Mystery of the Rainbow Capacitor! - Page 1
Forum section: electronics / projects

author: timb
author_rank: Super Contributor
timestamp: February 07, 2015, 08:15:23 am
content: So I've repaired a good deal of vintage gear, but I've never come across capacitors like *these* before. They're circa 1974.What are they? Did they come from Willy Wonka's Capacitor Factory? Are they Gay Pride Caps? Enlighten me!Sent from my Tablet

author: calexanian
author_rank: Super Contributor
timestamp: February 07, 2015, 08:28:58 am
content: Those are old dipped tantalum capacitors. Those pretty much suck. The top color was the most significant digit, the middle was the least significant digit and the bottom color to the leads was the voltage. Some had a dot on the side that was a multiplier but that was pretty rare since few of those over 10uf were used due to cost.

author: EEVblog
author_rank: Unknown
timestamp: February 07, 2015, 10:10:50 am
content: Yep, old dipped tantalums. Not that uncommon in old gear. Sometimes not that dazzling

author: timb
author_rank: Super Contributor
timestamp: February 07, 2015, 10:32:11 am
content: Okay, I suspected they were tants based on the shape, it's just the color that wowed me. I thought they were partially melted gobstoppers at first. Leave it to Tek to put candy colored capacitors in their gear.And yes, those will be replaced. Maybe I'll paint the new ones hot pink and orange.Hmm, in the third and fourth picture, the blue cap looks to be a poly of some type, yeah? There's a yellow Sprague (cutoff) to the right in the fourth picture that looks to be a film cap.Both of those types should be fine with age, right? I normally leave ceramics and film caps alone in old gear unless I suspect an issue.God, this board had like two dozen dipped tantalums; I'm surprised it works at all. (It does, albeit poorly.)Sent from my Smartphone

author: rolycat
author_rank: Super Contributor
timestamp: February 07, 2015, 11:53:06 am
content: It wasn't only tantalum caps that came in candy colours.Here are some old polyesters I dug out of a parts drawer:

author: timb
author_rank: Super Contributor
timestamp: February 07, 2015, 12:58:50 pm
content: Holy crap! They're like like little ice cream sandwiches made of LSD and fevered dreams!I knew there were color codes for capacitors but why have I never seen them before? I've repaired at least 30-40 prices of gear from the 50's through 90's and never run into this before.Sent from my Smartphone

author: lapm
author_rank: Frequent Contributor
timestamp: February 07, 2015, 01:12:29 pm
content: Color codes on capacitors went way of dodo when technology to print small text improved (less ink used = saves money)...I most often run into these color coded ones, when tooking apart old 70- 80- television sets when i was kid...

author: Tube_Dude
author_rank: Regular Contributor
timestamp: February 07, 2015, 02:28:44 pm
content: Quote from: timb on February 07, 2015, 12:58:50 pmThese are Philips polyester capacitors, very popular in old European devices. Maybe the fact you live in USA, can explain why they are not so familiar to you...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2015, 03:36:07 pm
content: That looks like Tektronix, but too many ICs to be a 475... contemporary signal generator perhaps?Going to guess the colors are 4, 7, 6 = 47uF, and voltage maybe 16 or 25V? Probably for a 5V rail.There's nothing wrong with dry slug or dipped tantalum, you just want to avoid using them where power surges and high currents are available.Tim

author: jaxbird
author_rank: Frequent Contributor
timestamp: February 07, 2015, 04:13:26 pm
content: Somewhat unrelated, but I used to be a bit of a jerk, using the larger of the candy colored caps, stick them in a mains sockets, then throw them at a friend and say, Hey catch, then watch them getting shocked by the charge as they heroically dive to catch the capacitorGood times, not even feeling any remorse.

author: timb
author_rank: Super Contributor
timestamp: February 07, 2015, 05:35:23 pm
content: Quote from: T3sl4co1l on February 07, 2015, 03:36:07 pmBingo, Tek TG501 Time Mark Generator. 5S to 1nS outputs. It's a TM500 module, originally released in 1972; this is a 1974 vintage with the 5MHz TCXO (or perhaps OCXO, not sure, it's in a metal can) option.The 1nS and 2nS outputs don't appear to be working and the variable timing control rod is broken.I'll have to take a picture of the top board for you guys, it's 50% PCB trace inductors. They look like crop circles. It's a work of art. If I didn't need to use it if have it framed.Also the component density in the board on the pictures above rivals some modern SMD boards I've seen. It's amazing to think this board was laid out by hand.Sent from my Smartphone

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2015, 07:45:49 pm
content: Quote from: calexanian on February 07, 2015, 08:28:58 amWhat's the default multiplier? I doubt it's nF that's too small, probably uF so the yellow violet one is 47uF.Are you sure about values greater than 10uF being rare? 1uF to 100uF is the usual range for dipped tantalum capacitors which aren't that bad as long as they don't get subjected to over voltage. I'd say dipped tantalum capacitors are more reliable than aluminium electrolytics because they don't dry out and can withstand higher temperatures.

