[FIXED] Industrial Square Label Model M 1388032 Controller Problem

[ekeppel]

Hi folks,

(I apologize for the long post, but I wanted to document everything I've tried so far as well as the troubleshooting process I followed.)

I recently picked up a 1388032 Industrial Model M with the black square label.  It's very similar to Chyros' that he reviewed in one of his Youtube videos, however this one has a standard English LED window and normal keycaps, rather than the ones with odd legends that he has on his.  Here's a picture of it:



Anyway, the problem I found immediately when I plugged it in for the first time was that all three LEDs would come on and then blink quickly and randomly together.  When this was occurring the keyboard was unresponsive and sent only a very few random scan codes when a few keys were pressed.

I immediately was able to rule out the cable.  To rule out any membrane damage, I swapped out the main keyboard portion with a known good one, but that didn't fix it.  This points to a problem with the controller. 

Once I knew where to look, I started troubleshooting the controller board and realized that this is a seemingly intermittent issue.  Sometimes, I can power up the controller and it will work fine (with two caveats, see below), other times it will lock up and start blinking the LEDs like crazy again. 

When I do manage to get it to power up without blinking the LEDs, all of the keys work correctly *except* for the RIGHT_CTRL and RIGHT_ALT.  When RIGHT_ALT is pressed, instead of seeing a Keydown_18 Keyup_18 scan code, I get Keydown_18 Keydown_243 Keyup_18, and then the key acts as if it's being held and does not release.  A similar thing happens with the  RIGHT_CTRL key, except instead of Keydown_17 Keyup_17, I get Keydown_17 Keydown_251 Keyup_17.

Since it was simple to do, I put a scope on the crystal and didn't notice any intermittent issues there, just a nice steady 4 MHz signal, so I'm wondering if there isn't an issue with one of the ICs on the board.  I also temporarily swapped both of the larger capacitors (C1 and C3) with new ones to rule them out.

I have a couple of extra controllers to harvest parts and ICs from if needed, as well as a de-soldering station, but I would like to get some advice on where to go next.  I really would rather repair the controller than simply replace it, since it's original to the keyboard and is one of the less common 1st generation ones with the early style crystal oscillator.  Here's a photo of it:



If anyone can help with troubleshooting ideas, I would greatly appreciate it. 

[taylorswiftttttt]

removed

[ekeppel]

I have no idea how to fix it.

But where did you get that amazing keyboard??

Found it on eBay. I'd been looking for one of these for a long time.  They are pretty scarce, I guess.  This one was made in early December of 1985 and is one of the first few thousand Model Ms IBM manufacturered that were compatible with a modern PC.  This model came out before both the 1390131 and 1391401 were made.

I'm currently checking each pin on the controller ICs to see if I can find anything that leads me to a solution.  I'm beginning to think that it may be something wrong with the 6805 MCU, so I might end up swapping it and seeing if that fixes it.  I just don't know.  If I had another 1st generation controller board, I'd probably just use that, but the only close one I have is a 2nd gen one from 1986 with the regular crystal.  It doesn't truly belong in this keyboard, so I would rather have at least the correct generation controller in it.

Anyone happen to have one sitting around collecting dust?  If so, please PM me! :-D

Edit: Just saw your edit.  I could live with the right ALT and CTRL sending odd scan codes, but there's definitely something else intermittently wrong with this controller causing the lock up and blinking LEDs. :-(

[chyros]

They are gorgeous keyboards <3 . Possibly the nicest-looking boards IBM ever made IMO.

Did you test the membrane portion of the keyboard on a working PCB, by any chance?

[ekeppel]

They are gorgeous keyboards <3 . Possibly the nicest-looking boards IBM ever made IMO.

Did you test the membrane portion of the keyboard on a working PCB, by any chance?

Thanks for joining the fray, Chyros. :-)   Yes, I eliminated both the membrane and the cable right off the bat.  The membrane is working great on another PCB, I just wish I could get this old 1st gen PCB working.  It's always nicer for things to be entirely original, even under the hood.

[TerryMathews]

Check your caps. Based on age, you might have failed capacitors.

[ekeppel]

Check your caps. Based on age, you might have failed capacitors.

That's just about the first thing that I did.     It's all in the OP, but it is rather long, so I don't blame anyone for not reading it start to finish.  I swapped in new electrolytics even though the originals tested good on my ESR meter.  The rest were mostly just small tantalums which rarely go bad, so I didn't bother with them.

As an update to this repair, however, I am happy to report that this keyboard is now working!

Both of the LED driver ICs were good, so I tackled the only other IC that was left -- the HD6805.  I ended up swapping it with one taken from a 1390131 PCB and it seems to be working great now.  The wonky right ALT/CTRL scan codes and lock behavior are gone too.  My best guess is that the MCU ROM got corrupted or damaged somehow.

I returned the PCB to the way I found it when I started, so the 1388032 is now back up and running with all its original components (aside from the new MCU).   

Thanks for all of the suggestions everyone! :-)

[chyros]

They are gorgeous keyboards <3 . Possibly the nicest-looking boards IBM ever made IMO.

Did you test the membrane portion of the keyboard on a working PCB, by any chance?

Thanks for joining the fray, Chyros. :-)   Yes, I eliminated both the membrane and the cable right off the bat.  The membrane is working great on another PCB, I just wish I could get this old 1st gen PCB working.  It's always nicer for things to be entirely original, even under the hood.

I completely agree! Sorry I couldn't be of more help but glad you got it working . It's always nicer to keep things original, especially one of these particularly old models.