Lisa / Mac XL Motherboard choke inductor values

[elemenoh]

I'm attempting to restore a Mac XL that suffered some pretty bad rusting. The IO area of the motherboard all needs to be replaced. Does anyone here know what the values of the 8x choke inductors at the mouse connector?

Photo

 

[stepleton]

Based on "DETAIL 'A'" in http://bitsavers.org/pdf/apple/lisa/hardware/lisa-motherboard-enhanced.pdf, they would appear to be 10uH.

 

[elemenoh]

Perfect. Thanks @stepleton!

 

[elemenoh]

I'm having a hard time locating the MLB resistor packs on this and the other available schematics. Are they 3.3k like the packs on the IO board?

 

[stepleton]

I'm not sure what MLB stands for. Do you mean the resistor packs like RP1 in your photo? Let me know and I might be able to take some measurements.

 

[elemenoh]

@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!

 

[stepleton]

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.  

 

[lisa2]

These parts are not just resistor networks, they are "RC Networks T-Filters"  AKA "EMI/RFI Filters" 

My modern 2/5 and 2/10 clone boards use Bourns PN# 4120R-601-250/500

More info here:

https://www.bourns.com/pdfs/601.pdf

Hope this helps.

Rick

 

[elemenoh]

@lisa2 It looks like that part is discontinued and out of stock. Would 4120R-601-250/201L be a suitable alternative?

https://www.digikey.com/en/products/detail/bourns-inc/4120R-601-250-201L/3593700?s=N4IgjCBcoGwJxVAYygMwIYBsDOBTANCAPZQDaIALGGABxwDsIAuoQA4AuUIAyuwE4BLAHYBzEAF9CAVnoAGRCBSQMOAsTIgAzHABMMKQhYgOXXoNETCAWh0Kl-AK5qSkclOZtOkEBMkhbriAASgDCAAQAFAAqVlEAnqy4AJRhAKIAsgCSYQBiApjsuHzM4kA

 

[lisa2]

Yes, it appears that Bourns have EOL these last year, but Mouser still has them in stock ( the RoHS version ):

https://www.mouser.com/ProductDetail/Bourns/4120R-601-250-500L?qs=izxORn%2FBzZchDGI8%2FT4C4Q%3D%3D

Rick

 

[stepleton]

Thanks @lisa2, I didn't know about these parts.

It's nice that you could make this network out of discrete components in a pinch. 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?

 

[lisa2]

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".

Rick