author: rolycat
author_rank: Super Contributor
timestamp: February 07, 2015, 10:45:13 pm
content: Quote from: Tube_Dude on February 07, 2015, 02:28:44 pmThese particular capacitors were formally known as Mullard C280s, and are familiarly referred to as 'tropical fish'They are still in considerable demand amongst restorers of old electric guitars such as the Stratocaster, and for use in guitar effects pedals. The musicians think they sound better than modern caps, so they can be found on eBay at inflated prices(Mullard was an old British valve (US tube) manufacturer bought by Philips and subsequently used as a UK brand.)

author: calexanian
author_rank: Super Contributor
timestamp: February 08, 2015, 12:02:04 am
content: Quote from: Hero999 on February 07, 2015, 07:45:49 pmTantalum capacitors used to, and still are VERY EXPENSIVE compared to electrolytic ones. Their value was really in high accuracy high stability timing circuits. They were also very popular as bypass caps in digital electronics. We still use them in many of our products because of stability. In a well designed circuit they will outlast your average electrolytic capacitor and remain in spec. The problem is when things go bad. Their failure mode typically results in a small explosion and fire when connected to a rail, or lots and lots of noise if in a signal chain. I recall when a 100 uF 25 volt tantalum cap was over $5!! When i was a boy I would use our out of spec ones we could not use in timing circuits and use them as electric matches! They worked great. Your standard 7400 series bypass caps seen all over things in the 70's and 80's were 1 to 10uf tantalums. The most popular being 1, 2.2, and 4.7 uf. I have nothing against them apart from their destructive nature in failure mode and cost.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 08, 2015, 04:24:31 am
content: I thought you were talking about the white tubular ones. Those are caps too.

author: Stonent
author_rank: Super Contributor
timestamp: February 08, 2015, 05:54:58 am
content: Because it's probably a Tantalum:

author: timb
author_rank: Super Contributor
timestamp: February 08, 2015, 06:56:30 am
content: Quote from: Alex Eisenhut on February 08, 2015, 04:24:31 amReally? You sure about that? I thought they were inductors...Sent from my Tablet

author: SeanB
author_rank: Super Contributor
timestamp: February 08, 2015, 07:26:46 am
content: White with the wire wrapped on the outside end then dipped are ceramic tubular capacitors. Made from an extruded tube of ceramic, with a silver coat that is then ground off to get the right value, then dipped in paint then marked with dots. They use the same colour code as the polyester ones, in pF, and have a pretty good tolerance and tempco. Limited in value but really low self inductance, and very stable, though you do get some with a very well defined tempco used for compensation. Limited to around 470pf or so just because of size.The inductors are in moulded packages, looking like big resistors, though you can clearly see the mould line on them.

author: timb
author_rank: Super Contributor
timestamp: February 08, 2015, 08:58:47 am
content: Huh. I'll be damned. Learn something new every day!Sent from my Smartphone

author: Stonent
author_rank: Super Contributor
timestamp: February 08, 2015, 09:25:46 am
content: For the woman who has everything, and loves you in spite of the fact that you spend more time looking at circuit boards than her.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 03:03:41 pm
content: There is a tant chart at:http://www.hamradio.cc/electronics/tantalum_capacitor_color_codes.php

author: Artlav
author_rank: Frequent Contributor
timestamp: February 08, 2015, 03:46:45 pm
content: Quote from: lapm on February 07, 2015, 01:12:29 pmInteresting.As a matter of inversion, the retailers here are still selling off the stock of Soviet resistors, and these aren't colour-coded (Modern one in the back for comparison).Oh, and we still call capacitors condensers.

author: Mysion
author_rank: Regular Contributor
timestamp: February 08, 2015, 05:41:36 pm
content: I now have the urge to get some of those caps off of ebay and frame them. Gay pride flag explosive tantalum edition!

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2015, 06:44:07 pm
content: Quote from: calexanian on February 08, 2015, 12:02:04 amI'm aware of their failure mode. They typically short circuit the power supply, draw loads of current, glow red hot and melt. I found that out when I deliberately connected one to a higher voltage and watched it meltdown, no explosion though.In the real world they rarely fail, as long as they're properly used. Always use a fuse, don't operate a tantalum capacitor near its voltage rating and respect the maximum ripple current specification and it'll be fine.I've never designed anything with a tantalum capacitor that's failed like that. Aluminium electrolytic capacitors on the other hand will almost certainly fail at some point, no matter how generously overrated.

author: timb
author_rank: Super Contributor
timestamp: February 08, 2015, 06:53:27 pm
content: Yeah, I took apart the TM503 mainframe last night. It's got these giant 18,000uF and 4300uF flange mount caps in it (with 5 terminals for PCB mount stability!) that I have to figure out how to replace. (The 18,000uF cap is mounted horizontally in a snap in bracket with wires directly soldered to the terminals.)Good old Mallory and Sprague.I gave the entire thing a nice shower (transformer and all) and have it drying in an oven now. 40 years of dust, dirt and grime is not something you can blow out with compressed air!Sent from my Smartphone