Tektronix 485: what's this encrustation? - Page 1
Forum section: electronics / repair

author: tggzzz
author_rank: Super Contributor
timestamp: January 23, 2018, 08:19:46 pm
content: I've just picked up a Tek 485 for an unbeatable price; naturally it doesn't work due to the PSU "ticking". But that's boring until proven otherwise.A surprise is that the photocopied manual is rather different to the manual from the usual places on the web. There's no contents pages, it istypewritten, and contains "pages 13-37 will appear in the permanent manual". The IC's datecodes are all 1972, and the s/n is B060xxx.There's a little mould on the front panel, since the scope had been in an unheated seaside attic for a few years. On opening it up, at first glance the boards are pleasantly clean with one exception. The inter-board "5-way fingers" (and nothing else) have a brittle white encrustation, as shown in the pictures below.What is the encrustation? Is there a better way of removing it than (gentle) suction, pulling the "fingers" out, and just brushing it off?

author: bd139
author_rank: Super Contributor
timestamp: January 23, 2018, 08:33:44 pm
content: Never seen that before!

author: Gyro
author_rank: Super Contributor
timestamp: January 23, 2018, 08:47:19 pm
content: I've seen it on old berg strips stored in cold conditions in the garage. I've never figured out what it is though. It brushes off and doesn't appear to be conductive. Either some sort of mold growth or plasticiser related?

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 23, 2018, 08:51:11 pm
content: Ok i never seen anything like that on electronic component but after studying the pictures it becomes clear.1.It is some kind of fibrous material.2.Considering point 1. it could be form of fungus and it can grow in similar fashion.3.Why do it only appear on the IC's is a bit of mystery and could have something to do with possible coating etc.Also such fibrous "growth" Could possibly be loft insulation but why it exists in only those areas would be mystery.Any case most likely safe to remove and see if the IC's are damaged. Most likely not.

author: tggzzz
author_rank: Super Contributor
timestamp: January 23, 2018, 09:41:26 pm
content: Quote from: Gyro on January 23, 2018, 08:47:19 pmIt isn't mold or fungus; those are neither brittle nor sharp-edged.It is almost certainly related to an ingredient in the plastic, but beyond that - what?

author: Andy Watson
author_rank: Super Contributor
timestamp: January 23, 2018, 10:42:04 pm
content: If it weren't so selective in its deposition I would have said it was sea salt.Is the manual the military version ?

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 23, 2018, 11:09:42 pm
content: Dried up dead mold is brittle and can be quite hard.But if the substance is very hard and sharp it is crystalline formation of some kind.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 23, 2018, 11:11:08 pm
content: Quote from: tggzzz on January 23, 2018, 09:41:26 pmDried up dead mold is brittle and can be quite hard.But if the substance is very hard and sharp it is crystalline formation of some kind.

author: tggzzz
author_rank: Super Contributor
timestamp: January 24, 2018, 12:24:52 am
content: Quote from: Andy Watson on January 23, 2018, 10:42:04 pmThat thought crossed my mind!QuoteHaving another look at it, it looks more and more like a cut-and-shut manual. There are different typefaces/typewritten bits, but the schematics have been neatly fan-folded. So Ireallydon't know.I'll just regard it an another alternative view of what the circuit might be

author: tggzzz
author_rank: Super Contributor
timestamp: January 24, 2018, 12:50:50 am
content: Quote from: Bashstreet on January 23, 2018, 11:09:42 pmCrystalline is a more accurate description of the feel, and of appearance in the photos.

author: james_s
author_rank: Super Contributor
timestamp: January 24, 2018, 01:58:20 am
content: You could collect as much of it as you can and run some tests. Thinking back to my jr. high chemistry class I can remember some easy ones that could be done at home. Solubility in water, acetone or other solvent. Density, flammability, colors produced when placed in a flame, melting/burning point, odor, etc. That could narrow things down a bit.

author: BravoV
author_rank: Super Contributor
timestamp: January 24, 2018, 02:15:27 am
content: Looks like its similar to my few connectors, its like something was slowly oozed out from the black plastic material. I just blew it off with compressed air + few strokes of brush cleaned pretty well.

author: pinkman
author_rank: Regular Contributor
timestamp: January 24, 2018, 04:51:01 am
content: This is really interesting. There is a fungus, "phanerochaete chrysosporium" which can allegedly digest some phenolic resins but I know little more than what I've previously read on the 'net. I found my way to that topic while reading about how there is less plastic waste collecting in the ocean than expected, and some scientists believe that it may be due to some plastic-eating microbes consuming it. It seems crazy to think that we could be feeding some massive growth in this plastic-eating-microbe population and later end up with some unintended consequence... Just imagine it... You're floating along in your favorite inflatable alligator when suddenly your long-beloved plastic pet is no more and you're swimming back to the beer cooler on your own

author: tggzzz
author_rank: Super Contributor
timestamp: January 24, 2018, 08:04:49 am
content: Quote from: BravoV on January 24, 2018, 02:15:27 amThat does indeed look similar.I'd prefer to suck than blowBlowing risks having the "crystals" end up in any of the many exposed rotary gold finger switches. Accessing the timebase control is a pain.

author: tggzzz
author_rank: Super Contributor
timestamp: January 24, 2018, 08:16:00 am
content: Quote from: pinkman on January 24, 2018, 04:51:01 amThat looked interesting, particularly the "crust-like fruiting bodies". Then I saw that the dimensions were inches rather than sub-millimeter!

author: BravoV
author_rank: Super Contributor
timestamp: January 24, 2018, 08:38:35 am
content: Quote from: tggzzz on January 24, 2018, 08:04:49 amWell, at your situation there, you better suck then.Mine is just a small board with all digital stuff, no exotic nor precision analog mechanical things around it.That was few years ago when I took that photos, just fyi, although they look like fine crystals, physically actually its more like flakes, as a gentle touch will collapse and turned it into powdery stuff, be careful not to let those thingy slip into those switches or openings.

author: ealex
author_rank: Frequent Contributor
timestamp: January 24, 2018, 12:29:01 pm
content: I've seen that as well in some old gear, "growing" on pin-headers and nothing elselook at the pin headers at the bottom of the image just above the edge connectoror under the top sockets.The board is the CPU of a Rhode & Schwarz SMS frequency generator.Only those pin headers behave like that, so I assume it's something leaking of the plastic / decomposing it.There are no signs of corrosion on the metal pins / solder joint or pcb tracks.I might pull the board out to take another photo, just to be sure