The Model M line of keyboards made by IBM beginning in 1985 has a deserved reputation as the best ever shipped. But both vintage model Ms and some of the descendants produced at the same factory by Unicomp have interoperability problems with modern hardware. There is also one known mechanical failure mode as the devices age.

This document collects information on causes and solutions. It also includes tips on testing, cleaning, repair, amd noise reduction.

We also cover the Model M’s predecessor, Model F, in this document.


The Wikipedia article on the Model M has a "Features by part number" chart that may help you identify the precise variant you are troubleshooting. The model number can usually be found on a sticker applied to the lower surface of the case.

The following table may help in identifying the manufacturing generation of your keyboard, and expresses the reduction in unit weight over time. It is originally from Thomas Ran (chyrosran22). Each model number is representative, but there are typically many other models per generation. The sample size for each was 1; weights may vary slightly from those listed. // It’s from 9.09 of his "The Ultimate IBM Model M review"

Model Badge Year Generation Total wt Case wt Plate thickness


square black





1.3mm, rainbow


square silver





1.2mm, rainbow


beige oval





1.2mm, rainbow


beige oval





1.0mm, gray


gray oval





1.0mm, gray


gray oval





0.8mm, gray







0.8mm, gray


If you are running Linux and have doubts about whether any keys are operating, xev(1) will display keypress/key-release events and what keystroke X interprets them as. Here’s the command:

$ xev -event_mask keyboard


One special tool is required for disassembling all model Ms prior to Unicomp’s "New Model M" reissue in late 2020 - a 5.5mm (7/32in) socket hex driver with walls thin enough to fit in the screw wells of the case. Some of the recent Unicomps with "Ultra" slimline cases have the screw heads more exposed, but a thin-wall tool is still what you need to get past the case shoulders near them.

Note that while these screws look like generic PC case screws, they’re not. They’re a little smaller and not interchangeable with them.

These drivers are a common tool in the radio-controlled-airplane hobby. A web search for "5.5mm hex driver" turns up many sources.

The Unicomp New Model M (both full-sized and SSK "Space Saver Keyboard" variants) dispenses with the 5mm hex-head screws in favor of Pozidriv screws (a variant of Philips).


Grubby keycaps can be effectively cleaned by popping them off with the tip of a butter knife (or you can be fancy and use a keypuller), dropping them in a glass of lukewarm water, adding denture cleaner or dishwashing soap, and letting them soak for a while. Rinse and let dry thoroughly before reinstalling. Don’t use hot water as it can deform the keys.

While caps are off is a good time to blow out dust and hairballs with a can of air or spray-painting compressor.

The plastic case halves can be washed with warm soapy water. Isopropyl alcohol can be useful for going after serious stains. Note that removing or damaging the provenance label on the bottom of the case will reduce the board’s value in case you ever want to sell it; you may want to be careful of this when washing the bottom case half.

Be wary of inverting the keyboard with the buckling springs bared, as they can fall out. If this happens they can sometimes be reinstalled by spearing them on a sharpened chopstick and twisting the wire tip of the spring coil into its attachment point on the barrel plate.


If at all possible, don’t store these keyboards with the cable insulation in contact with the case. Given prolonged contact, plasticizers in the cable can leach out and damage the case.

In all other ways model Ms are very stable in storage.


Model Ms have only one serious long-term failure mode: the plastic rivets securing the back plate to thebarrel plate can shear off over time. See the disussion of bolt-modding later in this document for details.

The Model F backplate is secured with metal locking tabs rather than plastic rivets and thus never requires a bolt mod. However, it does have a foam-rubber spacer mat which can degrade over time (more likely if exposed to heat and moisture) and may need to be replaced. A new one is relatively easy to make from thin sheet foam with a blade and a 12mm hole punch, or they can be purchased from Model F Labs.

Almost all Model M and F keycaps are made from PBT and will not yellow with age. Very occasionally you will encounter spacebars that were cast from ABS and have yellowed. Retr0bright will fix that up. Alternatively, Unicomp can sell you a PBT spacebar.

AT Model M issues

Many older model Ms have an AT keyboard connector, using a 5-pin DIN plug larger than the PS/2 mini-DIN.

