Tektronix 760A & 1781R capacitor questions (solved) - Page 1
Forum section: electronics / repair

author: Smokescaped
author_rank: Newbie
timestamp: March 26, 2024, 04:38:51 am
content: Forgive what might be a trivial problem, it's my first time tinkering with electronics.Background:I acquired two Tektronix oscilloscopes from an old lab, 760 & 1781R.What happened:After powering them on they both had a capacitor blow after about 10 seconds.Both fuses are OK.Pics attached.Questions:Is it something I did!?When capacitors are left to sit for decades, is this just something that can happen?Are these capacitors in particular known to be a common failure point?When I try to replace them, are there any other capacitors I should replace preemptively?Any other recommendations for how I should care for an oscilloscope that hasn't been powered on in 20 years?Tektronix part number:285–1222–00Description:CAP,FXD,PLSTC:0.068UF,20%,250VIt also has 565-1 & LD2 & 40/085/56 written on it.cheers

author: indeterminate
author_rank: Regular Contributor
timestamp: March 26, 2024, 10:01:28 am
content: What you have are not oscilloscopesThe caps that failed are X2 mains rated capacitors , that particular vintage tended to blow up.it needs to be replaced with an X2 250V mains rated capacitor not just any old thing with the same value.look for leaking or bulging electrolytic capacitorsclean off the dust and post sum pictures of the pcb's and people can tell you what other components are likely to have failedthere will be around 20KV floating around the tube & is power supply

author: srb1954
author_rank: Super Contributor
timestamp: March 26, 2024, 12:44:38 pm
content: What you are seeing is a classic example of a RIFA capacitor failure, often described as "RIFA madness". This brand of capacitor is notorious for exploding like this or emitting large quantities of noxious smelling smoke. It is due to the epoxy encapsulation cracking over time and allowing moisture ingress severely degrading the insulation resistance of the paper dielectric in the capacitors.You should replace all such RIFA capacitors with the pale yellow epoxy encapsulation immediately as they will all fail in time. These need to be replaced with a reputable brand of safety rated capacitor, usually an X2 type. The smaller 0.01uF capacitors may be a Y1 or Y2 rated type and should be replaced with similarly rated safety capacitors.You should also replace the Schaffner line filter as these also incorporate RIFA capacitors and will eventually fail in the same manner only with an even more noxious smell and a very sticky tar-like substance oozing from the filter.If you search though the forum you will find numerous other examples of this RIFA madness.

author: factory
author_rank: Super Contributor
timestamp: March 26, 2024, 01:26:38 pm
content: There are at least two more RIFA madness paper based capacitors hidden in the 1780/1781 series PSU underneath that. The one I had used TDK line filter, but some may use Schaffner madness instead. It's a TV waveform monitor & vectorscope;https://w140.com/tekwiki/wiki/1781RThe 760 is a stereo audio monitor;https://w140.com/tekwiki/wiki/760ADavid

author: Smokescaped
author_rank: Newbie
timestamp: March 27, 2024, 07:14:07 am
content: Thank you! Here's the additional photos requested.I'd be hugely appreciative if anyone has the time to do a sanity check on my choices for these Capacitor (0.068uF, 0.022uF 0.01uF) and line filter replacements.For the 0.068uF caps I'm looking at these X2 replacements:https://au.element14.com/kemet/pme271m568mr30/cap-0-068-f-20-paper-radial/dp/9717455For the 0.01uF caps I'm not certain of the class but other thread indicate they might be Y2. So maybe this?https://au.element14.com/kemet/pme271y510mr30/cap-0-01-f-20-paper-radial/dp/9717560There's also a thinner version of the 0.01uF cap, if anyone thinks that's preferable (replacing 250VAC with 300VAC should be ok?). The fitment on one of them will be pretty tight.https://au.element14.com/kemet/r413i21000000k/cap-0-01-f-10-pp-radial/dp/2835663As for those two hidden caps in the 1781R (thanks David). I googled the model and found the specs to be 0.022uF, 250VAC. Note that mine are stamped "L0" rather than "H8".Plugging those details into the website filter + Y2 class got me to one result:https://au.element14.com/kemet/pme271y522mr30/cap-0-022-f-20-paper-radial/dp/9717579Lastly for the line filter I looked up the old model and Mouser pointed me to a possible replacement, seems to be correct.https://au.element14.com/schaffner/fn9226-1-02/iec-c14-inlet-filter-1a-pcb-mount/dp/2449537

author: factory
author_rank: Super Contributor
timestamp: March 27, 2024, 09:18:27 pm
content: Most of us look for X2 and Y2 safety capacitors made from plastic film, as the paper based ones will only last about 20 years or sometimes less before failing.Also consider if you actually have a use for these, TV waveform monitors from the 1980s 1990s seem to be almost worthless over here in the UK, the audio monitor is probably more useful.The 1781 also has a battery backed Dallas NVRAM IC, these are a known issue in some test gear, I don't know if it stores vital calibration data in these, probably impossible to recalibrate without obsolete Tek software.I don't know if the other unit has one of these, I can't see one in the pictures you posted.David

author: Smokescaped
author_rank: Newbie
timestamp: March 28, 2024, 02:23:43 am
content: Quote from: factory on March 27, 2024, 09:18:27 pmI'll look up plastic film alternatives cheers.And you make a good point regarding the usefulness...The 760A appears to be really well documented among hobbyists as a creative outlet for displaying music, that's my intended application.The 1781 is more niche, it just happened to be available with the salvage so I took it home too.I have a familiarity with vectorscopes & waveform monitors from colourgrading so it’ll be fun to play around with it.