EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: george.b on May 29, 2019, 02:23:25 pm
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Hi all, thought I'd share my solution for fixing the keyswitches on a HP 4145B Semiconductor Parameter Analyzer.
I've recently had the chance to acquire a HP 4145B for really cheap. It wasn't in the best of shapes: it was missing some bits (most notably everything in the way of cables/test fixtures), didn't have the system disk, and all I knew about it is that it would power up and spit an error about missing said disk.
Writing a system floppy disk was easy enough, thanks to a good Samaritan who uploaded an image of it somewhere, and with it the unit actually worked, except for the fact that its front panel keyboard had turned to crap. Most keys seemed stuck and pressing them wouldn't register anything.
I came across this topic (https://www.eevblog.com/forum/testgear/hp-4145b-semiconductor-analyzer-repair-with-photos-(display-garbage-dead-rom)/), with a video showing an easy repair for those switches, and I proceeded to take the front panel apart to try and implement the fix. In my case, however, most of the steel leaf springs that give the switch its bounciness were corroded, some to the point of disintegration, so no easy fix for me, I'd have to replace those springs instead of just poking at them like in the video.
It occurred to me, however, that instead of steel leaf springs, I could try using a plastic sheet of some description. I enlisted the help of a good friend and, after experimenting with a few different types of sheets, we settled on a transparency, of the kind used with overhead projectors. In about two hours' work (plus a pizza break) we cut acetate pieces to size and replaced all leaf springs, so we had a mostly working keyboard. There isn't as much tactile feedback as there would be with the original steel springs, but IMO it feels nice to the touch, if a bit mushy, and they definitely actuate easier than before. A few keys are still unresponsive, so I'll get in there with some contact cleaner later on.
https://www.youtube.com/watch?v=UFN7NMz6l6k (https://www.youtube.com/watch?v=UFN7NMz6l6k)
Now to find them triaxial cables for a price that fits in my broke university student budget... :scared:
Edit: After copious amounts of contact cleaner and some wiggling, all keyswitches work properly :-+
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Nice fix of the keyboard! I've grappled with many HP keyboard springs. Although there has often been minor corrosion, the real problem is that
they never seem to stay fully engaged into the plastic retainers. In my opinion, the small ridge in the plastic is too small to hold them properly.
Most of the time, the actual switch housings are heat-staked onto the board underneath. They are not easily reusable if you ever pull one off (which
sometimes happens when trying to pull the key cap off). If you ever need some switches, be on the lookout for an old HP 3437A high-speed voltmeter,
which usually go on eBay for $25 to $50. At least on the ones I have, the switches are not heat staked - they are held on with a bracket. Buy the
HP 3437A, take out the dozen or so switches, and throw the rest of the piece away. Instant source of replacement parts.
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Aha, that's good to know. However, I would probably try to model and 3D-print a replacement first. I would feel bad for gutting a voltmeter for parts :( and shipping to my country isn't exactly cheap anyways.
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Hi George and Wn1fju,
Do you have diskette for this model?
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Hi George and Wn1fju,
Do you have diskette for this model?
Hi,
I have the disks for this model. I purchased my 4145B without the disc. Even worse my disc drive was defect. I started to manage using a USB Floppy Emulator with this analyzer. In the mean time I've purchased a 16500A LA which seems to have the same floppy drive. So I took the LA apart and use the floppy as spare part. Now with swapping between the USB Floppy Emulator and a working floppy drive I was able to create a fresh floppy disc. If my expenses are covered I'm willing to make a copy of the disc end send it.
Cheers
Michael
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Nice keyboard fix there. I do have more gear that uses these same switches so il keep it in mind if any fail.
I do have a working HP 4145B (That was my thread about fixing it)
I can make copies of the system disk using mine if you want, but it does have the more modern 3.5 inch floppy drive. Tho i think all B models have that so i'm guessing yours also has the modern floppy drive in it.
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My apologies for being a couple of months late. If I recall correctly, this is the disk image I used to write the system disk for my 4145B. I used ImageDisk and a DD floppy in a normal PC floppy drive to write it. I got it from here: https://torlus.com/floppy/forum/viewtopic.php?t=1296
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Oh nice, so a PC can create a copy.
Any tips on what you used to make it happen? Like did you need a old DOS machine with a physical drive, or can it be done on a modern PC with a USB floppy drive. Also did you have actual DD type floppy disks or can it work with a HD floppy since those are the most common kind that is easily bought today.
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I used a physical drive on an actual machine running DOS, yes - ImageDisk needs complete access to the floppy controller (so no USB drives), because the format isn't standard. I had an actual DD floppy, not sure whether it would work with an HD floppy, but could be worth a shot.
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for 4145B owners:
if you have an eprom programmer and if you have the possibility (I know it is soldered and therefore it is a bit boring), dump the eprom U6 and post it on the site www.ko4bb.com (http://www.ko4bb.com), a security copy is always a good idea.
I, as soon as I received my 4145A, is the first thing I did.