Inventories - Summary of inventories (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Inventories
Raw material and common parts	$ 14,707,000	$ 9,088,000
Work-in-process	4,158,000	5,123,000
Finished goods	2,723,000	2,482,000
Inventory, Gross	21,588,000	16,693,000
Less inventory reserve	(1,437,000)	(1,543,000)
Total inventories	$ 20,151,000	$ 15,150,000