Inventories - Summary of inventories (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Inventories
Raw material and common parts	$ 17,599,000	$ 14,707,000
Work-in-process	4,700,000	4,158,000
Finished goods	4,531,000	2,723,000
Inventory, Gross	26,830,000	21,588,000
Less inventory reserve	(1,677,000)	(1,437,000)
Total inventories	$ 25,153,000	$ 20,151,000