.

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