VT52 key repair advice?

[fritzm]

So, the ESC key on my VT52 seems not to be functioning (no key click or character sent, depressing with a little extra pressure and/or wiggling the key while depressed doesn't seem to help).

Any advice on repairing/replacing these keys before I jump in this weekend? Can the keycap be removed from the front to check the mechanism, or will I need to remove the keyboard PCB and desolder the key mechanism to asses? Are compatible replacement mechanisms available if needed?

I was in the VT52 recently to make some repairs, so I am aware of the disassembly that will be required to get to and remove the keyboard PCB.

thanks much,
--FritzM.

 

[Lou - N2MIY]

Fritz,

Yes, the keycap comes off. You can pull straight up. Do pull straight because the plunger can crack at the corners. Chances are that some of your plungers have cracked corners anyway if you have any loose keycaps or keys that appear to stick.

Under the keycap you will see the plunger. A spring pushes the plunger up to return it to the up position after the key is released. The plunger has a bar across the middle inside that allows two leaves (contacts) to touch only when the key is depressed. When not depressed, the bar holds the leaves apart. The leaves can get dirty on the contact surfaces, but usually they were plated. It is also possible that someone destroyed the leaves when removing the plunger, or reinserting the plunger in the past.

This keyboard mechanism was used on VT52 through VT100. I have restored many keyboards and have two VT52s myself (and replaced lots of plungers.) HP83 and 85 also use the same mechanism.

So, pull off the keycap, look down inside the plunger, and tell us what you see. Maybe take a photo with a macro lens.

Lou

PS. Since for some reason in some other thread I had reason to post photos of the back of a key and side of a cracked corner plunger, here are links to the key: http://www.vcfed.org/forum/album.php?albumid=51&attachmentid=4405 and plunger :http://www.vcfed.org/forum/album.php?albumid=51&attachmentid=4404

 

[jackrubin]

Hi Lou -

Please do tell us more about repairing the keys! I was just walking by my VT/78 (basically a VT-52 with an Intersil 6100 inside) and noticed a stuck key and a couple loose ones. What remedies have you discovered?

Thanks,
Jack

 

[fritzm]

Thanks for the tips, Lou!

I pulled the keycap and was able to see the leaves and plunger, which appear to be in good shape (not bent or broken, still shiny gold plate, no schmutz or corrosion in the mechanism, leaves meeting and separating reliably as plunger is depressed and released).

I tried applying a very slight pressure to the leaves while they were closed, and also bridging them with a conductive probe, but neither action caused a keystroke to register. However, if I depressed the plunger and then applied firm pressure with a probe around the key mechanism outside the leaves (enough to cause PCB flexure) I did get a keystroke to register. So I'm suspecting a mechanically broken solder connection or trace at this point. The ESC key is in the corner of the keyboard, so it's not unlikely that it could have taken a hit at some point over the years I guess...

I will go ahead and pull the keyboard PCB tomorrow, inspect and reflow the solder on the misbehaving key and see where that gets me.

 

[Lou - N2MIY]

Jack,

The problem is the plungers. They crack at the corners. They can be pulled out and replaced with ones that aren't cracked. You just need a donor keyboard.

This page is absolutely fantastic and shows the problem in one of the photos.

https://deskthority.net/wiki/Hi-Tek_linear

I just found this page now after doing a google search on HI-TEK, which is marked on the donor junk keyboards I got from Vulcan Scrap of Stamford CT (don't know if they still have any, it was a few years ago.) I think the donor keyboards I have were from Perkin-Elmer since Vulcan used to get a lot of PE surplus.

Lou

 

[fritzm]

Finally got around to digging in on this today. Re-examined the plunger and the switch leaves carefully, and they seemed in good shape. Went ahead and pulled the keyboard. Hooked it up to my bench supply and hardwired AC0-AC6 on a breadboard to select the key in question, then put a logic probe on it so I could see when the key activated and not.

It was flaky, but noticed again that the key would work reliably if I put a bit of downward pressure on the corner of the *entire* key bed. So I reflowed the solder behind the key in question, even though it looked fine, and bingo -- 100% reliable now!