Inventories - Summary of inventories (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Inventories
Raw material and common parts	$ 12,539,000	$ 9,088,000
Work-in-process	5,047,000	5,123,000
Finished goods	3,781,000	2,482,000
Inventory, Gross	21,367,000	16,693,000
Less inventory reserve	(1,488,000)	(1,543,000)
Total inventories	$ 19,879,000	$ 15,150,000