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Lisa / Mac XL Motherboard choke inductor values

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  • 68kMLA Supporter

@stepleton Yes I'm referring to the 'motherboard' which is basically a backplane. There are two resistor packs on it, including the one in my photo. I haven't bothered measuring since everything in that area is so damaged. The schematics for resistors packs on the other boards state 3.3k, but the packs aren't shown on the drawing for the motherboard.


Thanks in advance for the help!

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I've now measured RP1 and RP2 on a Lisa 2/10 motherboard --- I expect the values should be similar for a 2/5 motherboard.


The resistor packs are 20-pin DIPs. Oriented vertically, call the top-left corner Pin 1. Using a conventional DIP numbering scheme:


- Pins 1, 10, 11, and 20 are all shorted together


All remaining pins appear to be connected straight across the package with 50 ohms of resistance. So:


- Pin 2 to Pin 19: 50 ohms

- Pin 3 to Pin 18: 50 ohms

- Pin 4 to Pin 17: 50 ohms

And so on to

- Pin 9 to Pin 12: 50 ohms


Note that this arrangement is symmetric in all axes, so it doesn't matter if you get the resistor pack the wrong way round and put "Pin 11" up at the top left.  

Edited by stepleton
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  • 68kMLA Supporter
2 hours ago, stepleton said:

Do you know what the risk would be if you were to "Muntz" these filters away? Are they for signal integrity, safety, or compliance, or some mixture of the three?


My guess is that these are for EMI compliance, I don't think that if you jettisoned these parts it would have much effect.  Many-many serial designs ( including the Tecmar Lisa four port card) do not use filters like this and work fine.   The Macintosh does have these also, on the schematics they are listed as "DEGLITCH PACKS", and the SE the silkscreen on the Logic board says "FILTER".



Edited by lisa2
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