Tektronix Technical Forums are maintained by community involvement. Feel free to post questions or respond to questions by other members. Should you require a time-sensitive answer, please contact your local Tektronix support center here.

Keithley 614 zero check oscillations

200, 210, 220, 486/7, 60x, 61x, 64x Electrometers
409, 41x, 42x, 44x, 48x -
14x, 18x, 1801 Nanovoltmeters
15x, 18x Null Detectors/Microvoltmeters
Post Reply
geda7325
Posts: 7
Joined: May 24th, 2012, 2:56 am
Country: Italy

Keithley 614 zero check oscillations

Post by geda7325 » May 24th, 2012, 3:05 am

Dear All,

I bought a used Keithley 614. Sometimes, after pressing zero-check button with 0.2 V buttons pressed, I noted wild oscillations of the reading on the display. I was wondering whether all this is normal or not. I thought that with zero-check I should expect a fixed shift with respect to .00000 V, not a wildly oscillating one.

Thanks for reply,
Germano

RaymondR
Keithley Applications
Keithley Applications
Posts: 204
Joined: May 18th, 2012, 12:12 pm
Country: United States
Location: Cleveland, Ohio
Contact:

Re: Keithley 614 zero check oscillations

Post by RaymondR » May 24th, 2012, 7:01 am

Hi geda7325,

The 614 is an electrometer and all electrometers have extremely high input impedances (the 614 has >50 TOhms in voltage mode). This means the reading will jump all over the place without a load connected across the terminals. Enabling zero check changes this input impedance to ~10 MOhm (for Voltage measure mode) which protects the sensitive circuitry of the electrometer from charge buildup and overload. Disabling zero check changes this back to the normal 50 TOhms. See page 2-2 of the 614 user's manual: http://www.keithley.com/support/data?asset=955.

I suggest connecting a 1 MOhm resistor across the terminals. If the oscillations don't go away then there's a problem with the instrument.

One thing you should always do is keep zero check enabled until you are ready to make a measurement.

geda7325
Posts: 7
Joined: May 24th, 2012, 2:56 am
Country: Italy

Re: Keithley 614 zero check oscillations

Post by geda7325 » May 24th, 2012, 7:25 am

Dear RaymondR,

Thank you for very quick reply! Thank you also for advice. My problem is that even with zero check enabled (actually, not always) the reading oscillates with apparent no reason. This makes me difficult also to tune the zero volt pot (just under the zero check button). When the input cables are short-circuited (and zero check disabled) i have no problems (no oscillations). Is it possible that even when the zero check is enable, the electrometer circuitry could suffer RFI interferences, depending on the environment?

Thank you,
Kindest Regards,
Germano

RaymondR
Keithley Applications
Keithley Applications
Posts: 204
Joined: May 18th, 2012, 12:12 pm
Country: United States
Location: Cleveland, Ohio
Contact:

Re: Keithley 614 zero check oscillations

Post by RaymondR » May 24th, 2012, 9:38 am

Well, your 614 can be upwards of 30 years old so it's entirely possible that something is broken inside the instrument. While it is possible for the instrument to - up EM interference, I'm assuming this is the only instrument in your work area that's giving you stability problems. Keithley instruments are shielded by a metallic case which means that there would have to be rather powerful RF waves traveling through your work area to see any interference.

You can try the following:

1. Disconnect everything from the instrument (except the power cord).
2. Turn on and wait at least 1 hour for the instrument to warm up.
3. Enable zero check.

If that doesn't work then you can try cleaning out your instrument as 30 years on the shelf is a long time. If you can manage to remove the case then take a can of compressed air (for cleaning electronics) and clear as much dust from the components as possible. Also look for obvious signs of damage, such as busted capacitors (the large cylindrical ones may look like they're leaking something or the top may have mushroomed out) and signs of burning (browning of the circuit board, charring of components, etc.). Don't spend too much time looking, just see if anything is obvious and apparent but do make sure to clear as much dust as possible. Don't remove or disassemble anything other than the case, though.

Unfortunately, Keithley does not service this instrument any more so if you do discover damage then you're out of luck.

After cleaning, reassemble the case and retry what I suggested above. If you're still getting stability issues then something may have just gone bad.

geda7325
Posts: 7
Joined: May 24th, 2012, 2:56 am
Country: Italy

Re: Keithley 614 zero check oscillations

Post by geda7325 » May 24th, 2012, 10:50 pm

RaymondR wrote:Well, your 614 can be upwards of 30 years old so it's entirely possible that something is broken inside the instrument. While it is possible for the instrument to - up EM interference, I'm assuming this is the only instrument in your work area that's giving you stability problems. Keithley instruments are shielded by a metallic case which means that there would have to be rather powerful RF waves traveling through your work area to see any interference.

You can try the following:

1. Disconnect everything from the instrument (except the power cord).
2. Turn on and wait at least 1 hour for the instrument to warm up.
3. Enable zero check.

If that doesn't work then you can try cleaning out your instrument as 30 years on the shelf is a long time. If you can manage to remove the case then take a can of compressed air (for cleaning electronics) and clear as much dust from the components as possible. Also look for obvious signs of damage, such as busted capacitors (the large cylindrical ones may look like they're leaking something or the top may have mushroomed out) and signs of burning (browning of the circuit board, charring of components, etc.). Don't spend too much time looking, just see if anything is obvious and apparent but do make sure to clear as much dust as possible. Don't remove or disassemble anything other than the case, though.

Unfortunately, Keithley does not service this instrument any more so if you do discover damage then you're out of luck.

After cleaning, reassemble the case and retry what I suggested above. If you're still getting stability issues then something may have just gone bad.
Very clear. Thanks!

Post Reply

Return to “Older & Obsolete Instruments”

Who is online

Users browsing this forum: No registered users and 1 guest