To make these work, you’ll need an AT-to-PS/2 adapter. These are no longer common, the AT connector having been phased out after the introduction of the PS/2 in 1987, but can still be found with a web search in 2021.

Then follow the next set of directions for hooking up a PS/2 Model M.

Alternatively, if your Model M has a detachable cable (later versions went to an integral cable to cut manufacturing costs), the Ethernet-like connector on the keyboard is a style called "SDL". It is sometime possible to find SDL-to-USB cables (with active converter logic) on eBay and elsewhere. Search for "USB to SDL Model M" or "Soarer’s Converter, but note that there are at least three different variants of Soarer’s converter and make sure you get the right one.

At time of writing, there’s a particular eBay seller named "orihalcon" who has a reputation in the keyboard-enthusiast community for shipping excellent converters. He carries these cables.

PS/2 Model M issues

You may be able to plug your PS/2 Model M directly into a PS/2 port, if your machine still has one. But maybe not - a IBM or Lexmark Model M draws more power (100-120mA) than some modern PS/2 ports are designed to supply. Nominally PS/2 pin 4 is supposed to ship +5 V DC at 275 mA but a lot of later PS/2 implementations took advantage of the low amperage requirements of more modern keyboards (3-50mA) amd cheaped out on power management.

The high power draw remained true of older PS/2 Unicomp keyboards made to the Model M design, which had the same core keyboard electronics as late Lexmarks. The main chips on these, the Motorola 6805 processor and Intel 8042 PIC, are old and power-hungry designs.

You have two routes to recover. One (if your Model M has a detachable cable) is the aforementioned active SDL-to-USB cable. Otherwise you’ll need a PS/2-to-USB adaptor. However, not all of these will work. Passive USB adaptors, which just pass through the PS/2 signals, will fail.

The aforementioned orihalcon carries active PS/2 to USB converter cables.

Alternatively you may be able to find and use a "blue cube adapter" (web search for that phrase) that does active conversion. There’s even a technique where you take the shell off a blue cube and mount it inside the Model M case, soldering the leads on one side to the lead in the original cable and on the other side to the lrads of a USB cable which you then run out the back.

USB Unicomp issues

Unicomp keyboards prior to the 2020 "New Model M" could be purchased with a USB interface, but had continuing interoperability problems.

Boot-mode problems

The UB HID standard describes two ways that keyboards can behave on power-up. There is a "boot mode" which interrupts the CPU every time the keyboard is polled; this is intended to allow the BIOS to handle a USB keyboard in the absence of a USB-aware operating system.

Some (but not all) older Unicomp model Ms didn’t come up in boot mode. Suspect this if yours won’t talk to a PC at boot time and has to be unplugged/replugged to establish communication with the PCS’s internal hub. Alas, there is no fix; the workaround is the unplug/replug.

We have one report that even the prootype SSK version of the New Model M (part number UB40E7A) needs to be unplugged and replugged after boot. Possibly this was fixed in production versions; I have seen no such issue with my full-sized production New Model M (UT40U4A).

(The CY8C24493 the prototype SSK is using as a controller draws 250mA, which is pretty high for a keyboard without RGB lights, but runs on anything between 1.7v and 5.5v so this is definitely not an undervoltage issue.)

A subtler problem arises because some PC hubs fail to transmit the command required to switch a USB keyboard in boot mode to normal mode after boot. If this does not occur, the keyboard will be limited to 6-key rollover rather than supporting the N-key rollover of normal-mode USB. This limit is never pushed by older model Ms, which due to the way their sensor matrix is constructed are limited to 2KRO, but may be an issue with the Unicomp Mini-M SSK which has a reworked larger matrix with individual traces for the WASD cluster.

Controller incompatibility

I have checked operation of two Unicomp keyboards on each of two test machines. One, snark, uses an Intel NM10/ICH7 (rev 01) USB controller. The other, minx, uses an AMD SB7x0/SB8x0/SB9x0 USB controller.

  • A UB40PGA (the nipple mouse variant, "EnduraPro") manufactured in Oct 2012 works reliably on both machines.

