Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2021	Apr. 30, 2020
Statement of Cash Flows [Abstract]
Net income (loss)	$ 680	$ (10,026)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes	0	8
Depreciation and amortization	3,301	3,319
Non-cash lease expense	(281)	214
Provision for losses on accounts receivable, inventories, other assets and warranty reserve	(117)	1,385
(Gain) loss on marketable securities	(28)	1
Loss on sale of fixed and other assets, net	39	143
Employee benefit plans expense	1,495	1,052
Stock-based compensation expense	273	312
Changes in operating assets and liabilities:
Accounts receivable	(752)	1,970
Costs and estimated earnings in excess of billings	5,005	(283)
Inventories	3,002	17
Prepaid expenses and other	714	878
Other assets	(584)	(518)
Accounts payable - trade	(205)	374
Accrued liabilities	765	947
Loss provision accrual	(690)	748
Income taxes refundable	405	(350)
Other liabilities	(865)	(1,598)
Net cash provided by (used in) operating activities	12,157	(1,407)
Cash flows from investing activities:
Purchase of marketable securities	(2,452)	(5,489)
Proceeds from sale or redemption of marketable securities	2,498	3,539
Capital expenditures	(1,239)	(1,483)
Net cash used in investing activities	(1,193)	(3,433)
Cash flows from financing activities:
(Repayment of) Proceeds from PPP loan	(4,965)	4,965
Net cash (used in) provided by financing activities	(4,965)	4,965
Net increase in cash and cash equivalents	5,999	125
Cash and cash equivalents at beginning of year	3,808	3,683
Cash and equivalents at end of year	9,807	3,808
Supplemental disclosures of cash flow information:
Interest	123	99
Income taxes	$ 477	$ 467