Old HP 1222A Oscilloscope Repair - Need Help! - Page 1
Forum section: electronics / repair

author: Philipp F.
author_rank: Contributor
timestamp: April 29, 2014, 07:59:56 pm
content: Hey EEVBlog Forum,I am a student and electronics hobbyist, i have the most basic knowledge in electronics, but for this I might need some help from you.A while ago my old HP 1222A Oscilloscope stopped working, i checked the fuse and it was blown, i didn't have the right fuse so i set it aside for a while but now I am making an attempt to repair it.Before installing a new fuse i took the case off to see if anything was visibly broken, and indeed there was!A strange looking cap right next to the transformer and the voltage selector spew out all its guts.In addition to the faulty cap, i messed up when trying to unsolder this thing, i pulled out a contact from the voltage selector switch that was solderd to the cap and a couple of other wires.I am sure I can just bodge repair that on the backside, but i am unsure where to place the jumper.I attached pictures of everything, also one thing i noticed, it has settings for 100, 120, 220 and 240 volts.I live in Germany where we have a voltage of 230V, when i got it it was set to 220V i hope this is still in range (maybe that's why the cap blew?).I hope you can help me place that jumper and finding a replacement cap, i googled the number on it but these kind of caps are unknown to me.Also, can I ignore the possibility that something different might be broken or caused the fault and just install a new cap? My best guess is that it's just the age that made it faulty.

author: krivx
author_rank: Frequent Contributor
timestamp: April 29, 2014, 08:06:36 pm
content: Do you have a schematic? If you can't replace the voltage switch you could just remove it and hard-wire the scope for 230V use.

author: Philipp F.
author_rank: Contributor
timestamp: April 29, 2014, 08:16:28 pm
content: I did find one online.http://cp.literature.agilent.com/litweb/pdf/01222-90908.pdfI tried searching for the part but i got lost in all the documentation.Ok, i found it in the part list, its A4C1, yet this doesnt help me very much.

author: JOERGG
author_rank: Contributor
timestamp: April 29, 2014, 09:03:53 pm
content: Hi and welcome to the forumGoogle "Entstörkondensator" find the values you need 0,2uF and 2 x 2500 pFDo not spend 15,- € on the part at Ebay. If you can not find the exact thing then i probably just would give this one a try:http://www.pollin.de/shop/dt/MjMyOTk3OTk-/Bauelemente_Bauteile/Passive_Bauelemente/Kondensatoren/Entstoerkondensator_ERO_F1740_347_3512.html?utm_source=google&utm_medium=pla&utm_content=200767&gclid=CN7D69bKhr4CFWjpwgodjb0A9w

author: Bob F.
author_rank: Regular Contributor
timestamp: April 25, 2015, 06:45:21 pm
content: Over a year now since the OP, but I had the same issue today. Nothing from the scope, a dead fuse and the same capacitor had leaked all over the board - looks like it may be a thing... So, for reference should anyone else have this happen in future:Looking through the user/service manual I noticed that cap was drawn on the schematic by hand. Looking in the Manual Changes section of the manual, it seems earlier serial numbers of the scope did not have that capacitor so I have simply removed it. The scope has been working happily all day now.Does anyone know what the purpose of this dual capacitor (A4C1 on the attached diagram) across the primary of the mains transformer?Cheers, Bob.

author: tautech
author_rank: Super Contributor
timestamp: April 25, 2015, 07:57:46 pm
content: Welcome to the forumIt's a mains suppression cap pair.Google x y cap.Many just continue use without them, it won't affect performance, just your scope won't be at all protected from in-coming nasties from the mains supply.If you can't find a similar 3 lead cap, just bodge the correct values together, insulate well and attempt to keep leads as short as possible.You might have parts available from your junk pileall modern eguipment should have x-y caps.Ensure the meet voltage spec.

author: Philipp F.
author_rank: Contributor
timestamp: April 26, 2015, 01:44:46 pm
content: Hey, i replaced it with the Cap JOERGG suggested, sorry for not posting this but it took me some time and i completely forgot itI also replaced the broken fuse and it worked fine and still acts as my main oscilloscopeAt the moment I'm looking to upgrade to a Rigol DS1054Z as I'm planing to become an electronic engineer

author: Bob F.
author_rank: Regular Contributor
timestamp: April 26, 2015, 10:52:16 pm
content: Thanks tautech.I did try searching but without much success (other than this thread!) - now I know the name of the type of cap, google becomes a font of knowledge!I'm coming back to hardware after many years elsewhere so my junk parts box needs extensive development () so I'll either search out the cap or do as you suggest and hack it out of two caps. I've ordered a Rigol for delivery next month (they seem to be constantly out of stock in the UK) - it's been so long since I did any serious hardware, I have never used a digital scope before so it should be a fun week, or several...Cheers, Bob.