Guess the hidden 74 series chip! - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 27, 2022, 06:46:07 pm
content: This is a little hard to explain but on the board I am attempting to repair is an interesting stack of two chips.Basically a resistor pack has been stacked on top of another with the pack providing pull-ups to the output pinsSo the game... guess what the chip underneath is... I'm speculating it is a 3 to 8 line decoder...It's difficult to know which way up it is (since I cannot see it!)... but I am going to presume it is orientated such that...It has 16 pins... pin 8 is GND and pin 16 is VCC. If this orientation is correct then I believe pins 15, 14 and 13 would be inputs... with the other pins being decoded outputs and maybe the odd enable pin somewhere.Any ideas? Normally 3 line decoders have their inputs on 1,2,3 so this is a bit back to front... I don't believe it is upside down... but these days... anything is possible.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 27, 2022, 07:20:02 pm
content: I think I've worked it out... 74LS42 and the resistor pack is used for ORing together outputsSN7442A perhaps... otherwise ORing the outputs might not be too clever?

author: harerod
author_rank: Frequent Contributor
timestamp: January 27, 2022, 07:41:13 pm
content: Is this worth the time and risk guessing? Since this is a digital application, I'd suggest:- one row is only connected by two pins- get your ESD-strap, unsolder those two first- then the opposite rowWe have done worse in the 1980's - e.g. unmounting 16 piggy-back riding DRAMs when upgrading an 1MB Atari ST to 2.5MB:https://atariage.com/forums/topic/159321-how-hard-would-it-be-to-get-a-520st-to-1mb-ram/page/2/?tab=comments#comment-2013581And as far as wired-and / wired-or goes - I still admire what the late Sir Clive did for the Specci.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 28, 2022, 12:46:14 am
content: Quote from: harerod on January 27, 2022, 07:41:13 pmI have other more pressing parts to fix so I won't be removing it. A BCD decoder makes sense so I am content with my guess.It is a decode of A11, A12, A13 and NOT (A14 OR A15) and then partitions these into chip enable lines for 5 EPROMs... pin 1&2 drive a 4K EPROM, pin 3&4 another 4K EPROM, pin 5&6 another 4K EPROM and then pin 7 drives a 2K EPROM and finally pin 9 drives a finally 2K EPROM... an interesting approach... but seems to work.

author: edavid
author_rank: Super Contributor
timestamp: January 28, 2022, 03:33:39 am
content: Quote from: NivagSwerdna on January 27, 2022, 07:20:02 pmThe 7442/74LS42 don't have open collector outputs, so they can't be wire OR-ed.(The 7441/74141 have open collector outputs, but are not logic level.)

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2022, 09:10:48 pm
content: It is probably a 74LS138. I have a similar fuzzy logic address decoder on a wire wrap board.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 28, 2022, 09:33:35 pm
content: Desolder the two pins of that resistors array, by gently bending them away from the IC's pin 9 then pin 16, one by one, then flip the R aray sideways a little, just enough to read the IC underneath, without desoldering the rest of the R array pins.Why do you want to guess instead of looking?