I've got a 1989-issue VT420 terminal I used to use as a secondary console for my Linux machine. After a couple of years, I tried to bring it back to life. After switching it on, the well-known line and box patters flowed over the screen, then came a Wait for a couple of seconds and finally a VT420 OK. The LK401 keyboard itself seems to work (I hear the clicking sound when pressing a key, and LED #2 goes on when pressing Caps Lock).

But: When trying to get into the setup menu by pressing the F3 key, nothing happens. Not even the click sounds! The other local function keys from F1 thru F5 seem to be dead, too.

VT420 switched on

What's wrong here? Is this a hardware fault and my terminal trash?

UPDATE: After fiddling around with my VT and minicom on the other end, I found out that some other keys on the LK401 have stopped working, too: the caret key (above the Tab key), and the '1' key next to the caret. So does this look as if a part of the keyboard PCB is broken?

I've opened the case and made some pics of the keys in question:

Row of keys + PCB

Key close-up

Images in full resolution

UPDATE #2: I managed to disassemble the whole keyboard and found out that te conductive strips printed on the lower plastic sheet were corroded! I've already had such an issue with my old Model M:

Corroded consuctive strips

So I used some conductive varnish to make the strips conductive again, reassembled the board, connected it to the terminal, and found out that the keys in question now did work, but emitted the wrong keycodes:

F1 → s; F2 → e; F3 → w; 1 → a

This was obviously not what I wanted. What's happened here?!

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    1 - Setup and other special local functions can vary depending on the keyboard type. Which keyboard do you have? 2 - There are ways to disable and/or reprogram some of the special keys, so a previous user may have messed things up for you. 3 - Selected terminal emulation may also change the function of some of these keys. Commented May 22 at 20:15
  • You say that you used to use it as a secondary terminal. Has anyone else used it since then (and potentially messed with the settings)? Did the Fn keys work previously? How was it stored (covered, in a dry place)? Could some liquid have dripped or been spilled on to the top part of the keyboard? Any sign of water damage on the casing? Have your opened the keyboard to inspect the traces/switches (corrosion)? Please edit and update your question to address the issues raised in the comments. Also, a photo might be useful (plus it would be nice to see the unit and its condition). Commented May 22 at 20:46
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    @Greenonline I am the only one to have access to the device, but haven't switched it on for a couple of years. F1 thru F5 used to work.
    – Neppomuk
    Commented May 23 at 19:24
  • @Neppomuk If you open up the keyboard then you could use an oscilloscope to see if a signal is generated when F1-F5 are pressed. If not, suspect the circuit board traces in the keyboard, but do check any cables first, especially any solder joints at the ends as they may have gone bad with age. It can be repaired. If there is a signal, maybe the firmware needs to be re-installed in the display unit. I have no idea how to do that. Commented May 24 at 14:02
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    @Neppomuk Yes. If you don't have an oscilloscope, even a $20 multimeter will at least let you check for broken traces, cracked solder joints, and determine whether any of the key switches might need something like contact cleaner. (True story: I had to use DeoxIT to fix the microswitches in a modern Logitech mouse because, apparently Chinese Omron clones pinch pennies by not including a coating to prevent oxidation like the Japanese ones in another Logitech mouse from 2007 that's only needed a replacement for the scroll wheel rubber which probably got eaten by anti-COVID alcohol vapors.)
    – ssokolow
    Commented Jun 2 at 23:04

2 Answers 2


In Installing and Using the VT420 Video Terminal, Chapter 5 "Using Set-Up", section "Keyboard Set-Up Screen", it describes the F1 to F5 keys can be set to preform their local functions, send a function key sequence to the host, or be ignored.

The descriptions of the settings for the F3 key includes the following instructions: "To enter set-up after you disable this key; log out, turn off the terminal, wait 10 seconds, then turn on the terminal and make F3 the first key you press."

