TRS-80 Model 1 Expansion Interface question?

Peter Cetinski pete at
Mon Mar 20 20:10:26 CDT 2017

> On Mar 20, 2017, at 8:46 PM, Win Heagy via cctech <cctech at> wrote:
> Is there any reason why this chip would be removed?  I see a number of them
> on ebay for around $25.  The expansion interface hardware manual indicates
> it is an FD1771B-01, but the service manual indicates a couple
> possibilities....FD1771 A/B -01 -11.  Any considerations to look for here?

The EI may have had a double density board which plugs into the FDC chip socket.  The FDC chip then plugs into the doubler.  Without the doubler you only have single density.  Original Percom or Tandy doublers are fairly rare to find today so someone may have pulled it out to sell.  Ian Mavric in Australia does make reproductions though.

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