Keithley 191 multimeter teardown video

[ivan747]

I finally finished editing this video I shot back in february. It is the teardown of my Keithley 191 multimeter. It is special because it has a very high accuracy (0.005% ±3 digits) and it is very old, form 1979. No Amp range, CAT ratings or anything fancy, but the construction is very different from your typical HP 3478A. Take a look:


http://youtu.be/lu2HytegneI

Feedback is appreciated.
Thanks
-Ivan

[free_electron]

Aiaiaiai......

You have clearly not looked at the schematic of the 191 have you ?

A lot of your explanation is completely wrong!
The jfets are range switches. There is no discrete buffer opamp. That to100 package is the first amplifier.
The jfets are on teflon standoffs to INCREASE impedance. Not to make it low impedant.

And the trim pots do not adjust the hybrid. The hybrids are accurate. The trimpots are part of the divider chain. They allow tu ing i. A narrow band the tap of the divider.
The mechanical switches combined with the jfets switch the tap nodes around.

I don't want to discourage you , or talk down, don't misunderstand me.
The problem is that in this kind of teardown you need to try to be accurate. Re-film a segment if you goofed up. (Like the tantalum for a ptc ).  Or the 10 ohm capacitors you talk about....

Re-film it. Re-mix it.

The eprom explanation is also wrong. Its a mask rom. Back in the old days eproms were much more expensive and smaller tthan mask roms. So the board has two footprints for development. Come production time only one mask rom was needed.

The silicon area of an eprom is larger than a mask rom because you need room to get the uv light into the gate structure if you want to erase it. A mask rom is a simple vertical diode that is either there , or not.
The rest is a grid structure. No need for read amplifiers, charge injectors for programming and all that other stuff that eproms need.

[ivan747]

The amount of factual errors you pointed out are too many. I might as well remake the video. I was trying to do this with little planning ahead but it gets hard.

Next time I will plan ahead and check the documentation. The copy of the manual I have has some illegible schematics, so I couldn't follow those for too long.

I should do an easy tutorial next time.

[free_electron]

again , i don't want to discourage you , so don;t get me wrong. i like teardonw videos with some description thrown in.
But before you tackle such things ; dig through the service manual a bit , read up on the machine and then go off making the video.

Dave can pull it off live on video because he's got years of experience with all kinds of electronics and recognises parts and their function just by looking at them. Things like these teflon standoffs are well known tricks to 'old-salts', the same for jfet input switching... So as soon as we spot that in a design , within a few seconds we know exactly what they are doing there without even needing to look at the schematic.

The same with confusing capacitors for ntc's ( although dave goofed up in his plasma review where he mistook potted inductors for capacitors ( even though the lettering 10uH was a dead giveaway ... ) ).

Tearing down a machine is one thing. aCtually making a meaninfull technical explanation of what they are doing requires a bit of upfront planning or, a truckload of experience...

Let me guess : this machine was made even before you were born . right ?
Not having experience is not a bad thing, you can substitute by upfront planning ( go throught the manual , try to find the functional blocks , read the principles explained in the good old service manuals and then do the teardown.

[ivan747]

Comment noted 
And yes, the machine is twice my age.