Strange Plus Mod?!

RetroTechGuy

New Tinkerer
Feb 25, 2023
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I opened up this Macintosh Plus today and came across this mod - never seen it before! Anyone know what it does?

18A7BCD6-F8DA-42E7-B9C4-E0C9960006C7.jpeg
 
That looks like a capacitor, maybe some sort of hacky way to stabilise screen during floppy access , rather than address the fault directly.
 
Looks to me like they relocated that capacitor since it would be physically too big to fit on the analog board - it would interfere with the CRT.
 
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Sorry yeah, I should make the picture into a ‘find the differences’-challenge… !

Here is an up close of what seems like the automotive capacitor.

The case was also lined with copper (see pic), something I’ve never seen before - I was wondering if and how this is relevant to the giant capacitor.
 

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I wonder where that thing came from and what it was used for. That's some serious RF filtering going on there. Maybe it was used in a lab of some sort. My guess would be all that was to keep it from interfering with sensitive equipment.
 
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Can't think of what they were called offhand, but bucket could be from one of the security hardened setups for Alpha-Bet Soup Agencies. Backside and front bezel had cover plates while the sides looked stock, copper on the inside of the bucket makes sense for that.
 
There are several reasons to use copper as a lining:
  1. EMI shielding helps reduce the amount of electromagnetic interference (EMI) that enters the case and interferes with the computer's components, such as in labs or industrial areas, where there are high levels of EMI caused by high-frequency equipment or radio transmitters. In other words, putting the computer in a Faraday cage to block EMI.
  2. Copper is an excellent conductor of heat, so lining the inside of a computer case with copper can help heat dissipation more efficiently and prevent overheating.
  3. Since copper can transmit electrical signals quickly and reliably, hence the likelihood of the signal transmission between the computer components is not disrupted or degraded.
 
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Thanks all!

It’s also dead as a door nail so I’m going to try to coax it back to life - maybe I’ll start by putting a normal size cap in C1…I’ll save the cool shielding though!
 
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