Inventories - Summary of inventories (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Inventories
Raw material and common parts	$ 17,469,000	$ 14,707,000
Work-in-process	5,684,000	4,158,000
Finished goods	3,511,000	2,723,000
Inventory, Gross	26,664,000	21,588,000
Less inventory reserve	(1,569,000)	(1,437,000)
Total inventories	$ 25,095,000	$ 20,151,000