REVENUE - Disaggregation of Revenue (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019
Disaggregation of Revenue
Amount of impairment losses			$ 0	$ 0
Net sales	$ 6,094,000	$ 4,750,000	19,232,000	14,097,000
Domestic
Disaggregation of Revenue
Net sales	5,945,000	4,205,000	18,629,000	12,860,000
Foreign
Disaggregation of Revenue
Net sales	149,000	545,000	603,000	1,237,000
Commercial Aerospace
Disaggregation of Revenue
Net sales	2,277,000	1,998,000	7,436,000	6,741,000
Defense
Disaggregation of Revenue
Net sales	3,595,000	2,517,000	11,292,000	6,641,000
Industrial
Disaggregation of Revenue
Net sales	222,000	235,000	504,000	715,000
Magnetic components
Disaggregation of Revenue
Net sales	3,230,000	2,440,000	10,394,000	6,983,000
Potted coil assembly
Disaggregation of Revenue
Net sales	1,299,000	1,321,000	4,226,000	4,186,000
Electro-mechanical assemblies
Disaggregation of Revenue
Net sales	$ 1,565,000	972,000	$ 4,612,000	2,631,000
Large transformers
Disaggregation of Revenue
Net sales		$ 17,000		$ 297,000