  • A UB404LA (the trackball variant, "Ruffian") manufactured in Oct 2012 has flaky behavior on both machines. On minx it disconnects from the hub every few minutes, needing to be replugged. On snark it has once failed to register at boot time but is generally reliable.

We have a confirming report from one Alex Lustenberg, in a comment here, that the UB404LA is flaky even as the only device on his machine and doesn’t enumerate on boot.

Alex also reports the UB404LA failing to initialize correctly with these host controllers: NVIDIA MCP55 (rev a1/a2), Intel series/C200 series (rev 05), and NEC uPD720200 (USB3).

Undervoltage sensitivity

Alex Lustenberg reports "according to some accounts, the USB controller in the unicomp isnt exactly USB compatible; undervoltage within USB spec will cause it to not init properly"

This post and its comments display some evidence that undervoltage probably isn’t an issue, but a keyboard controller failure to respond properly during the initialization sequence may be.

This theory is reinforced by experiments with two cheap hubs containing the Terminus Manufacturing 4-port USB hub chip (1a40:0101) and a hub containing a chip by Genesys Logic, Inc. (05e3:0608). The UB404LA fails to initialize on powerup connected to either of these, but becomes visible when unplugged and replugged. The UB40PGA exhibits normal behavior.

I disassembled the UB404LA and found that the only chip in it is a Cypress 24794 SOC (manufacturer data sheet here) sitting on a mini PCB behind the LED panel that is wired to both the USB cable and the keyboard proper.

I also disassembled the UB404PGA. There are two chips on the mini-PCB in this one, an LM393 differential comparator and a CY763513-PVC (data sheet here).

The key point about these Cypress chips is that while they’re both USB controllers they’re different SOCs; it could easily be that the USB stage in the first is flawed but the second is not.

We have a report that flaky failures on a UB40B5A (the 122-key variant) were resolved by plugging it into the sort of high-amperage USB2 port used to recharge cellphones.

On the other hand, the undervoltage theory gets a boost from one Andrew Hutchings, who reports: "I’ve just come back from a micro electronics lab where we have been testing [the Cypress 24794 SOC on a Ruffian v2 board] and it seems the problem is to do with voltage. If the USB port is 4.9-5.0v it works fine, 5.1-5.2v the handshake does not succeed." A day later he said "We jury-rigged a TPS73150 regulator in front of the USB power input to the PCB. It now works perfectly on every PC and powered hub I have. Not sure how permanent a fix it is but it will do until I can buy a replacement PCB from Unicomp."

I can no longer recommend buying any of these older Unicomp models. The New Model M appears to have solved the USB incompatibility issues of older Unicomps. In addition, it is plainly visible from the finish of the case and keycaps that they bought new tooling to produce these and the build quality has gone way up.

Cable failure due to nonexistent strain relief

It has been reported in a comment attached to this G+ post that one UB4044A (Classic Black with no pointing device) has exhibited intermittent USB disconnect issues which turned out to be cable failure; there is no strain relief where the USB cable enters the keyboard.

I have opened up the UB404LA and UB404PGA, both manufactured in October 2012, and verified that there is no strain relief on the USB cable. It looks like this is a consistent design error across the Unicomp USB line - up to and including the full-size New Model M - that may account for a lot of problems.

One product that does not have this issue is the narrow tenkeyless (SSK) version of the New Model M. It has a USB A connector in the rare locking variant on the keyboard chassis, and a cable that mates with it And has a conventional Model A connector on the other end.

Power usage

The UB404PGA reports 98mA draw and says it is "Bus Powered"; the UB404LA reports 100mA draw and says it is "Self Powered". The USB 2.0 spec allows a 500mA draw.

On opening up these keyboards, I discovered that both use Cypress programmable SOCs, replacing the 6805/8042 found on older Model Ms. These are low-power chips that should have no issues drawing from a USB 2.0 controller.

On this evidence, I think the theory that the Unicomps still have the Model M’s physical power-draw issues can be dismissed. However, there is evidence that some sort of logic error triggered by under-voltage or under-amperage may have mimicked such problems in several Unicomp models pror to the New Model M; refer to the previous section on USB Unicomp issues for details.

