So I was playing around with a bunch of NOS LM311 chips and an ACI card with different audio sources.
What did I discover, not all LM311 chips are created equal. Even ones from the same manufacturer and date code.
The all worked with a real cassette player, but at slightly different volumes. Some didn't like iPod and iPhone audio. Some wouldn't load at all and some would put intermittent bad data into ram. This was very apparent when loading Apple the 30th. You'd get corrupted images. My guess is this is related to the fact the iPod and iPhone can't drive as much volume as a real cassette adapter. I think there will also be some variations related to your power transformers and supply components, since the LM311 uses unregulated negative voltage to the LM311.
So if you have a problem with your ACI with an iPod or iPhone, before you go crazy, swap out your LM311 and if it works don't throw out the original one. It may work in another ACI on a different board.
then phenomenon you recognized might be resulting from another fact:
The LM311 is in fact not really a simple OpAMP.
Instead it's in fact a comparator ( special variation of OpAMP
designed for switching ) with what you may call a "overdrive magnification"....
so in fact the problem is related to the one input where you feed the
"reference value" to which the other pin is compared to.
Old chips had a relative large margin of voltage that you may call
"indefinite area" between "high" and "low" ( in general between 0,9 Volt and 3,6 Volt ).
If the values of the resistors that create that "referece / compare voltage are not well
selected or if one of that resistors is at the "outer boundary" of its specified limits -
the voltage to compare to - drifts also out of limits and the comparator
gives bad results by switching where it should not switch.
This can sometimes be corrected by using better resistors with less tolerance.
bear in mind that most resistors ( specialy at the old ones ) have a tolerance of
20% - meaning they may be up to 20% off from specified value !
I thought the variance in the resistors is why 1% ones were used to make sure this wasn't an issue. My point was if you are having problems with your ACI, try a different LM311, but if you find a working one, it doesn't mean the old one was bad. Just might not work in your setup.
I'm just curious - what input capacitor value(s) did you use in your testing?
nowadays 1% resistors are rather common in circuits, if critical conditions are expected.
In old days they have been rather uncommon because they have been far
more expensive in those days. And i also agree with the fact that also within the LM311
there is a large variance resulting to the viewed effects.
I just wanted to spot out, that the variance of the LM311 is not the only possible
just to expand the point ( that i mentiond about the "illigal state" between 0,9 Volt and 3,6 Volt ):
If you have 2 equal resistors and the supply source is 5 Volt the "middle point to compare" would be 2,5 Volt.
With 1% resistors this may change within boundary from
2,475 Volt to 2,525 Volt.
With 20% resistors this may change within boundary from
2,0 Volt to 3,0 Volt.
If another LM311 is not availiable at the moment it might be usefull
as alternate solution to change the value of one resistor to pull down the "compare voltage"
from 2,5 Volt to a value between 1,75 and 2,0 Volt.
But thats rather more a solution for experienced Users.
.01 Cap that was baked past the Curie point to reset it.
I usually don't upgrade the cap on the ACI. With the iPod/iPhone as an audio source it's usually not needed. With an actual cassette player, I have seen a difference.
I was thinking you probably were using the .01 uF cap. I'd say that with a .1uF cap, LM311 characteristics would be less critical. As designed, with a .01uF cap, performance is marginal. Designs with good margins will normally not be so sensitive to IC selection.
Wow Corey, resetting the ceramic cap is fascinating.
How did you heat the cap? I understand that 125 degrees C is enough but I've never done it.
I have heard that heating both cap leads with a hot soldering iron might do it.
Now that we can accurately measure capacitance, checking an old ceramic cap before and after heating might be very instructive!
Question: Would other methods of heating to the Curie point change the outside appearance of the cap?
I actually use an air oven at 300'F for 30 minutes. I have a rig setup to do sets of caps at a time.
They physically don't look different after the baking. They just reset back to their original values.
I have a friend who is an engineer and I'm pretty sure that if you were to call him he would say that in any circuit, the correct value of EVERYTHING has to be used.