HP 140A Oscilloscope Not Lighting Up

[marc11657]

Hey,

I've just bought a potentially dud HP 140A Oscilloscope from a car boot for £4. It's in a pretty poor state but I thought for £4 it was worth the gamble and if it didn't work I'd eventually get around to fixing it up.

Anyway it switches on and lights up except for the CRT screen which just stays entirely blank! I was thinking about replacing most of the caps first before replacing the CRT or do you think it's likely to just be the CRT anyway?

 

[radardoug]

Contrary to popular opinion, IT ISN'T ALWAYS THE CAPS!

You need to get a schematic for the unit, and a voltmeter, and start measuring. A scope will have very high voltages around the tube, so BE CAREFULL!

 

[mjrippe]

Here's a link for the 140B schematic, should be close: http://bama.edebris.com/manuals/hp/140b/

And +1 to radardoug's warning - even if you are used to tube amp B+ voltages, a 'scope can have over 1kV on the tube!!!  Your heart and most voltmeters will not survive this voltage.

 

[PRR]

It is NOT the CRT.

I've pulled a 1937 'scope from a ditch and the CRT was fine.

FAR more likely, a power rail has quit, a connection has tarnished, or somehow an amp-stage is so far off center that the trace DOES work, except it is deflected outside the faceplate.

Study the service data and check voltages at key points.