It was a no brainer – at first. Circa 1985 equipment still in use by telcos, the DUT was a remote-operated twisted-pair telephone cable analyzer. We had extensively reverse-engineered and repaired several of these a couple years ago, the OEM was defunct. The customer was so happy with our previous work that another one of these antiques recently came in for repair.
As the SME (subject matter expert) who did the initial reverse engineering I was handed the project. Pulled out my notes and tried to refresh my memory with what I had studied 2 years ago. Easy fix – power supply problem. This was a simple DC-DC converter with no regulation feedback: -48 volts in to a push-pull center-tapped step down transformer, linearly regulated +-15Vdc and +5 Vdc out. Several resistors were burned to a crisp, a collector-to-emitter clamp diode was shorted, the 7815 and 7915 linear voltage regulators had failed leaky to ground (fortunate since this hopefully spared the load circuits from damage), five electrolytic capacitors were dried out and measured open on the scope. Non-electrolytic capacitors all tested good. As a precaution, I replaced all power supply silicon, even those that still tested good since they had likely been stressed and were now prone to imminent failure.
The obsolete TIP51 NPN BJT push-pull driver transistors were a bit difficult to find replacements for, but finally found the NTE394 which looked electrically suitable. It was a larger TO-249 package instead of the original TO-218, but it was the same bolt-to-a-heatsink style and all mechanical dimensions looked workable. In this case the metal chassis enclosure itself was used as the heatsink.
The original TIP51 transistors used rubbery conforming thermal insulators and plastic shoulder washers to isolate their collector flanges electrically from the chassis, but these insulators were too small to be re-used with the new transistors. Each NTE394 came with a large mica insulator, requiring a thin layer of