Bolt modding

The Model M is a notoriously rugged device which routinely clocks in a service life of over 30 years. However, it has one known mechanical failure mode. Its metal backplate is secured to the barrel plate holding the keyswitches by a large number of plastic rivets in backplate screw holes.

Over decades these can degrade and break, affecting key feel and in extreme cases rendering the device inoperable. You can tell this is required if you hear rivet fragments rattling around when you shake the keyboard.

Function can be restored with a procedure called a bolt mod that replaces the rivets with bolts (or sometimes screws). There are helpful videos of the procedure on YouTube.

Floss Modding

The Model M (and even more the Model F) is quite noisy compared to modern keyboards, and this is not to everybody’s taste. There’s a procedure called a "floss mod" that consists of inserting little lengths of dental floss or paracord core fibers down the middle of each buckling spring. This considerably reduces the action noise, especially the pingy high-freqency transients.

Be careful to trim the fibers flush with the spring tops rather than allowing them to protrude, as protruding ends can degrade the crispness of the key feel.

The Rump mod

A particularly brave mod is this one by Chris Lee that replaces the Model M’s controller with a tiny custom PCB running open-source keyboard firmware.

Note: If you have PCBs made for this mod, please drop me a line because I want some myself.

Model F

The very first buckling-spring keyboard, the Model M’s predecessor, was the the Model F. This was the keyboard attached to the original IBM Personal Computer, the PC XT, and early versions of the PC AT. It is nowadays referred to as the "XT keyboard", though it predated the XT. The PC version had 83 keys; some industrial versions had 122 keys.

Some of the very earliest variants of the Model M (1390148, 1390639, 1389969, 1390120) spoke XT protocol as well. Not much is known about these and it is possible they never shipped in the US.

Keyboards speaking XT protocol are not directly usable with modern machines, even with the dual-adaptor setup described for the Model M. IBM changed the Model M’s keyboard scan codes in a way seriously incompatible with the Model F.

(This change wasn’t done gratuitously; the microcontroller technology of the time made it expensive to assign scan codes in violation of a natural sequence determined by the wire layout.)

One variant of the "Soarer’s Converter" family of active cables bridges the gap from the F to USB.

New Model F

Model F Labs will sell you a hand-crafted recreation of the Model F with a somewhat more modern ANSIfied layout. While it is eye-wateringly expensive, it speaks USB natively and no interoperability issues have been reported with modern hardware.

Reaching Unicomp technical support

I haven’t found a way to reach a live human being yet. The Unicomp website carefully avoids giving any technical support email addresses or phone numbers. The company has a main number at (859) 233-2130, but you can’t get to an engineer that way.

You can open support tickets through and I have received replies.

Wikipedia on the Model M

History of the device and how to identify variants.

Care and feeding of the IBM Model M keyboard

Just what it says on the tin; applies to Unicomps as well. Particularly good stuff about how to clean the beast.


The only place to buy new Model Ms.

Resells vintage IBM Model Ms (and occasionally Model Fs) scavenged from various sources.

Various vendors on eBay sell used and reconditioned Model Ms. Quality is much more hit or miss than if you buy from, beware.

Model F Labs

Produces an authentic though expensive clone of the Model F.

orihalcon’s eBay store.

Direct conversion of a fixed-cable Model M to speak USB using a Blue Cube adaptor

Tactile Keyboard FAQ

Many links to related resources can be found at this FAQ.

Video of a restoration and bolt mod with detailed instructions.

Thomas Ran’s "The Ultimate IBM Model M review"

Power Consumption of IBM Model M Keyboards


1.20: 2021-05-00

Explain boot-mode issues.

1.19: 2021-05-08

Added Thomas Ran’s table.

1.18: 2021-05-05

Note existence of Model Ms speaking XT protocol. Describe bolt, floss, and spacer modding. How to clean and test.

1.17: 2021-04-28

The full-sized New Model M has no strain relief, alas.

1.16: 2021-04-26

Note the existence of Soarer’s Converter for the old Model F. Mention the new Model F. Add some links.

1.15: 2021-04-24

Controller incompatibility on the Unicomp SSK, correct "Phillips" to "Pozidriv", add history section.

1.14: 2021-04-18

Update for the Unicomp New Model M.

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