Help resurrect my Xerox Alto

[Brent Hilpert]

On 2010 Sep 20, at 3:15 PM, Dave McGuire wrote:

> On 9/20/10 2:49 PM, Brent Hilpert wrote:
>>> Was a great time and really a blast to dig into the Alto. We 
>>> definitely
>>> improvised, not having a stock of uA709 opamp on hand... we just 
>>> bypassed
>>> that regulator circuit and stuck a 7805 there instead, which was 
>>> pilfered
>>> from Vince Briel's spare parts stock for his Altair8800micro kits.
>>> Too much fun.
>>
>> If the 709 was a TO-99 (can) package and you need to replace it, I 
>> have
>> lots of 741's in TO-99.
>>
>> Just mentioning it in case it is difficult to find a 709 in TO-99. 
>> (DIP
>> should be easy to find something to substitute.)
>
>   TO-99 709s are difficult to find, but not impossible.  For most 
> applications I believe a 741 can be used as a drop-in replacement for 
> a 709; this being a voltage regulator I'd guess it probably isn't all 
> that demanding on the 709's particular specifications.
>
>   It wouldn't be 100% historically correct, but if you think about it, 
> that would likely have been a reasonable repair method during this 
> machine's service lifetime.
>
>   I'd say do it.

The 741 was well available by the time of the Altos introduction, and 
simpler to use than the 709. Mildly surprising to hear the Altos used 
the 709, perhaps the 741 was still a fair bit more expensive.

Pins 1 & 5 (TO-99) connect to external compensation components on the 
709, they are for offset null on the 741. One should probably snip off 
or leave open those pins if replacing a 709 with a 741.

I think I have one 709 in TO-99 from my first op-amp project (a 
square-wave generator) ca 1972.