As guessed, the inductor sharpens the edge that's formed when the transistor turns off and the load resistor is in charge of switching levels.
     I added 100 pF of load... I have no idea what the real loading is, but that seems like a possible value for a long wire and a couple of terminations.
     If someone can stick a probe on a relatively clean logic signal and send me a photo, maybe we can calibrate the model a bit more carefully.
     As usual, my first reaction was "an inductor makes it faster?? Can't be!" But of course that's what it does.

/guy

This is a bit of a surprise since the peaking coils seem basicly:
     - maybe 50 turns of fine wire around a non-conducting slug
     - two wire leads coming out the ends
     - covered by epoxy and value identifying paint stripes

See photo below

On Dec 14, 2017 23:13, "Robert Garner"  wrote:

    Robert,

    Thanks for dissolving away the outer shell or our failed inductor!
    Are the photos posted somewhere, or re-forward?

    Interesting that the failure point was where the external leads attach to the interior/inner wiring.
    Can you see if it was a solder connection?
    Perhaps heat expands the encapsulate, breaking the solder joint?

    Good idea, we should probably replace the core memory power inductors.
    (The entire 1401 may harbor over a 1000 inductors, so we won’t be replacing all of them.  ;-))

    Thanks again,

    - Robert

wrote:

The sulfuric acid basically dissolved the outer shell, so the inner core was left alone. I can unwrap it if you'd like to see. The wiring does look somewhat messed up. Half of it actually looks nice, but the rest looks a bit wonky.

Dialog

Inductor, used as Peaking Coil

- Exposed

- Exposed

 
  It was also telling that during the procedure, the wire broke at both points 
  where they join the leads. Definitely a weak point. 
  You might consider replacing the other inductors 
    which almost certainly will have the same problem at some point.

---

On Thu, Dec 14, 2017 at 10:39 PM Ken Shirriff  wrote:

    (Robert Baruch has been decapping chips and agreed to look at the bad inductor from the 1401.
      He runs project5474.org)
 
      Nice pictures. It's interesting to see what's inside the inductor. 
      Thanks for dissolving it. The winding looks a bit haphazard 
        - is that how it was, or did the wires shift around during processing? 
      Is the core a different material or did you just take the inductor out 
         before everything dissolved?
 
      Based on your previous comment that stressing the lead fixed the inductor, 
         it seems like a bad connection between the lead and the internal wire 
         would explain the problem. It seems like a random failure, 
         not connected to the suspected power surge.
 
      Ken