National Semiconductor CDIP-14 chip identification, circa 1984 - Page 1
Forum section: electronics / projects

author: russell--
author_rank: Newbie
timestamp: March 13, 2025, 12:15:09 pm
content: Hi, I'm trying to figure out what a chip is on a board I'm reverse engineering. It is on the "Function Board" of an IBM Wheelwriter 5 typewriter from around 1984. I found a link from a company that says they have stock:https://www.americanmicrosemi.com/product/SA879971/.Their are photos consistent with the one I have. There is a National Semiconductor logo, and the IBM part number is "849 3328", then the next line has "SA 879971". On the Function Board, it has a role in mediating power for battery backed SRAM (where the typewriter stores settings like tabs and margins and other state, up to 8KB of data. The pinout is odd and hasn't matched anything I've been able to find. See attached circuit sketch. It smells like some kind of op-amp. When the +12V and +5V power rails are turned on, I measure the following voltages:pin1: 12.57 pin14: 0.061pin2: 5.733 pin13: 0.005pin3: 4.995 pin12: 0.705pin4: 5.732 pin11: 12.95pin5: 12.59 pin10: 5.731pin6: 5.732 pin9: 6.353pin7: 4.958 pin8: 6.358delta V across the 74ohm resistor connecting to pin 1, I measured 0.430V.I have googled pretty hard, asked chatbots, and come up without an identification, much less a datasheet. Any ideas?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 13, 2025, 04:14:16 pm
content: Digchip list "High collector-emitter voltage (Base open) VCEO" and several other transistor parameters under their part description.https://www.digchip.net/datasheets/quote.php?action=search&pn=SA879This and the unusual power-supply pins lead me to GUESSING that this part is a transistor array. The pin-out of the common CA3046/MC3346 fits well and forms a circuit that would provide +5V output on pin 3 with the power sourced from the +12V supply to pins 1 and 6. The regulation being performed by reference to the +5V supply on pin 7.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 13, 2025, 04:45:03 pm
content: One piece of advice;I also post hand-drawn photos taken with my phone, but I also crop them prior to posting, to eliminate all the areas which are blank or don’t contain meaningful details.

author: Whales
author_rank: Super Contributor
timestamp: March 13, 2025, 09:23:52 pm
content: To quickly fix the giant image: delete the [attachimg=1] text from your post. The image will still be attached, but will be shown as a little thumbnail at the bottom of your post instead. People can then click on that to see the pic.The only other mention of this part number I can find is for a 4033-001 IBM print server board. It looks like the two chips are near the "TR" (token ring) connector. This might align with the theory that they're just transistor arrays?https://www.ardent-tool.com/4033/I can't find any PCB photos or schematics online, but it looks like you can buy 4033-001 boards. Depending on your budget it might be worth it for helping with your reveng, just to find out more about this one chip.https://www.argecy.com/product/10961/4033-001-marknet-print-server-external-refurbished-with-90-day-warrantyhttps://www.recycledgoods.com/lexmark-token-ring-print-server-4033-001/https://www.ebay.com.au/itm/142019970057https://www.ebay.com.au/itm/196847298531

author: russell--
author_rank: Newbie
timestamp: March 14, 2025, 12:23:49 am
content: Fixed the obnoxiously large image by removing the tag as suggested. thank you.A reply on the other topic (with the same name), now locked, suggested that it might be an LM3045, which prompted me to try out my cheap-o component tester on the legs, three at a time. I need to recheck, but it reported a bunch of thyristors. The last three pins (12, 13, 14) were different. I need to redo the tests more carefully, and can report the results. I think the LM3045 is close, but maybe not the exact thing.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 14, 2025, 12:54:03 am
content: It's a transistor array.Find youself a datasheet for a '3046 or '3045 (CA3046, LM3046, etc - the prefix changes with manufacturer) - the pin-outs are the same. I believe the 3046 varient is the only one that is readily available today. If you want an exact match, you will probably have to go to one of the distributors that advertise the SA879971. Otherwise, you can take the transistor parameters from the vendor that I highlighted above and try to find a NOS part that fits.It's a transistor array! It still baffles me why people ask for help and then ignore the answers (twice !)The attached shows how the array maps to the circuit you reversed.SA879971.Circuit.pdf(14.45 kB - downloaded 33 times.)

author: Andy Watson
author_rank: Super Contributor
timestamp: March 14, 2025, 01:17:17 am
content: Quote from: russell-- on March 14, 2025, 12:23:49 amThe individual transistors can be checked (individually) with any meter/DVM that has a diode test function. The transistors are fabricated on a common substrate which, when biased the wrong way, will introduce parasitic junctions and SCR structures that will confuse your component checker.

author: russell--
author_rank: Newbie
timestamp: March 14, 2025, 02:50:48 am
content: As it happens, and apologies for not including this earlier, I measured pair-wise diode drops with the chip out of circuit, and here's the matrix I got. Things look pretty consistent with the LM3045 from pin1 to pin8, but I'm having trouble matching the results from pin 9 to 14. But... I'm not super confident the measurements are correct. I'm redo'ing them. Meanwhile, I do think the 3045 is likely the correct answer.

author: russell--
author_rank: Newbie
timestamp: March 14, 2025, 03:15:14 am
content: Quote from: Andy Watson on March 14, 2025, 12:54:03 amI wasn't ignoring the answers, they just didn't seem completely consistent with some of my (maybe faulty) measurements.Your diagram looks correct, except that the resistor value between 12V and pins 1 and 5 is 74 ohms, not 47k.Thanks all, for your help!