Where did you get those TIL311's from? A local surplus store
here has put a bunch of them on ebay that match this
description. If you did buy these from Active Surplus, I know
the store manager there and I'll do what I can to help.
Date: Sat, 28 Jun 2014 23:49:00 -0400
Subject: Re: [N8VEM-S100:4412] v20-16, z80-20 and chip
I purchased two Z84C0020PEC's that were also questionable.
One was dead, one runs ok at 8mhz (I haven't tried faster
clock speeds yet). They had identical top printing and date
codes, but the molding marks on the bottom were different.
The dead chip was replaced without question with one that
also works fine at 8mhz.
Also a recent batch of TIL311's were used pulls (advertised
as new) and 3/10 were dead in whole or part. They had
shortened tinned leads and obvious signs of removal.
I could write pages about counterfeit capacitors - my
primary hobby is vacuum tube audio gear. My experience is
that 100% of Chinese capacitors are faked.
If Mouser or Digikey carries the parts it's worth the extra