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The Wikipedia page on Mostek, one of the original fabricators of Z80 chips, includes the following claim:

When Vin Prothro, President, and L. J. Sevin, Chairman of the Board, discovered that Zilog had modified the recipe for Z80 chips to keep the yields low, thereby buying Zilog time to build their own fabs, Mostek sought a new microprocessor partner.

The article is poorly referenced, and I haven't been able to find any corroboration of that claim.

The Computer History Museum's Zilog oral history includes a little about the Mostek relationship and the genesis of Zilog's own fab, and briefly mentions a problem with the first wafer - but (perhaps unsurprisingly) this particular issue doesn't come up.

So, (1) is there any corroboration of this claim? And (2) how might Zilog have 'modified the recipe'?

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    I've never heard of this before, and I think a Wikipedia editor has misremembered something here. There is a similar story, and it's included in one of Wikipedia's linked sources ("Computer History Museum - Oral History of Robert Proebsting", pages 41-43), but it tells of Mostek engineering a chip (a DRAM in this case) such that when other manufacturers copy it their yields would be low, not the other way around. – Jules Apr 1 '18 at 22:54
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    Also worth noting: Mostek continued manufacturing Z80s for many years after Zilog had their own production in place. Mostek Z80s were used in many of the early Sinclair computers, for example -- the Issue 3 Spectrum I had used a Mostek processor, and I believe was manufactured in 84 or 85, so they were still producing them quite a while after any such problems would have been discovered. – Jules Apr 1 '18 at 23:04
  • @Jules I think those comments would make a perfectly serviceable answer here, unless someone knows something more first-hand. – Jim MacKenzie Apr 1 '18 at 23:56
  • I remember in the early 80s, lots of people thought Mostek and MOS Technology (6502) were the same company. – cup Jun 3 '18 at 17:38
  • I worked for Mostek in Texas for almost a year in the 70's, worked in the clean room acid washing wafers, oh joy.. – Moab Jun 22 '18 at 2:12
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To quote @Jules's comments:

I've never heard of this before, and I think a Wikipedia editor has misremembered something here. There is a similar story, and it's included in one of Wikipedia's linked sources ("Computer History Museum - Oral History of Robert Proebsting", pages 41-43), but it tells of Mostek engineering a chip (a DRAM in this case) such that when other manufacturers copy it their yields would be low, not the other way around.

Also worth noting: Mostek continued manufacturing Z80s for many years after Zilog had their own production in place. Mostek Z80s were used in many of the early Sinclair computers, for example - the Issue 3 Spectrum I had used a Mostek processor, and I believe was manufactured in 1984/85, so they were still producing them quite a while after any such problems would have been discovered.

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