SE/30 PDS Card stacks & CRT Anode EMI

[zigzagjoe]

So a while back, @Bolle worked on a couple Micron Xceed Macrocolor cards with one or more bad VRAMs. He theorized that it was potentially due to these cards living in very close proximity to the CRT anode, especially when stacked on a maccon or one of his clones.

This seemed like a sound thought process to me, so as I'm looking at stacking cards, I did an unscientific test with my meter's AC setting. Didn't do a good job keeping the meter in view, but you can definitely see that it's getting noticeable noise & apparent voltage fields when the probe is nearer the anode. There's a small change in tone from the CRT, too. Both my machines seem to do this (fully recapped).

My solution was to add a bracket for mechanical support of the card stack as well as to shield for the parts of the board closest to the CRT. There's about 1 cm of clearance between the bracket and the card, enough for airflow.



Has anyone else looked into this kind of thing? I'm curious what other folks do to fit these stacks in & support them, I had to bend the original card supports slightly out of the way and coat them in kapton tape.

 

[Boctor]

Yes, I have this issue with my SE/30. I inserted a TwinSpark with a Diimo 030 (IIci cache slot type), then an Asante MacCon on top. The Asante's passthrough connector is just a tad too close to the anode cap and emits quiet, continuous static noises, similar in intensity to running one's hand in front of the CRT. Even after a discharge, thorough cleaning, reseating prongs, and fresh dielectric grease. It seems like it's not the cup's edges or anything, it's just the suction cup itself, even the back part, as if the insulator is holding a static charge. I am hoping this is different from more serious phenomena like arcing. It may also be worth noting that my living space is incredibly dry at this time of year.

If both your machines are well-kept and exhibit the same thing, maybe this is a general trait of CRTs? I've not yet found examples of any CRT/all-in-one devices out there where PCBs are in close vicinity to the anode cap. In many larger TVs, the cap even points upwards, positioned atop the tube. But CRTs were in their twilight years during my coming of age, so I am not an authority.

I kludged an insulator/barrier with a thin, dense piece of cardstock (similar those attached to the analog/neck boards) which prevented static from making the leap, suppressing the noise. Your support bracket idea is much more elegant. Of course, maybe I should keep an eye out for Bolle's excellent adapters, if those would keep the SE/30 PDS Tower of Power™ small enough to forego worrying in the first place. (I only want to use Diimo+Ethernet.)