Back around 1993, I bought a Tektronix Type 647 Oscilloscope for $100. I thought it would be a good way to learn about electronics. It didn't get much use at first, mainly because I didn't know what to use it for. Then I discovered a problem. Sometimes it would blank the screen, and later a bright flash would occur.
Suddenly it became interesting, since it was clearly malfunctioning. At first I thought it might be just dust burning off, but the problem persisted. I opened it up and looked inside the high voltage (HV) area. There were five tubes, but only four were lighting. I found a local electronics store and replaced the non-working tube. But the malfunction continued.
Tektronix had no information on the scope, since it was manufactured in the early 1960s. Instead, they referred me to Deane Kidd, a former Tek employee, who could provide further advice. He sold me an instruction manual (which contains schematics), and recommended replacing transformer T820 for $25. That also didn't fix it. Over the next few years, I also replaced C838 ($7.50), Q814A/B, and Q804. I considered replacing the tubes in the voltage tripler circuit with solid state diodes, but didn't want to put modern parts in this classic.
Eventually, it started working again, though unfortunately I'm not entirely sure why. It might be related to my replacement of Q804 -- I don't remember if it experienced a fault after replacing that part or not. Another possibility is that the electrolytic capacitors healed themselves.
Since this appears to be a classic, with very few produced, and there are no images available online, I've taken a few shots.
