Macintosh SE PSU IC identification - Page 1
Forum section: electronics / repair

author: gkmaia
author_rank: Frequent Contributor
timestamp: November 30, 2020, 05:37:12 am
content: That is probably for someone that is familiar with these old macs as there are no PSU schematics for the SE.I got an IC that is faulty. I know that because I have two of these PSUs and by swapping the IC it works.Could it be a high voltage optocoupler or a 177 opamp? Identification markings are 177 u8918. I could not find anything online so far... possibly remarked by manufacturer?I wish I could tell more but that is all I know so probably someone that if familiar with this PSU and have had similar problem?! Maybe?

author: bob91343
author_rank: Super Contributor
timestamp: November 30, 2020, 06:52:46 am
content: 8918 is the date code, 18th week of 1989, maybe May.Since it's an 8 pin DIP I vote for an op amp or even a dual opamp. Most optocouplers would have only 4 pins.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 30, 2020, 10:32:35 am
content: HiTry the attached schematic

author: Haenk
author_rank: Super Contributor
timestamp: November 30, 2020, 11:02:31 am
content: Can't make out any part numbers >300, so it's not the same schematics.#deleted#Looks like an Unitrode OEM part...

author: JKKDev
author_rank: Regular Contributor
timestamp: November 30, 2020, 11:45:02 am
content: Wouldn't it be rather straight forward to test the working IC? Does it act as a diode on one side and a transistor on the other? If you drive the diode, does the transistor turn on? Then you have an opto.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 30, 2020, 01:34:43 pm
content: The problem is that there is more than one variant of a Mac SEGeorge

author: Haenk
author_rank: Super Contributor
timestamp: November 30, 2020, 02:07:25 pm
content: Quote from: JKKDev on November 30, 2020, 11:45:02 ama mil spec opto is unlikely IMHO

author: CJay
author_rank: Super Contributor
timestamp: November 30, 2020, 02:10:45 pm
content: Given Unitrode tended to be *big* in SMPS controller ICs and that's from a PSU, I reckon there's a good chance it's something like one of the UC384x family, a wider pic of the rest of the unit and maybe the back of that board might be useful...

author: gkmaia
author_rank: Frequent Contributor
timestamp: November 30, 2020, 06:57:53 pm
content: Thanks! Ive tried a 8dip opto and no good. I will test the working one as an opto and let you know.I am off on vacation and will send a photo when i am back. What i got so far are some plots of it. The ic in questio is within a sub board conected on test points p10,12,12,13.

author: gkmaia
author_rank: Frequent Contributor
timestamp: November 30, 2020, 06:59:11 pm
content: Is the image opening? I cant open it from my phone

author: fzabkar
author_rank: Super Contributor
timestamp: November 30, 2020, 07:06:13 pm
content: You have uploaded a zero byte file.

author: james_s
author_rank: Super Contributor
timestamp: November 30, 2020, 08:21:52 pm
content: Draw up a schematic of the parts surrounding the IC and post that, it should provide some clues. I usually start by trying to identify the supply pins, ground is typically easy and Vcc is usually not too hard either. Then see if you can figure out what's an input and what's an output, there are a lot of parts that share common pinouts.

author: Haenk
author_rank: Super Contributor
timestamp: November 30, 2020, 09:36:03 pm
content: I have found a a link to 1987 / 1992 parts, not a lot of DIP-8 in there:http://bitsavers.informatik.uni-stuttgart.de/components/unitrode/_dataBooks/1987_Unitrode_Linear_Integrated_Circuits_Databook.pdfhttp://bitsavers.trailing-edge.com/components/unitrode/_dataBooks/1992_Unitrode_Product_and_Application_Handbook.pdf

author: gkmaia
author_rank: Frequent Contributor
timestamp: December 01, 2020, 05:01:04 am
content: The IC is on a sub board connected to the main board.Not sure if it comes or goes to it but the +5v and the +12v (with signal for the CRT) goes through it.

author: gkmaia
author_rank: Frequent Contributor
timestamp: December 01, 2020, 05:02:25 am
content: Quote from: Haenk on November 30, 2020, 09:36:03 pmAwesome will dig those!!!

author: fzabkar
author_rank: Super Contributor
timestamp: December 01, 2020, 05:58:54 am
content: There is an 8-pin uPC358C dual op-amp in that other circuit which is functioning as a voltage detector for the +5V and -12V rails. I suspect the "177" IC is also some kind of voltage or current sensing circuit.

author: gkmaia
author_rank: Frequent Contributor
timestamp: January 01, 2021, 06:17:21 am
content: I am still stuck with this one.I've tried a precision opamp op177 and no success. Also tried the conventional single and dual op amps no success...Would the IR2175 current sensing IC be an option?I am gambling... I have absolutely no idea what this little daughter board does.

author: CJay
author_rank: Super Contributor
timestamp: January 01, 2021, 09:38:36 am
content: Quote from: CJay on November 30, 2020, 02:10:45 pm

author: gkmaia
author_rank: Frequent Contributor
timestamp: January 10, 2021, 08:38:12 pm
content: Quote from: CJay on January 01, 2021, 09:38:36 amSpot on!!! Thanks, the PSU works now!

author: CJay
author_rank: Super Contributor
timestamp: January 10, 2021, 10:11:03 pm
content: Quote from: gkmaia on January 10, 2021, 08:38:12 pmSO, which chip was it? Only that might be useful for other people in future?

author: vincomhk
author_rank: Newbie
timestamp: May 25, 2024, 08:23:54 am
content: I also have issue on the psu of Macintosh SE. Searched on web and found this post. Just to share the photo of that board with a relatively clear image of that IC for reference.

author: threepwood
author_rank: Newbie
timestamp: June 27, 2025, 09:29:25 pm
content: Quote from: CJay on January 10, 2021, 10:11:03 pmIn fact it would be very helpful for a similar repair. Is there a way to contact the user?

author: picburner
author_rank: Frequent Contributor
timestamp: June 28, 2025, 03:21:30 am
content: Based on this last photo IC 301 is an LP1842 (UC1842) and the schematic should be the one attached.Edit: When I wrote the text I was still sleepy.... I mistook the "I" for an "L": the exact name of the device is IP1842.

author: threepwood
author_rank: Newbie
timestamp: June 28, 2025, 02:01:35 pm
content: Quote from: picburner on June 28, 2025, 03:21:30 amSo if I'm happy with running it from 0° to 70°, I can use a more common UC3842. This information is very useful because the military grade "177" Unitrode is found on many Astec power supplies of those years.

author: wraper
author_rank: Unknown
timestamp: June 28, 2025, 02:17:38 pm
content: Quote from: threepwood on June 28, 2025, 02:01:35 pmVariants with wide temperature range are the same ICs, just with tighter parameter binning guaranteed. The vast majority if not virtually all UC3842 will work in the same wide temperature range just fine.