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    Already done that. Even disconnected KB and mains, reconnected both, and switched on. No use.
    – Neppomuk
    Commented May 23 at 19:17
  • Just to make sure: It seems like you saw the part about turning the terminal off and on, but maybe you missed the part about "and make F3 the first key you press"? (Turn on the terminal, but do not press anything on the keyboard yet. Wait until the VT420 OK message appears, then press F3.) The key click not sounding for just the F1 to F5 keys suggests the terminal might be ignoring the F1 to F5 keys on purpose...
    – Bavi_H
    Commented May 23 at 21:13
  • I've already done that this way. No use.
    – Neppomuk
    Commented May 23 at 21:15
  • @Neppomuk Did you wait 10 seconds, as written? It's not impossible that there is a timing circuit involved. Commented May 25 at 17:09
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    @Neppomuk Then you should start searching for the firmware for the terminal and how to update it as it could be that it has suffered from bit rot, IMHO. Maybe bitsavers.org has something useful for you. Commented May 27 at 19:25

If other keys are non-responsive too, that suggests that something has gone wrong beyond just a misconfiguration, but it's not necessarily the PCB:

  1. The keyswitches could need service (Stick a multimeter in continuity mode on a working keyswitch and press it as a control test, then repeat with the problem keys.

    If that's the problem, depending on the mechanism, apply either DeoxIT (metal leaf spring contacts) or something which restores conductivity to conductive rubber pads (membrane mechanisms) like a rubbing of graphite from a freshly sharpened pencil... HB works, but the softer you go, the less often you'll need to turn to the sharpener to loosen up more graphite powder.

    I used the former technique to fix microswitches in a Logitech mouse and the latter to fix keys in a Yamaha keyboard. The 8-Bit Guy used the fancy, expensive paint-on repair compound for membrane contacts to fix keys in a Commodore PET.

    (I'm assuming that, since the rest of them work, they're not foam-and-foil switches. Those tend to all decay in parallel and so badly that you need to order replacement parts.)

  2. There could be one or more broken traces on the PCB (For easiest diagnosis, try to find a circuit diagram to see if there are any current paths that are only used by the problem keys without having to chase traces on the actual PCB.)

    I fixed a SNES controller that had been rage-gripped so hard that one of the support posts for the PCB cracked some traces by using a pick to scrape off the solder mask surrounding the crack and then applying a blob of solder to bridge it.

  3. I don't know enough about the VT420 to rule out ROMs going bad, but it's theoretically possible.

    While I haven't done it for my Voodoo 3 3000 PCI yet, a lot of 3DFX Voodoo cards refuse to POST these days and reviving them is usually just a matter of re-flashing the card's BIOS. EEPROMs, EPROMs, and PROMs are more robust, but even mask ROMs can go bad.

    (I have a Nintendo 64 Gex cartridge where the mask ROM has gone bad and, given that both of the copies of Paperboy 64 I returned had the same problem and I've never seen any other catridges suffer that problem, it seems to be down to bad manufacturing runs similar to how, from what I read, disc rot in music CDs is very strongly correlated with a specific time period and a specific manufacturing plant.)

  • Good advice, thank you. In fact, I used to have such an issue with my old Model M, where several keys ceased working. Conductive traces on a plastic sheet had become rusty and could not be restored. Neither were the sheets themselves available as spares any more. This was the death sentence for my Model M, unfortunately. :(
    – Neppomuk
    Commented Jun 5 at 19:36
  • @Neppomuk Do you still have it in a closet somewhere? We're starting to see interesting repair options. For example, Unicomp sells replacement Membrane Assemblies and their Class 2 Repair Service is listed as being for IBM-branded keyboards.
    – ssokolow
    Commented Jun 6 at 21:31
  • No longer, sorry! Ehen it broke down, Unicomp lo longer had any replacement conductive sheets for this particular models to offer anymore.
    – Neppomuk
    Commented Jun 7 at 20:57
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    Too bad. What brought it to mind is that I saw... I think it was Level1Techs... announcing that they have plans to release a Model F conversion kit for modern Unicomp boards and that sort of approach certainly avoids the need to manufacture new membranes.
    – ssokolow
    Commented Jun 7 at 21:24

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