I got a tektronix TAS 350 oscilloscope from my university when they were clearing out an old building. It was broken however, it didn't turn on. As it's a 1GSa/s 200MHz oscilloscope I was happy they left it behind. And it turned out the repair wasn't particularly difficult.

Looking around

The first thing you notice when opening this oscilloscope is that it's nearly empty. On the top you see the power supply and that's everything you can see. There's room for a possible option that provides a GPIB connection but unfortunately it isn't installed.

The oscilloscope opened up

Besides the power supply there's the front panel and the mainboard. The frontpanel is obviously flat behind the front and all the buttons and knobs are on it. The mainboard is hidden underneath. It's the smallest pcb but does all the work. Why they made such a big oscilloscope for such a small mainboard is a question worth asking.

The mainboard

The repair

Teh oscilloscope did react to the power button, the fan made a slight movement. I suspected some of the caps of the power supply had gone bad as they seemed to have expanded, so I took them out one by one and measured their capacitance. They were all allright, so that wasn't the problem. Then I measured the resistance between the ground and the power supply pins. And then I found the +5V line was shorted. I traced it's lines around the board, testing components along the way. And the diode in the switching part of the power supply was shorted out. I couldn't find one with similar voltage specifications, so I took a random diode out of an old computer power supply and replaced it with that.

The faulty diode of the power supply

After replacing the diode the power supply worked fine and the scope is now on my desk for regular use. I sold my HP 54502A, this is now my newest oscilloscope. It has a nice update rate my other scopes don't have but it lacks the GPIB connectivity. But I still have my HP 54111D (1GSa/s, 500MHz) and obviously has GPIB, so I'm not left out completely.

The oscilloscopes works fine again