Badly damaged Keithley 168 multimeter

[ealex]

Hello

I received a "free" Keithley 168 multi-meter that looks like it's been thrown down the stairs, burned and used for growing fishing bait in, all at the same time.
The case is completely unusable: cracked and warped and covered with some sort of fine black powder.
The electronics inside look a little better, except for the input range switch - it's got some corrosion and it seems stuck / broken.
Also, the transformer looks bad - like it's seen a lot of high temperatures.
The black dust is also covering all the main PCB.

The good part: it seems to at least power up - i can get the segments to light up.

By the way it looks, the function switches are ruined - can i get independent segments for that kind of switches ?
Any idea on how to proceed on getting it to work enough to check if it's got other defects ?

I can adapt some generic box for it, and i can also get a custom made transformer once i find the schematic.
For now i can power it up from a lab power supply - it seems there are only 2 voltage rails

Here are some pictures:

Event the pins that keep the display in place are broken - easy fix with some CA glue


The transformer and marks left by the previous inhabitants


The input section ( bypassed fuses ) and the stuck range buttons.
The on/off / charging buttons work properly.
The internal plastic part of one of the blue switches looks like it's broken and stuck at a strange angle

[Kleinstein]

The transformer does not look bad - just a little dust and corrosion from the outside. That is nothing to worry about. It also works, so not coils burnt. Unless the transformer gets really hot (e.g. due to shorted turns) I see no reason to relayce that.

For the switches, one could try cleaning them, use some contact cleaner and move them several times. I might just be corrosion products and dirt that hinders operation.

[ealex]

the blue switches are stuck - tried to push them, etc
i'll spray some contact cleaner and try to push them harder. i don't think i can damager them worse than that.
also, i'll start searching for replacements - i can live without the interlocks, etc.

if i can't get them unstuck i'll try to somehow hard wire them so it can measure DC volts - so i can at least perform some basic tests.

[Gyro]

It's a way of bringing fuses back from the dead. Never a good sign. I've seen it done on mains fuses too! 

[ealex]

no idea - that's the way i got it.

just powered it up - the switches are stuck on the DCV range - it seems to measure properly but it's constantly +50mV higher. The black dust that's all over it might be ~ conductive enough or the thing just drifted

now i know that at least some part of it is still working.

next step is to get the switches loose for some more testing and to clean up the board.

the case needs to be replaced, it's crumbling - i will search for a plastic box with a similar size, and find a way to fit the front panel

[ealex]

Update:

the switches are not fixable - at least i can't find any way of getting them unstuck.

i will search new switches - any keyword on how to look for them ? Independent units will be ok - i can live without the interlocks, etc

[dhodapp]

For the switches, look up "push button switch 4 pole" (I think they are 4 pole...looks to be so) . I don't know how many you need but here is a set of 10 interlockable switches.

http://www.ebay.com/itm/1-pc-Interlock-Push-Button-Switch-10-Keys-4-poles-w-Ear-/130337410156?hash=item1e58b70c6c:m:maho0tCLX--tX9uhql6aQUw

[grumpydoc]

Update:

the switches are not fixable - at least i can't find any way of getting them unstuck.

i will search new switches - any keyword on how to look for them ? Independent units will be ok - i can live without the interlocks, etc

The 168 had some sort of lock out to stop "illegal" combinations of buttons being pressed - is that the problem?

If replacing the switches you'll need to unsolder everything and put new switches in the frame so, if you haven't already, I'd dismantle the switch frame and try the individual switches to see if they are actually the problem.

[klunkerbus]

Something like "pushbutton bank switch" is probably what you want to search for.  I recall that at the test equipment company I worked at in the early 1980's, we had a development kit where we could prototype up just about any switch like that as far as number of poles and number of buttons.  We were limited though in what we could prototype as far as interlocks between buttons.  Production switches were purchased against a specification drawing.   Driven by the switch size, configuration and interlock, they weren't standard parts that we ordered from a catalog.   

Finding a drop-in replacement could be a challenge. If that doesn't pan out, you might have to do what grumpdoc suggests and (painfully) remove the switch assembly from the board, take the switch assembly apart, and see if you can figure out what's broken about it. Taking the switch assembly apart would involve bending up on countless tabs on the metal brackets at the front and rear of the switches, and removing the button, spring and c-clip from each button post.  The individual switch elements should then be removable from the brackets.   If there were interlocks intended to keep more than one button pushed in at a time, there should be a plate at either the front or the rear of the switch that slides back and forth inside the bracket to "lock" switches in and then release them when another button is pushed.  If you are lucky, something about that latching plate is just bent, gummed up or otherwise stuck.  More troublesome would be a broken or missing flat spring usually at one end of the interlock plate. I think I see one of the flat springs on the front left, blue end of the switch assembly. 

Our test equipment switches never had the metal brackets at both the front and rear of the switches.

EDIT: Well, my point at bending up tabs on the brackets applied when there were gaps between the switch bodies.  With switches pushed up against each others like yours, that part of my post wouldn't apply.  I

[Brumby]

I found this teardown



It shows the operation of the switches and their interlocking, if that might be of any use.

[ealex]

hello
spent some extra time with it:
The AC and DC buttons are pressed at the same time.
The spring on the AC button is fully compressed and it feels like it's glued in.
The amps button moves freely -it latches when pressed but does not
The mA / ohm HI and ohm LOW are stuck - pressing them as hard as i dared did nothing.

If i can find those switches as stand-alone units i'll just replace them all, and drop the interlocking. I don't have the skill or patience to rebuild the switch stack from scratch.

priority now: find appropriate replacement switches

[ealex]

Short update: after some probing / poking i managed to get the switches unstuck to some extent, enough to test the volts and ohms range.
The volt and ohms range seems to work. The amps range is still stuck and i need to manually move the interlock plate to engage that range.

As an idea: can i replace the switches with some small signal relays ?
Assuming i can get enough relays for cheap - does the work require worth recovering that meter ? ( relays + push buttons + a small micro to replicate interlocks, etc )

[lowimpedance]

Short update: after some probing / poking i managed to get the switches unstuck to some extent, enough to test the volts and ohms range.
The volt and ohms range seems to work. The amps range is still stuck and i need to manually move the interlock plate to engage that range.

As an idea: can i replace the switches with some small signal relays ?
Assuming i can get enough relays for cheap - does the work require worth recovering that meter ? ( relays + push buttons + a small micro to replicate interlocks, etc )

I can appreciate the 'want' to get the multimeter working , but this one is really pretty much beyond help and not worth any more time.
 Are multimeters in general that hard to come by where you are ?.

[ealex]

no, i can afford a new decent one. ( got this one as junk )

with this thing it's more like a challenge - how much do i have to do to get it working again - no idea why i'm stuck on it.

i think i'll keep it as a side project for now.

i'll keep updating this post if i'll make any more changes to it.