Electronics > Metrology

Strange asymmetric nV noise of Keithley 148

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--- Quote from: Whitefoot on May 18, 2023, 05:56:31 pm ---I will try to go thru the adjustment procedure and check cal

--- End quote ---
The important part is not try to calibrate it until it is absolutely clear that it is out of cal. You need just to make sure that the amp gain is high enough
so that it is possible to zero it and the response time is ok*, but not too high to avoid too much of the "ripple oscillations" at the linear output.
*the response time is ok: the amp response time should be about 0.2-0.4sec for the 100nV step signal at 100nV scale and also see img5 picture c of my initial post

I found that both of my 148's have power supply problems.
In unit nr 1 the -18V is about -11.4V.
In unit nr 2 the -18V is about -10V.
I didn't go any further yet. I don't know when or if I will try to fix this.


--- Quote from: Whitefoot on May 19, 2023, 02:26:16 am ---I found that both of my 148's have power supply problems.
In unit nr 1 the -18V is about -11.4V.
In unit nr 2 the -18V is about -10V.

--- End quote ---
* on the Rear Panel check the position of line switch S220 (117 volt or 234 volt),
* remove both lamps on the front panel, disconnect the internal batteries,
* check again the -18V, Test Point A, Figure 20 (K148 manual, !!!be careful!!!: Test Point A is at the bottom of the rotary switch, and above are 2 red wires, they are at the line voltage),
* check -+12 and +1.2 power supply voltages,  Test Points B,C,D, Figure 21 (K148 manual).
What are these?

The line switches are correct.
Both lamps are burned out.
They don't have batteries.
I'm going to put them back on the shelf.  I have too many unfinished projects laying around.

Before putting my K148s back on the shelf, I checked the regulated PS voltages.

Unit nr 1:
+12V  --  +11.8V
-12V   --  -12.0V
1.2V   --  +1.17V

Unit nr 2:
+12V  --  +12.00V
-12V   --  -12.05V
1.2V   --   +1.21V

These are all within spec. It appears that maybe the -18V is only important for the battery charging circuit. Neither unit has batteries.

For a quick functional check I placed a 10ohm resistor across the inputs. I connected a battery at 6.2V thru a 1.0Mohm resistor in series with the 10ohm resistor. This gives roughly 6.2uA current thru the 10ohm, and both 148s measure 62uV, so they look to be basically functional. I haven't yet tried anything more with the low ranges -  that gets time consuming.


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