An ADR68k showed up with multiple problems and a common symptom: no function, except from the remote telling it cannot find the main unit. The first obvious fault were broken ZIF sockets and an EPROM travelling the 19″ case. Replacing the sockets did not help much, as the power supply was starting to develop a high current smell.
The crowbar circuit tried to force down the linear +5V regulator which is good for up to 10 amps, causing the SCR to get very hot until it finally shorted out. The regulator uses a sense circuit with the power and regulator ground being seperately connected to the main board via a connector – this one:
I know those connectors in a similar or worse condition quite well from pinball games. In this case the resistance of the power return has increased, the regulator tried to compensate and finally lost against the crow bar.
A careful rework of the PSU, including replacement of previously installed cheap capacitors, soldering the wires directly to the main board brought the unit back into operation.
But there still was a input level indication without any signal. It turned out that the PCM53-I DAC in the ADC circuit had an offset on the output and needed to be replaced.