Tantalum Cap 3uf 150v replacement for Tek 465? - Page 1
Forum section: electronics / beginners

author: Porch
author_rank: Contributor
timestamp: February 02, 2015, 06:29:38 pm
content: I am trying to bring a Tektronix 465 back to life. Most of the wet caps are out of specification and the ripple is nasty, so I am replacing them.I did find a Tantalum 3uf 150V that is leaking. Ya, I didn't think a tantalum could leak, but it has. It's mounted across the 55+ rail and ground. C1534Mouser is the only one who has them at 70$. Can I sub them with something cheaper?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2015, 07:20:16 pm
content: Eww, high voltage tantalum... I take it, the original was a wet slug type, then? Trouble is, the $70 parts really are wet slug (or mil spec at least). And yeah, they're still around, and yeah, they're still arm-donatingly expensive...But, I would think a low ESR electrolytic should do about as well. I'm not familiar with the circuit so I don't know what exactly is most appropriate. Also possible you might use a film capacitor that size, if you can squeeze it in there. Film or electrolytic only need to be rated a bit over nominal voltage, not three times like tantalum, so it doesn't have to be obscenely bulky or expensive.Tim

author: tautech
author_rank: Super Contributor
timestamp: February 02, 2015, 07:48:06 pm
content: You might want to also add a 0.1 uF ceramic as a bodge across any non-tantalum replacement to catch any HF stuff.

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2015, 07:49:20 pm
content: 4u7 350V electrolytic will fit there, and will probably be the same size. Capacitance is about right, ESR will be around the same range ( not high, not too low) and will last pretty much forever though it likely will be a radial type and not axial. Common value as well, as they are used in SMPS high voltage sections.If it is a wet tantalum with a rubber or epoxy overfilled seal yes it will leak with time, they were the lower cost ( not quite as expensive as an equal mass of gold) option over the glass frit sealed ones. Those ones last till they rust through, you overvoltage them or you apply reverse voltage for long enough.

author: Porch
author_rank: Contributor
timestamp: February 02, 2015, 08:19:40 pm
content: Here is what it looks like.From the diagram, it looks like it's just filtering out the HF ripple. Would a ceramic be a better long life choice?http://www.digikey.com/product-detail/en/FK11X7S2A335K/445-8274-ND/2815204

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2015, 11:26:31 am
content: No; the ceramic will be maybe 50% of rated value at that voltage, and it will age (over a decade, maybe down by half again). If the value is important for compensating / dampening the regulator, that wouldn't be real great. Also, the ESR may simply be too low! (Which is also a problem with substituting film capacitors here.)FWIW, shopping for ceramic capacitors, you either want to find C0G of nominal voltage and as large a value as possible ($$$, many in parallel?), or no worse than X7R (any lower number or higher letter e.g. Z5U, etc. has worse temperature, aging and voltage dropoffs), at either triple the voltage rating and nominal capacitance, or nominal voltage and triple the capacitance.Tim

author: Porch
author_rank: Contributor
timestamp: February 03, 2015, 06:25:29 pm
content: Thanks for the info.4u7 350V electrolytic it is then.

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2015, 06:32:30 pm
content: It is the cheap wet tantalum..... Replace with modern electrolytic, there you probably can get away with a 4u7 160V modern one, it is smaller and will fit. The old one you have is 37 years old, keep it when you get it out and open it to see the lovely sintered slug inside. Careful the liquid left is pretty nasty to skin, so wash in soapy water.

author: mzzj
author_rank: Super Contributor
timestamp: February 05, 2015, 08:29:46 am
content: If ordinary aluminium electrolytic doesn't work for some reason(leakage current when hot...) you could try 3uF 150v plastic film cap with a 1-10 ohms series resistor.

