unknown part on 128k analog board?

[reallyrandy]

Can anyone point me to the correct part on Digikey or Mouser or even Console5 for this part at L2 on a 128k Analog board?
It's right next to a power plug that fried and melted this one.

 

[JDW]

Custom part not available from Mouser, Digikey, Console5, etc. Replacements must be extracted from gutted boards.

Honestly, it would be nice if we could get a newly built replacement for those, since old parts won't last forever.

 

[joevt]

A normal inductor won't work? https://en.wikipedia.org/wiki/Inductor

The schematics say L2 (connected to -12V) is 27µH
L1 (connected to +12V) and L3 (connected to +5V) are 28 µH (toroid)
These three are connected to the Power connector J7
Actually, I think that's the schematic for the main logic board. Is there a schematic for the analog board?

There's a Mac Plus analog board schematic which shows L2 is a variable or adjustable inductor that controls the width of the display.
https://vintageapple.org/gamba2/schematics.html

Actually, the site that has the 128k main logic board schematic has schematics from Beck-Tech that include the analog board.
https://www.macdat.net/files/pdf/apple/schematics
It shows an L2 like the Mac Plus.

What's the range for the L2?

 

[JDW]

I do not have technical info on L2 in order to answer your question about "range." And whether or not a standard part would work is not something I have tried. In fact, I've never read about anyone trying it, probably because most people salvage L2 from a dead or otherwise junked Mac.

So if you give it a shot, you would be breaking new ground. And if you find a good replacement candidate, that benefits the whole community. So please don't allow anything I've said to date hold you back. I just cannot give you all the technical specs for that custom made coil because it was probably specially made for Apple and therefore no datasheet exists.

But yes, that part does allow you to change the CRT display WIDTH, and that is shown on the vinyl jacket too:

 

[reallyrandy]

I wonder if the internals could be extracted and a new plastic housing could be 3D printed? I might try that since it's junk now anyway. I unfortunately don't have a donor board to extract another inductor from.
I think the main problem with that is 3D printed items are not heat resistant at all.

For context, this is a 128k Mac that has a Levco Monster Mac board installed and a Corvinus external hard drive. I'd love to get this piece of history running again!

 

[JDW]

I’m no expert on 3-D printing, but from what I understand, there are some plastics that are more heat resilient than others. ABS is an all around great plastic.

The Top 6 Most Heat-Resistant 3D Printing Filaments

Got a hot job? Heat-resistant filaments are the future of functional 3D printing. Let's look at the Top 6 Most Heat-Resistant Printing Filaments.

filamatrix.com

 

[JeffC]

I wonder if the internals could be extracted and a new plastic housing could be 3D printed? I might try that since it's junk now anyway. I unfortunately don't have a donor board to extract another inductor from.
I think the main problem with that is 3D printed items are not heat resistant at all.

For context, this is a 128k Mac that has a Levco Monster Mac board installed and a Corvinus external hard drive. I'd love to get this piece of history running again!

Are you sure it doesn't still work? I accidentally snapped the plastic housing on one and it still works fine.

 

[reallyrandy]

I slid the plastic out of the coil and it's still intact and the pot still spins inside. So I think I'm gonna try and glue it back together with some T-8000. It has a super fine nozzle so I think I can make it work. The burnt plastic is really brittle though so I have to be really careful. I'll take some close ups and post if it works.
The problem with 3D printing high heat plastics is it takes even higher heats to print them and that's a more expensive printer than I have. I have the Bambu Labs A1 Mini.

 

[Paolo B]

My two cents… the plastic part fulfills the function of allowing the ferrite core to be precisely aligned with the coils, allowing the fine tuning.
If the sheath of the coils is not damaged, and the ferrite core can still be adjusted, the component should still be fully functional.
I had an analog board, though, where the ferrite core became brittle to the point it crumbled when I tried to tune its position. In that case, it’s indeed game over.

 

[This Does Not Compute]

I'm not too knowledgeable about inductors, but could one conceivably use an LCR meter to measure the min and max values of a stock L2 part and use those to source a modern replacement?

 

[YMK]

You could find a plastic tube of equal outer diameter, then drill and tap it to match.

 

[Paolo B]

The plastic tube is threaded, for allowing the fine positioning of the ferrite core wrt to the coils.

 

[YMK]

The plastic tube is threaded, for allowing the fine positioning of the ferrite core wrt to the coils.

That's why I said "tap it to match". Taps make threads.

 

[trag]

Ten years ago, my efforts to reproduce. Unfortunately, the thread where Wally reported the inductance range is long gone. I want to say 0 - 44 uH, but I don't think that's right.

https://68kmla.org/bb/index.php?threads/anyone-have-a-dead-l2-adjustable-inductor.27941/#post-294686

 

[reallyrandy]

I glued it back together and although it's bit brittle, it should hold. You only need to adjust it once… I hope.

 

[daanvdl]

I have recently been searching intensively for this variable inductor. The range is from 0-35 approximately. After a long search I found a batch (NOS) that met the specifications. If you still need a replacement: https://airtalk.shop/product/l2-adjustable-inductor/