[SOLVED] Agilent 54502A CRT display broken

[brych34]

Hello everyone

I'm a newcomer to electronics and by luck I recently acquired an old 54502A 400MHz digitizing oscilloscope that was being thrown away: https://ibb.co/M9bfnMp

Unfortunately the cathode ray tube display is broken. When I use the 'self-test' function the entire display is supposed to light up, yet only part of it down the centre does: https://ibb.co/hKXV2F2

If it's just a case of replacing a component I'll try and repair it but if you think the display itself is busted I'll take it apart for parts.

Thanks for the help

[SoundTech-LG]

That CRT display looks great. Your problem is likely in the horizontal deflection. Likely the vertical mounted board to the left of the CRT. Unfortunately I don't think there is a schematic for it.

[brych34]

Thanks - I'll have a look at it tonight after I discharge the display.

[TurboTom]

These monochrome CRT displays usually run their horizontal deflection through a quite substantial bipolar electrolytic capacitor of a few microfarads. Over the decades, these caps will dry and ESR will increase until the horizontal deflection fails (almost) completely. You may want to search for such a capacitor on the CRT PCB and check its capacitance / ESR. Other components in its vicinity may fail as a consequence.

[brych34]

I'll see if I can get my hands on an LCR meter. Thanks

[brych34]

It works now! I replaced the 3.3uF bipolar capacitor on the board to the left of the CRT with one from AliExpress:


As far as I can tell it works perfectly now:



Thank you!

[GreyWoolfe]

 

[fcb]

Excellent repair. These are great scopes - especially for free.

Remember they are equivalent time sampling at 400MHz. Realtime to 100MHz.  But still excellent!

[SoundTech-LG]

Great Job!  I'll try to remember that fix. Lots of 54xxx scopes, logic analyzers, and the 53310A Modulation Domain Analyzer use that same CRT/board combo. How did you happen to find the cap???

[TK]

I had the same issue with an HP8714B and replaced the bipolar capacitor.  They come in a much smaller footprint now and it worked when I replaced it... but then later another magic smoke and the new capacitor was gone.  I asked the forum and I remember somebody recommended using a non electrolytic capacitor and that is what I did and it has been working for 3-4 years now.

[brych34]

How did you happen to find the cap???

Sorry for the late reply. I just searched for "3.3uF bipolar capacitor" on AliExpress and it led me to here: https://www.aliexpress.com/item/32594169451.html

So far it seems to be working well!