Re: Where to start on Missile Command PCB repair

Grant,

There is also the issue of "fan-out", which refers to the maximum number of inputs that can be driven from one output. Those fan-out ratios are quite different between the different "74" series chips, and can get more than a bit touchy when dealing with cross-series inputs and outputs.

A 74LSxx series output pin is generally held to have a maximum fan-out of 20 to other 74LSxx input pins. A standard 74xx series output maximum fan-out is 10 to other 74xx inputs.

However, in the case where you have a cross-series situation, things are a lot different. For example, in the case of a 74LSxx output driving 74xx inputs, the cross-series fan-out is only 5!

So, yes there are a lot of factors that go into deciding whether you can replace one series chip with another - the other two are propagation delay (already discussed) and threshold voltages (generally not an issue). True that in many (most) cases, you can get away with it. But there have been a few cases I've seen where you couldn't - those are the ones that you need to actually study the schematic to know it you're safe to substitute or not. The 74153 address decoder on Williams boards is one that immediately comes to mind where an LS series part just wouldn't work.

I'll admit that I haven't actually looked at your situation to know, but this is meant more for general discussion anyway.

I also think Jameco is a decent source for many of the 74 series chips. Their shipping via USPS is quite fast (to me at least in Minnesota, 2-3 days transit typically). However, a lot of their 74xx chips are "pulls", so they're essentially new-old-stock. I have run into some that have had some lead tarnish, but the trusty old Pennwalt micro soda blaster takes care of that in short order :)

--Brad

----- Original Message -----
From: "Grant Thienemann" <grant.thienemann@gmail.com>
To: "rasterlist" <rasterlist@vectorlist.org>
Sent: Friday, November 1, 2013 10:50:12 AM
Subject: Re: RASTER: Where to start on Missile Command PCB repair

Ok I've dived head first into researching 74XX series ICs, and here is what
I've figured out those far.

1st. 74XX can be replaced with 74LSXX because they are both the same speed,
10ns, the 74LSXX just draws less power
2nd. 74SXX MIGHT be able to be replaced with 74FXX series. 74SXX series is
a 3ns delay while 74FXX is a 3.4ns delay, still trying to figure out if
that is ok or not . .
3rd. 74HXX doesn't really have a good replacement because the delay is 6ns,
the closest thing I've found is 74ALSXX but its 4ns vs 6ns.

I'm just trying to figure out what I should order as a massive parts list
for all the various boards I need to replace, which includes Missile
Command, Asteroids, Asteroids Deluxe, Berzerk and Frenzy.

Thanks
Grant
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Received on Fri Nov 1 12:45:30 2013