TRS-80 Model I

[Pete Turnbull]

Ethan Dicks wrote:
> On 2/4/07, Pete Turnbull <pete at dunnington.plus.com> wrote:
>> Neither did I, for much the same reasons.  And I had to repair quite a
>> few that went out of alignment, and some that had released the magic
>> smoke when someone plugged the drives in "off by one" after they
>> disconnected them to move the machine.
> 
> That was a problem I also saw with the drives.  Several of my friends
> had Apple IIs (I had a 32K PET 2001-N), and ISTR more than one person
> did that.  Not cheap.  My memory is that at the time, the bare drives
> were $600 new, so even if one just replaces boards, it's not
> inexpensive in that environment.

Actually they were cheap to repair, because the most common failure by 
smoke emission was the 74LS125 and the next most common was the MC3740 
(I hope I've remembered that number correctly). The former cost a few 
tens of pennies and the latter several tens of pennies.  It was just a 
nuisance to have to do it, especially since at one time the Motorola 
chip was hard to source in small quantities.  One Motorola distributor 
even told me it didn't exist!

> The 6520 == 6821 FWIW.

Not quite -- they're functionally equivalent but the 6520 is a copy of 
the 6820, and the 6821 is a later more reliable part.  We regularly 
replaced the ones in PETs because one (I forget which of the two it had) 
used to get damaged easily.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York