A maybe interesting case of case yellowing (Centris 660 AV)

This may be interesting to those who are investigating case yellowing more thoroughly:

Recently, I got my first “real” desktop Mac back, a Centris 660 AV from 1993 (I think, I acquired it in Aug. 1993). It sat stationary on my desk until I phased it out in favor of a PowerMac 7300, which must have been around Easter 1997, and gave it to my brother, who had it in use for some additional years (so it had seen about 10 years of service life). So far, there isn’t much unusual about this.

What may be interesting, is the pattern of case yellowing, which isn’t too severe in general: while there is next to no yellowing overall, there’s slight yellowing on the left side of the top case and at the left few centimeters of the front panel. The same pattern is seen on the Macintosh Color Display, which is associated to the machine and the mouse (you can clearly make out the direction of the exposure on the beveled top, caused by light coming from the left). The Extended Keyboard II is another case and there’s some pretty noticeable yellowing on this. This pattern provides a pretty good indication for when the crucial exposure was happening: When I had the machine, I had it sitting stationary on a desk in a room facing to the South and the light coming from the right. When my brother had the machine, it was the other way round with the light coming from the left. Meaning, while the machine was exposed to some bright light in the first 3 and a half years, this didn’t contribute much to yellowing and all this happened only after this! – I guess, there must have been a first stage of degradation of the material, before the exposure to light could cause any harm.

As for the computer, it is in pretty good nick and, somewhat miraculously, the battery didn’t leak. (However, in its later years, it had been exposed to cigarette smoke and the inside of the PSU and of the monitor required some intensive cleaning.) Next plans: get a new battery and install AUX.

Something I didn’t remember: The entire assembly is based on sliding mechanisms. There’s just a single screw in this, securing the PSU, which is else held in place by a sliding mechanism as well. Even the the top case is secured by two tabs at the rear. – Meaning, unless you want to remove the PSU, you can dissassemble the entire machine and its component groups without a screwdriver. – The days, when Apple devices were service friendly…

Top case removed (after cleaning – the single screw is seen on the lower left, in the center of the lower edge of the PSU, note how the back panel is held in place by just some tabs as well)

The tabs at the rear and the Extended Keyboard II (ah, those magnificent Alps switches!) with cleaning efforts going on. (The color aberration seen here is more an artifact of the photo. In fact, this side of the case was never exposed to light with the monitor sitting on top. However, we can make out the faint yellowing on the left top edge – here on the right of the image as we are facing the machine from the rear.)

P.S.: If you ever attempt to open a Macintosh Color Display, make sure to get the service manual. The case is secured by just a couple of screws and is mostly held in place by two hidden tabs. The manual will tell you where to depress the case on the top in order to slide it apart. (I guess, otherwise, you’ll have a hard time trying to pry the case apart and are likely to just damage the case without any more reasonable effects regarding getting the thing apart.)


Those low-height pizza-box LC-style cases are a pleasure to open and give you fast and easy total access to the motherboard for upgrades. :slight_smile:

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As for size, the 660av is more in the middle between the LC boxes and the PowerMacs (just one expansion slot twisted horizontally) – and surprisingly heavy. (I haven’t found out why this would be in particular, none of the individual parts is of notable weight, but there’s some care applied to shielding, I guess, it just adds up.) The all sliding assembly was to me some of a surprise as I remembered that PowerMacs were all about hinged units and folding. They opened up pretty much like a book, once the top case was removed.

PS: Other things of note about the Centris 660av
The 660av featured a DSP (much like the NeXT machines), video capture with SVideo in and out and MPEG2 en/decoding in hardware. However, the MPEG2 encoder was still too slow for realtime encoding and the 230MB HD took you just that far, just a few seconds worth of raw video. Meaning, this was more about video conferencing than editing or effects, etc. Moreover, the 660av is one of the few machines for which there was a version of A/UX, which may be nice to try. (For those not familiar with this, A/UX was AT&T Unix with a Macintosh environment running as a process, years before there was OS X. However, there had to be specially adapted binaries for the particular machine. Therefore, there are just a few Macs which are able to run it.)

Now there’s something funny:
As often, the keyboard case shows the most of yellowing (due to material and exposure). I had given this a fair cleaning with a cleaner which is also a fat solvent (running here under the “Cillit Bang” brand) – and after a few days, I was under impression that there were what seemed to be some brighter areas near one of the corners. Could it be? I revisited the process and gave it, while perfectly clean already, another soak in combination with some intensive wiping by a micro fibers cloth. It could have been a placebo effect, but I was under the impression that this did have a bit of an effect – not much, but some. Two days after, I’m rather convinced that the borders, where a function key overlay had masked the case were actually more blurry than they had been before and the yellowing might be indeed a degree less. But keep in mind that there’s probably some bleach in this product and that my personal observations (which aren’t that positive anyway) are probably tainted by anticipation. On the other hand, if there is any noticeable change at all, is not related to abrasive removal of the surface. (I did a test on a small spot using backing powder as an abbrasive and there’s no difference.)

However, could it be that the process is – at least partly – related to oils entering the material? This may add up with the history of the particular machine, which had been for the first few years (with almost no yellowing caused at all) situated in one of the greener more gardenly districts of Vienna and was then moved to a typical inner city district, including increased exposure to aerosols caused by combustion and car exhausts. – Are there any reports on a distinction in, say, machines that were in a rural area, as opposed to those used in an urban environment?

Please note that the effect of this treatment, if there is any at all, is rather faint and this is by no means a perfect cure. So don’t rush to try it on your own, there’s not much sense to this. It only made me think…