Keithley yellowing

[Mr. Scram]

I've noticed that almost every Keithley that's not brand new has yellowed to some degree. Some have yellowed a little bit, many have yellowed a lot. I know the exterior looks aren't the most important part of high end test equipment, but it seems to be something that's preventable. Especially considering that other brands seem to suffer from the phenomena much more sporadically, which suggests that the units that show yellowing have had much more of a beating. What gives?

[TiN]

I had some mixed results using peroxide method to get FP's more white, however drawback is that silkscreen paint is really fragile during the process. Also older Keithley's have biege tint even from factory, they are not designed to be white. 

[helius]

The yellowing can be prevented by not using brominated ABS, however there are cost and performance issues with other structural plastics.

[kosine]

It's got NOTHING TO DO WITH BROMINE!!

Probably just plain old ABS or a similar styrenic material. It oxidises over time and goes yellow, heat and UV exposure are the main culprits. How badly and how quickly it occurs depends on the source material: Molecular length, monomer residues, manufacturing additives, how it's made, how it's stored, how it's processed at the factory.. lot of variables. Some sources are better than others, a lot now contain anti-oxidants to mitigate yellowing. (But it costs a little more so isn't always used.)

The bromine story is just hearsay. Fire retardant plastics aren't used unless regulations & laws demand it. It's expensive stuff, harder to process, weakens the material, and the bromine compounds that are sometimes used (there are many other options) don't even go yellow. They're actually quite stable and can survive recycling and reprocessing. (Bromine itself is volatile - and it stinks. If it was leeching out of the plastic you'd know about it!)

If you want an idea of just how expensive fire retardants are, I was recently quoted £17,000 for a tonne of halogenated additive. And you need to add 20-30% to your plastic to meet a UL94-V0 rating. Compare that with raw ABS, which is currently around £2,000 per tonne. Seriously, no one uses the stuff unless they absolutely have to. And when they do, they damn well make sure you know about it to justify the expense!

[macboy]

Whatever causes that yellowing, I have had great success removing it with peroxide.

I use "40 vol" (high strength) peroxide gel purchased from a beauty supply store; you won't find it at a corner pharmacy. I remove the front panel (and lens if possible), paint the gel all over the panel, place in a glass pan and cover with plastic food wrap. Place in full mid-day sun (not cloudy day), rotate and re-wet every 10 minutes or as needed. In 30 to 40 minutes, its done.

If you let the peroxide soak for many hours, it will soften the silkscreen to the point you can rub it off easily, so don't do that. You would also risk bleaching the dye in the plastic. If the process takes more than an hour, then something is wrong, so try a different type of peroxide and/or wait for a really sunny day. If your local weather forecast gives a "UV index" or similar, then wait for a high or very high rating and do it that day. The strong UV is absolutely essential to the process.

Here are a couple older posts with more info on the process including photos, plus a technique to restore dings and dents:
https://www.eevblog.com/forum/testgear/reversing-_yellowing_-of-70s80s-style-plastics/msg1242399/#msg1242399
https://www.eevblog.com/forum/testgear/restoration-glory-of-keithley-2001-dmm/msg937701/#msg937701

[gamalot]

I use this. 

[RoGeorge]

A search for 'restoring yellow plastic of vintage PC/monitors' will return many tricks used by vintage collectors.
Some restorations results are very good, after whitening the case looks brand new, yet I don't know if the same methods can be applied to Keithley instruments, too.

[kosine]

I would expect the Keightly facias might respond well to peroxide treatment, though in the polymer world the technique hasn't been all that well studied and is still considered somewhat experimental. There are many different materials out there, and some will respond better than others.

Here's an in-depth article if you want the gory details about what's going on: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4320144/pdf/40064_2013_Article_1415.pdf

In short, it's oxidation damage caused by the production of free radicals, either thermally or through UV light absorption. It tends to break the polymer molecules into shorter strands, and oxygen attaches itself to the breaks causing the yellowing. (The same thing happens to paper, as you may have seen in many old books and newspapers.)

Peroxide treatment stabilises the effect and reduces the yellowing (by effectively bleeching it), but it won't stop it happening in future, so the yellowing will return in time. Eventually the plastic will become weak and brittle, and there's no coming back from that sadly. Will take many years of course, although the heat from power supplies can considerably speed up the process on their enclosures.

If you're wondering why manufacturers still use styrenic materials instead of something better, well there isn't really anything that's as cheap and easy to work with. (Got to remember that a lot of electronic manufacturers can't even stretch to decent capacitors, so wasting money on non-functional case parts isn't going to be a high priority for them.)

[Mr. Scram]

I have to say that in this case I'm not too interested in restoration techniques. They seem a bit experimental and results are reported to be somewhat temporary. They definitely have a place in the retrocomputing world and other places where parts simply aren't available, but that doesn't really apply here.

What surprises me is that Keithley hasn't fixed the issue after what seems is quite some time. To me it appears to be an engineering issue that's essentially solved. However, it doesn't seem to be viewed by Keithley as an issue. While they're considered "budget friendly" devices in the TE world, they're hardly bottom of the barrel or low budget devices. It doesn't appear to be good for the brand if most devices in the field look worn or even junky, especially when competing devices don't suffer from the same problem as badly or consistently. Is it really a case of them being too cheap for their own good? Or am I completely misjudging the target audience and are minor cost savings worth the penalty?

[Brad O]

We did actually change our plastics in the last few years to a type that should be more resistant to yellowing.  The bottom left instrument in your image (a 2635?) is the newer, whiter plastic, the other instruments you have are really close to their original color. 

Most every plastic yellows over time, I agree though, it can be more noticeable on the old Keithley stuff.  Part of that is caused by the old instruments like you have in your picture using multiple formulas of plastics on the front panel that yellowed at different rates.  Most other T&M manufacturers, I think, traditionally used a single molded piece on the front panel so any color change is uniform across the front panel.  Newer Keithley equipment also uses one type of plastic across the front panel as we moved away from the VFD displays.

My personal opinion though?  The mechanical engineers just wanted the instruments to return to Keithley Brown.

[LapTop006]

My personal opinion though?  The mechanical engineers just wanted the instruments to return to Keithley Brown.

IMO the mechanical engineers were right.

At least mostly.

Some of the buttons used on kit of the BROWN era (and for that matter some of the earlier white kit like the 3322 I have) really weren't great.