Replacement chips for Stargate CPU board

shuyge

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I've been working on fixing my first CPU board and I've determined that chip 3D is bad. The local DRAM address line A2 is stuck low. I've got signal coming in, but nothing coming out.

The BOM indicates that the part is a 74153 which is a dual 4 to 1 multiplexer.

The question is which variant of 74153 do I buy and is brand important for the Williams CPU boards? Williams in their infinite wisdom decided to put stickers on all of the chips and I am unable to read the original markings on the existing chip.

Do I get a straight up 74153, a 74LS153?

Any advice from people who have repaired these boards already?

TIA,
Stan
 
To add to this -- a bit of explanation why you should stick with a 74153:
Each output of the 74153 is driving up to a whopping 24x 4116 inputs.
A typical 4116 is rated to have a worst case high level input current of 10uA and a low level input current of -0.4mA.

24x 10uA = 240uA for high level input
24x 0.4mA = 9.6mA for low level input

A 74153 can provide:
800uA (0.8mA) high
16mA low
Enough high level and low level current to drive all 24 inputs.
But still more than 50% loaded for low level current... I can see why these fail over time.

A 74LS153 can provide:
0.4mA high
8mA low
It only provides half of required current. Can sometimes work ... but for how long is anybody's guess.

Don't bother with HC or HCT type parts - they are low power devices and have zilch for current outputs.

In old computers that used banks of DRAMS such as this - it was not uncommon to use Schottky "S" or Fast "F" logic components. These parts are far faster than the 74153. But they are used for their current drive capability and not for their speed.
Both 74S153 and 74F153 can provide:
1mA high
20mA low
Can provide more current than the original 74153 and can often be used. Only downside to these was their speed...too fast. If the board designer did not include series resistors on the outputs then there could be electrical "ringing" on the address lines. And, Williams did not add these which can potentially provide problems depending on exact memory parts used.
 
Thanks guys.

When I was looking at datasheets, I failed to consider the current needed to drive all 24 ram chip address lines for some reason.

I installed a 74153 in a socket and she booted. I got a partial boot noise from the speaker and no other indications. I have no video and most likely a sound issue to sort out yet.

Stan
 
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