author: Porch
author_rank: Contributor
timestamp: February 08, 2015, 05:07:46 am
content: Good news. The scope is up and working. I replaced all the big caps in the power supply side, along with the leaking one shown in the photo above.Most of the caps tested good after I replaced them. The 5500uf 30VCD tests open or low capacitance on my ESR70 meter.The 3000MFD 35V test at 6621uf. Go figure that out. I was able to replace them without having to remove any boards.With the change, the noisy power rails went away, and the 15+ line came back up from 12+. After a lot of cleaning with DeoxIT D5, most of the switches and knobs are now stable. Not perfect, but not to the point I want to gut this beast to get to them.Thanks for all the help guys.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2015, 06:13:57 am
content: Electrolytics can gain value by thinning of the dielectric; after reforming, the value should stabilize again.It could also simply be that way. Most are +80/-20% or something ridiculous like that. It's only a little out of spec, if it is so rated.Tim

author: MrSlack
author_rank: Frequent Contributor
timestamp: December 30, 2015, 09:07:48 am
content: Raising a dead topic again here but I've got a 465B with what looks like the same sort of cap failed but on the 55v rail (C4303). Looks like a weak point.I managed to get it sweeping again last night (the original problem) due to an open resistor but there is a lot of noise on the display and the beam return doesn't turn off the beam. The 55v rail is at 54.9v but there's a lot of ripple and noise. Reckon this one has fluffed the thing:Now to find a replacement...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2015, 12:20:27 pm
content: Is that actually tant? I see crusties on the left solder joint, electrolyte?Tim

author: SeanB
author_rank: Super Contributor
timestamp: December 30, 2015, 12:52:10 pm
content: Wet tantalum capacitor. Replace with a 4u7 200V 105C electrolytic, preferably a Nichicon or other good quality unit. ESR on that old one will be in the 50R top 2k range, though the capacitance might still actually be in spec, but the sulphuric acid electrolyte has evaporated through the rubber seal.

author: MrSlack
author_rank: Frequent Contributor
timestamp: December 30, 2015, 01:32:09 pm
content: Was a tant by the looks. One of those rancid old ones that meltsManaged to grab a BC Components/Vishay 4.7uF 100v axial alu electrolytic from RS. I looked at higher voltage units but the dimensions were unsuitable.I put my peak voltage meter on it and the rail didn't hit more than 55.2v on power on. 100v looks OK for derating here unless there is something I missed.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 30, 2015, 06:32:26 pm
content: I have the same scope and was able to locate the *full* service manual and a few other things that I found extremely useful. You didn't ask, but in case you need them,here's a link to my copy drive with PDF copies.Feel free to download whatever you need. I still need to find a few hours to go over a careful calibration and fully check all the power rails.I also found a site that had a few mods that I plan on doing to my own when I can. As well as a repair that sounds similar to your issues. Maybe it will be helpful too.http://www.spurtikus.de/basteln/repair/tek465/index.html

author: MrSlack
author_rank: Frequent Contributor
timestamp: December 30, 2015, 06:50:08 pm
content: Fantastic thanks for this - much appreciated. I've got a half missing copy of the SM from the boat anchor manual archive so this is much better. I'll mirror this on my web site as well when I sort it outCurrently sliging together an ESR meter so I can test the caps properly. Decided to design one properly that doesn't have coils to wind, nothing but generic NPN transistors (no op amps), has no dual rail supply requirements and works with a DVM in volts mode I.e zero hassle one.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 30, 2015, 07:13:51 pm
content: If you need a decent ESR meter, might I recommend anifty little one designed by board member Jay_Diddy_B.I have one half built on my desk yelling at me to finish it since I have some caps to test.

author: MrSlack
author_rank: Frequent Contributor
timestamp: December 30, 2015, 07:59:06 pm
content: Looked at that actually. I don't have a meter movement floating around so am building one that knocks out a linear DC voltage via a similar bias scheme. Trying for 100mV/ohm. Have it mostly working in LTspice but will see if it works in real life. It's very similar in principle I will add.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 30, 2015, 10:11:27 pm
content: You dont have to use an analog meter. You can just use a DMM and note the relative change between a known good cap and a suspect cap. If I remember correctly, they even talk further in about how to change it a bit to operate completely with a voltage measurement being displayed.

author: MrSlack
author_rank: Frequent Contributor
timestamp: December 30, 2015, 10:23:29 pm
content: Fair points - will read the thread properly this time