This unit worked, but had an annoying defect. Occasionally on a cold boot it would appear to be completely silent, but worked just fine after a restart. A few things were investigated, which turned out to be unrelated. Some electrolytic capacitors were replaced unnecessarily. And finally the root cause was found that was missed somehow so far. it turned out that fault was caused by the -15V power rail DC-to-DC converter, which did not start sometimes. Its enable pin was tied to the input 12V through a 1M resistor, which seems odd. So using 100K resistor instead of 1M seems to fix the problem.