Keithley 260 Teardown

[Vgkid]

I have always wanted one of these Keithley Nanovolt Sources, but financially they have always been out of reach, until one turned up on our favorite auction site.
While the outside leaves much to be desired , mainly the extremely yellow knobs. The rest of the outside case was actually in decent condition. There were remnants of calibration void stickers that have long since been removed.

Sourcing voltages down to 1nV requires much attention to detail for shielding ,thermocouples, and avoiding thermal related drafts(creates thermal emf's). Therefore the output terminals are housed in a shielded enclosure.

Sourcing voltage to low levels requires Low thermal emf binding posts , and if Low Johnson noise is required a Low source resistance is needed as well. With Source resistances ranging from 100-Ohms (high range) , 1-Ohm(uv/nv range). For those who noticed the binding posts , yes , they are L&N ones.

Opening up the enclosure were are greeted with a small transformer , a circuit board , and a very large shielded enclosure.

Zooming into the circuit board reveals nothing special. A 1N706 acts as the voltage regulator for the power supply , while a 1N936 works as a voltage reference. The only interesting aspect to this board is that the it is a constant current output supply , as opposed to a constant voltage.


Removing the shield reveals the Current divider , polarity switch , and the shielded voltage dividers.
The divider circuit baffled me at first , but reading the manual helped me in that regard.
The ranges are 10,1,0.1 Ohm resistors. The 1/10 ohm resistors are in a series divider with 1-10 , and 10-100 ohm resistors respectively.. The 0.1 is by itself.
---Opening up the shielded divider will wait until after dinner...

[Vgkid]

part 2:

Going further we remove the will need to remove the screws holding the divider shield in place. The standoff is for attaching the main divider shield.

We are greeted by a calibration sticker from 9/11/1979.

After removing the thick shield , were are greeted with the metrological goodness , of copper , and gold everywhere.

The two large resistors are a pair of copper wirewound resistors. They are both 1/1k Ohm series resistors. They act as dividers for the microvolt , and nanovolt ranges.

The Trimpot is the only adjustable element in the main divider. It is in series with the 34.8k metal film resistor, which are in parallel with a 1k metal film resistor. These make up the millivolt divider.
… and thats it, for this teardown.

[TiN]

Thanks for teardown. Waiting for part 3 with test results
You have nV-meter?

[HighVoltage]

Thanks for the great pictures.
Nice to see the attention to detail on the wire crimping of the copper lugs and the great copper nuts.
Although the copper lugs and nuts are not gold plated, they have no problem with corrosion, even after such a long time. May be the copper is surface treated somehow?

I also wonder how stable such an old instrument is, when measured with a good nano volt meter these days.

[Vgkid]

Thanks for teardown. Waiting for part 3 with test results
You have nV-meter?

No nV meter for me. The k181 specs aren't the best when compared to newer meters , even though the price is a bit high(but affordable). The -bay seller with the EM amplifier never responded back to my questions

Thanks for the great pictures.
Nice to see the attention to detail on the wire crimping of the copper lugs and the great copper nuts.
Although the copper lugs and nuts are not gold plated, they have no problem with corrosion, even after such a long time. May be the copper is surface treated somehow?
I also wonder how stable such an old instrument is, when measured with a good nano volt meter these days.

Thanks for the complement. I was surprised at the lack of corrosion , though there was a bit of discoloration on the long lead of the resistors , overall there was no noticeable corrosion. To bad I didn't have everything else that would have come wit the meter. There must be a large amount of Cu crimp tubes/ring terminals/spades/Cd solder somewhere...
no nV meter here .

[fki82]

Wow, that copper looks like new and hasn't corroded a bit!
Is it coated with something?

[Vgkid]

I will be able to test this , but it willl be a few months before i get some time. Something is in the mail that I over paid for...

[Vgkid]

This is not what I meant to get earlier, but I did get an em A-22, and another new toy.
Its vintage.

[HighVoltage]

Nice new toy.
Finally you can test your 260
Hmm, but which one will be correct, the 260 or the 34420A?

Looking forward to your measurements

[Vgkid]

Need to order cables, and a shorting plug first...
Unless I want to try and see it cat-5 cable would fit.

[HighVoltage]

First I made my own shorting plug and it worked OK
Now I got an original Keysight plug and it makes a measurable difference.

https://www.eevblog.com/forum/metrology/buying-low-thermal-emf-lemo-connector-for-nanovoltmeter-(344202182)/