Not to engage in thread necroambulation, but I just came across this thread while looking into MPX units for a non-MPX tuner. I believe I have an explanation for the problem AC-11 issues, and thought I would share that in case it is helpful.
Note: this hypothesis is speculative as I do not have an AC-11, so I have no way to verify this. I hope someone with the unit performs this test. But I suspect my hypothesis explains the cause of the drift and poor functioning.
IF cans are typically hybrid creatures, containing:
(a) a coil with a tuning slug
(b) a silver-mica capacitor at the base. The capacitor is not encased and is created using a sheet of mica silvered on two sides.
Over time electrochemistry resulting from potential difference plus moisture causes silver ions to migrate through the mica; this is the infamous "
silver-mica disease". The migration ruins the capacitor, and may even bridge the primary and secondary. The leakage of B+ to the secondary eventually becomes a short, but prior to that point the leakage couples all manner of noise from primary to secondary. In an radio it sounds like crashing waves of static. In addition, sulfidation of the silver—silver sulfide is an insulator, unlike silver oxide which is a conductor—creates a poor contact and the capacitance continually varies as the frequency cycles. The effect in old radios and TVs is called "scintillation". Corrosion also causes impedance changes which results in frequency drift as the LC circuit changes its resonance point since the circuit is now RLC instead of LC, and the R is likely frequency dependent.
I suspect the replacement coil above described either did not have a defective capacitor or, more likely, had one less defective.
The solution is to remove the capacitor in the can and replace it with an external capacitor. Modern silver-mica capacitors use a silver alloy which does not migrate. I don't know how much better temperature stability would be obtained with SM vs. a modern PP.
Many times the capacitor value is undocumented, and the reported fix is to experiment with various values until the LC resonance point is hit. This surprises me because resonance occurs when XC(f) = XL(f), so if L is known and f (resonance point) is known, then XL is known and consequently XC must be known, and C can thus be calculated. I suspect a variable capacitor would be the easiest way to do the experimental calculation. In any event, once determined the new capacitor can then be soldered underneath the can.
In any event, I believe that deterioration of the capacitor in the IF can would explain the lack of frequency stability and other poor functioning.
The poor reputation for the AC-11 may have something to do with IF can issues.
I hope someone with an AC-11 will be able to verify this hypothesis and report back.