Keithley 228A "Recent" Schematics - Page 1
Forum section: electronics / testgear

author: icy.dev
author_rank: Contributor
timestamp: August 23, 2025, 12:31:25 am
content: Hey Y'all!Any chance there's someone out there with a very RECENT Keithley 228A schematic set other than the one from the instruction manual rev E that's online???If not, anyone know a good source??? I asked Tek (trouble ticket) but haven't heard back...(Gonna post on Group.io too...)Specifically looking for these board revisions:Power Supply Board: 228-152-02H (REV H)Digital Board: 228-142-02D (REV D)Analog Board: 228-106-02K (REV K)THANKS in advance!!!dc

author: Hydron
author_rank: Super Contributor
timestamp: August 23, 2025, 01:09:22 am
content: There are at least two different versions floating around, one difference is the newer schematic shows the daughter board (but not it's details) in place of the 555s (!!) that the older version used as MOSFET drivers in the PSU. I'm not sure what rev the ones I've seen get to though, might not be as high as you want.Out of interest what are you looking for in the newer ones anyway? I haven't noticed any major discrepancies between the schematic and the board in my unit, though I haven't messed with it much beyond putting a temperature controlled fan in and reducing the excessive output stage bias current. I do however plan to make some mods to the PSU though to eliminate the need for burning 60W at idle just to keep it in CCM!Edit: looks like I have analogue board rev J, PSU G and digital D, so you might have larger differences from the service manual than I'm seeing. What firmware version did yours come with?

author: icy.dev
author_rank: Contributor
timestamp: August 23, 2025, 12:52:45 pm
content: My big problem is that I can't get it to boot at all...Kind of chasing my tail as a notso newbie with this one.The biggest difference I've seen so far is that the digital board uses (2) 28 pin ROM chips (likely 6264s). CPU is a (ST)EF68B09P.Can't remember the Firmware from when it did boot...but the ROMs are labeled 228-800/1I'm also getting 15v off the 'DG' jumper to the Digital board vice the schematic's 9-10.5v(Schematic also doesn't show the 2nd capacitor/resistor pair on the DG + rail. -not a huge issue...just another difference...)I'm confident that my issues are on the Digital Board...but I'm doing the "smart" thing and starting at the line cord.+/-5v is fine. I'm def not making +/-15v nor +/-115v.Initial symptoms before I 'broke it' was that you had to cycle the power switch a couple of times before it would boot up...BL: Finding enough differences to see if there really is...-dc

author: icy.dev
author_rank: Contributor
timestamp: August 23, 2025, 01:03:21 pm
content: The schematic I have shows the daughter board...but not the schematic for the daughter board itself!?!? (228-186)Do you have that one???-dc

author: free_electron
author_rank: Super Contributor
timestamp: August 23, 2025, 01:53:23 pm
content: CHECK HERE :https://pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/23_Keithley/Keithley%20Manuals/There's several

author: Hydron
author_rank: Super Contributor
timestamp: August 23, 2025, 03:53:59 pm
content: Yep the rev E schematics annoyingly don't detail the daughter board. It's just a MOSFET driver plus some discrete parts, nothing exotic.I have also noticed that the digital input voltage is well above the nominal given in the docs - it's unregulated though so would always be a range, and the voltage regulator will sort it out. This is where I'm running the fan from so I'm actually happy to get more like 12V.I would expect the digital board and display to boot without the rest of the system if you give it 9V DC or so (though it would likely throw errors) - that's where I'd start debugging.Edit: I think that the switch mode PSU rails are started by the digital board so still points to digital issues

author: icy.dev
author_rank: Contributor
timestamp: August 23, 2025, 05:09:02 pm
content: Awesome idea Hydron!! Will try to boot the dig board separately!Probably will take till Mon evening though...gotta play with the Fam!-dc

author: icy.dev
author_rank: Contributor
timestamp: September 10, 2025, 02:02:18 am
content: Well...it seems like I found 2 issues that I wasn't really expecting...The power supply board: (Only getting +/-5v...no 15v or 115v)-It seams that there is no voltage on the enable pin between CR313 and R333 but have 5v on all 3 legs of Q304...-Also U302 (the TL494) does not have a consistent pulse rate on pin 5...-Then I finally took off the back and noticed that U301 had been previously replaced! (bad flux cleanup)---I'm planning to shotgun a lot here: Q304/U301/U302/C325/C332/Cap on the daughter boardThe Digital Board:-After mangling the ribbon cable and waiting for a new IDC DIP connector I hooked up the digital and display boards to my 9v bench supply...NADA...-Did a LOT of poking and noticed that along with the corrosion left from the battery...there was no connection between the CPU RESET (pin37) and the OUTPUT of U423 (pin4) - 0v on pin 4 but had 5v on R404p7 and U405p5!-Soldered in a bodge wire...still NADA!-Then I noticed that U423 was actually a ITS6728 not a ICL8211...They may be the same...but gonna zap that install a MAX8211So not a lot of progress...still no 15v & 115v on the PS and no display from the digital board...I really hope I didn't zap the ROMs or CPU...Waiting for parts now...unless y'all have any other suggestions?? 🤞🏼

author: icy.dev
author_rank: Contributor
timestamp: September 19, 2025, 09:44:45 am
content: Well...I got a wild hair...I finally got my T48 EPROM tester working and decided to test the ROMs...YEP!!! One of the ROMs went bad! Bad Address Pin (U414 pin 23)!!!I THANK whomever loaded the ROM images onto the TekWiki!!!! Got a new chip and loaded it and it started right up!!!I'm back in business!!! Now just have to "calibrate" it!!!(still gonna replace those old caps though...)-dc

author: Hydron
author_rank: Super Contributor
timestamp: October 07, 2025, 10:20:42 am
content: Great to hear that it's going! I didn't see any page for the 228(A) on TekWiki - is there somewhere with a stash of ROMs on the site? I know I uploaded some to eevblog a while back for this instrument.Edit: did you have a battery leak on that board? Looks like some ugly traces and the battery is gone. Maybe I need to get off my ass and whip up a NVSRAM adapter board so that I can eliminate my battery in this and some other instruments (FRAM probably isn't an option here as this is the main processor RAM that gets written to constantly, and FRAM does wear out, albeit very slowly).

author: icy.dev
author_rank: Contributor
timestamp: October 07, 2025, 04:25:03 pm
content: Well don’t I feel silly!!It’s YOU I have to thank for the ROM dumps Hydron!!!(Silly me was pulling roms from tekwiki and got confused)Those ROMs save my 228!! Thank you!!!…yea….battery exploded all over. Cleaned it good…just didnt think it would have roasted the rom like that. Yea…I was lucky to fine the other shirt too! I think that’s why the guy before me gave up.dc

author: Hydron
author_rank: Super Contributor
timestamp: October 07, 2025, 08:47:24 pm
content: No problem, glad they were of use. Asked as I wondered if there was a newer version available than D0. Man I wish Keithley released FW changelogs!