Dave saved this classic from the dumpster, a HP 1740A 100MHz dual channel analog oscillocope from the mid 1970's.
It wasn't supposed to be working so this is a look at the problem, a mini teardown, and some basic PSU troubleshooting.
Update: Even after 4 hours it still doesn't fail with the covers off. So likely to be some sort of thermal issue as suspected.
Forum: http://www.eevblog.com/forum/blog/eevblog-803-hp1740a-analog-oscilloscope/
Original HP Journal article on the HP1740A:
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-12.pdf

Acknowledgments
The 1 740A design group was led by Stan Lang until the start of pilot production when he transferred to another project. The design team included Jim Garner, mechanical design including the vertical
attenuator switch, Eldon Cornish, who designed the horizontal section, and Van Harrison who designed the CRT circuits, power supplies, and gate amplifier.

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