Please help identify component on keyboard logic board - Page 1
Forum section: electronics / repair

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 04:26:38 pm
content: Hello all,I'm trying to revive an old xt/pc keyboard that throws a keyboard error and started to poke inside it and found these two components encased in a clear case which look like fuses to me.Can someone please confirm? If that's the case they are dead as I put my meter in continuity mode and didn't get any beeps across the leads. Furthermore, what fuse value should I replace them with if that's what they really are?

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2022, 04:32:31 pm
content: https://www.eevblog.com/forum/metrology/mlccs-encapsulated-in-glass/

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 04:40:09 pm
content: Can you measure +5V at the keyboard connector? If not, look for an open polyfuse.

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 07:27:27 pm
content: Thank you both!I did find what looks to be a polyfuse. If this is what it is then it's dead (measured resistance across the leads/no continuity). What replacement fuse would I need for this application? I take something rated for 5v, maybe 500mA max?As for measuring voltage on the keyboard, do you mean while pc is powered on, or should I inject 5v into the end coming from the pc side?

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 08:19:21 pm
content: That's a ceramic capacitor.In my experience you should find a picofuse. (I was wrong to suggest a polyfuse, as these are resettable components.)Something like this, probably 1A ...https://au.rs-online.com/web/p/non-resettable-fuses/5413287pIn fact I was wondering whether your first photo has such a device.As for the keyboard connector ...https://old.pinouts.ru/InputCables/KeyboardXT_pinout.shtml

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 08:49:25 pm
content: As far as I can tell it's got: resistors, ceramic caps, zenners, schottkys and some tantalums.Also keyboard connector is proprietary not the standard DIN, pincount is same

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 09:06:34 pm
content: Find the +5V pin in the power connector, then perform a continuity test with each pin in the keyboard connector. Then power up the motherboard and see if you can measure +5V at this pin.Edit: If there is no continuity with the +5V supply at the keyboard connector, then this would suggest that there is an open fuse.

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 09:12:04 pm
content: Quote from: fzabkar on March 17, 2022, 09:06:34 pmGood idea, thanks!

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 09:52:27 pm
content: I cannot find any pinout diagram for this connector but I measured +5v, +5v and +12v on some of its pins.

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 09:59:06 pm
content: I've never seen an XT/AT/PS2 keyboard that required a 12V supply. Very strange ...

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 10:34:31 pm
content: Quote from: fzabkar on March 17, 2022, 09:59:06 pmThis is similar with what I have: somewhere in the article it says it runs on 12vhttps://www.reddit.com/r/MechanicalKeyboards/comments/mrzj1j/i_successfully_brought_this_compaq_portable_1/

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 10:54:02 pm
content: Quote from: newtekuser on March 17, 2022, 10:34:31 pmI still can't see which component needs 12V. :-?Edit:Is there a 3-terminal 7805 regulator under the extended metal area? If so, maybe the incoming 12V supply is being down-regulated by the 7805. I would measure those voltages.https://i.redd.it/ki4ja6hd6it61.jpg

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 11:07:42 pm
content: Quote from: fzabkar on March 17, 2022, 10:54:02 pmI measured voltage on all ICs and each is getting 5v. Didn't measure the voltage reg since I assumed to be working with everything being 5v downstream.

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 11:15:20 pm
content: I suspect that the keyboard controller is an 8048 or 8748 or another mask ROM-ed member of that same series. I would confirm its supply pins and try to read it in a programmer. Failing that, find a replacement chip from any other XT keyboard and see if it at least communicates. The row and column matrices will probably be different, though.Edit: I can see "8048" in the part number.

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 17, 2022, 11:50:05 pm
content: Quote from: fzabkar on March 17, 2022, 11:15:20 pmMine is actually using an Intel P8035LEdit: I found one on ebay, would a direct swap work or does this need to be programmed with the same code as the original? (Which I assume to be the case) stupid question, these are not eeproms and cannot be programmed more than once.https://www.ebay.com/itm/283861894704?hash=item42177cc630:g:X9EAAOSwxBNeqIZgEdit2: I found the datasheet for this controller and measured the reset pin and found it high (reading 5v)https://datasheetspdf.com/mobile/541972/Intel/P8035AHL/1

author: fzabkar
author_rank: Super Contributor
timestamp: March 18, 2022, 12:20:55 am
content: The 8035 has no ROM. Therefore there must be an external EPROM chip. :-?https://en.wikipedia.org/wiki/Intel_MCS-48Edit:I didn't see your pics before I posted.

author: fzabkar
author_rank: Super Contributor
timestamp: March 18, 2022, 12:32:17 am
content: Quote from: newtekuser on March 17, 2022, 11:50:05 pmThe reset pin is active low. Therefore it should be sitting at 5V when the uC is running.Do you have an oscilloscope? If so, check the oscillator, and look for activity on the address and data lines of the EPROM.Alternatively, if you have a programmer, dump the EPROM several times and compare the dumps. There may even be a checksum which we could verify.

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 18, 2022, 01:13:54 am
content: Quote from: fzabkar on March 18, 2022, 12:32:17 amThank you for the correction!I do have both, unfortunately my scope (tds5054) only plots a square wave (otherwise just noise) when hooking up the probe to its signal calibration pin and my programmer (mikroprog) only does 5v programming

author: fzabkar
author_rank: Super Contributor
timestamp: March 18, 2022, 01:23:17 am
content: Just read the EPROM several times. That only requires 5V. Then upload the dump(s) and we can look for a checksum.

author: newtekuser
author_rank: Frequent Contributor
timestamp: March 18, 2022, 04:20:45 am
content: Looks like the scope wasn't dialed in properly (I'm a newb). I do see activity when probing pins 2, 3