deskthority

I got my hands on a white label Model M for cheap. Two-part keycaps, faIrly clean with only a few popped rivets. It works, but the LEDs flicker when activated. I’m using the Monoprice PS/2 to USB converter which has been fine for blue label Ms and Unicomps. Checked the ribbon cable connections, blew out the dust and swapped out the SDLC cable and no luck. I did some searching for flickering LED issues on Model Ms but nothing turned up, so here I am.

I’m wondering if this is as simple as needing a better converter (maybe a voltage issue?) or something more complicated. Any ideas?

So the leds flicker, but otherwise the keyboard works well? And you can still turn them on/off by pressing numlock/capslock, but it just flickers?

Do you have a video of the flickering?
Do you have an oscilloscope to probe around with to see where the flickering is coming from?

Monoprice adapters work well unless they are faulty.

In my experience, the flickering is usually caused by a bad connector cable or a bad connection where it terminates on either board. In the absence of an oscilloscope, see if you can do a continuity test on the cable, or remove the end of the cable that connects to the keyboard PCB and apply a low voltage to each LED through the end of the cable. If the LEDs still flicker you will know whether your issue is with the cable/LED board or with the main PCB.

pandrew wrote: ↑

03 Jan 2021, 02:25

So the leds flicker, but otherwise the keyboard works well? And you can still turn them on/off by pressing numlock/capslock, but it just flickers?

Do you have a video of the flickering?
Do you have an oscilloscope to probe around with to see where the flickering is coming from?

Yes, otherwise the keyboard works fine. I can still turn the LEDs off by pressing the appropriate key, but when they see on they flicker. Caps Lock, etc. work as they should.

I don’t have an oscilloscope.

I suppose I could try some part swapping with my other M but it’s about five years newer so I don’t know if that is even possible (I know there are differences but I’m not enough of an M collector to know the specifics off the top of my head). That said, simple as these are, I don’t know if I want to risk messing with the good one to troubleshoot the bad one.

pandrew wrote: ↑

03 Jan 2021, 02:25

So the leds flicker, but otherwise the keyboard works well? And you can still turn them on/off by pressing numlock/capslock, but it just flickers?

Do you have a video of the flickering?
Do you have an oscilloscope to probe around with to see where the flickering is coming from?

No oscilloscope but I think I have a continuity tester around here somewhere.

The light from the LEDs was so faint at first (it almost seemed like they weren’t on at all) and gradually became brighter and more consistent as time went no (but never stopped flickering). It almost seemed like they were warming up, if that were a thing.

I would look at the solder joints on the PCB. Look for any joint that looks cracked or discolored. Also, look for components that wiggle (there may be some flex in the leads or the component itself. This is normal. The lead itself should not move in the solder though). I picked up an Omnikey off of eBay that wasn't working and the issue was that the crystal oscillator had pulled out of a cold solder joint.

A few more bits of information:

I pulled out an old laptop with a PS/2 keyboard port just to eliminate the USB adapter as a variable and the flicker persists.

At this point I also noticed that when the LEDs are activated they flicker in unison.

There is no correlation between any movement I make on the keyboard and the flicker. Typing, pressing keys, jiggling the ribbon cables, etc. have no effect.

Since all of the keys work and the Caps Lock and Num Lock work as they should I’m starting to think it is the LED module itself, it could be the PCB, but better to start small.

Well, I fixed it...?

I connected the PCB to the LED module on the newer Model M and the flickering moved. So, not the LED module.

I put it back and once again reseated the cables, etc. to no effect, when suddenly (when I wasn’t touching it) the LEDs stopped flickering.

So, maybe I did something or maybe since this keyboard was untested and hadn’t been used in 20+ years something on the PCB needed to warm up. Or it’s flaky. I hadn’t heard of this symptom before so I’m flummoxed.

I suppose the thing to do is leave it unplugged for a week and see if the flicker comes back. If it does then maybe I’ll just replace the PCB, if I can find one.

This really sounds to me like a bad connection (either connector or soldered) somewhere. Can you post a picture of the back of the board?