[volt-nuts] Keithley 2001 Multimeter Fault

Alan Scrimgeour scrimgap at blueyonder.co.uk
Sun Mar 7 01:35:17 UTC 2010


I hadn't realised the LM2940CT-5 was so dependant on the output capacitor 
capacitance and ESR for stability until  I tested one just now. Can anyone 
recommend an upgrade?

Alan

----- Original Message ----- 
From: "Brooke Clarke" <brooke at pacific.net>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Friday, March 05, 2010 7:13 PM
Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault


> Hi Alan:
>
> I have a bunch of combined capacitance and ESR meters that I'm about to 
> start selling.  See:
> http://www.prc68.com/I/ESRmicro.shtml
>
> Have Fun,
>
> Brooke Clarke
> http://www.PRC68.com
>
>
> Alan Scrimgeour wrote:
>> Thanks John, I plan to start the search in a few hours. Wish I had a 
>> thermal imaging camera!
>>
>> Alan
>>
>>
>> ----- Original Message ----- From: "John Lofgren" <jlofgren at lsr.com>
>> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
>> Sent: Friday, March 05, 2010 4:09 PM
>> Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault
>>
>>
>>> Yes, check capacitors.  I has something similar happen to the -5V 
>>> regulator on HP 55300A GPSDO.  The symptoms were very similar to your 
>>> description, but with the addition of a discolored heatsink.  It turned 
>>> out to be a shorted ceramic cap on the regulator output.
>>>
>>> Good luck and keep us posted.
>>>
>>> - John
>>>
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On 
>>> Behalf Of Laurence Motteram
>>> Sent: Thursday, March 04, 2010 7:04 PM
>>> To: Discussion of precise voltage measurement
>>> Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault
>>>
>>> Don't be misled by the working display, as it has its own
>>> microprocessor.  The digital PCB has two LM2940CT devices, but it sounds
>>> like the one in question supplies the digital +5V.  Naturally,
>>> everything connects to this, but you might like to start by checking
>>> capacitors.  If you are lucky, you could find a shorted tantalum.
>>>
>>>
>>> Regards,
>>>
>>> Laurence Motteram
>>>
>>>
>>> -----Original Message-----
>>> From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
>>> Behalf Of Alan Scrimgeour
>>> Sent: Friday, 5 March 2010 11:32 AM
>>> To: Discussion of precise voltage measurement
>>> Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault
>>>
>>> The little functionality the meter had when I got it enabled me to start
>>> a
>>> self test, but now I can't even do that as functionality has dropped
>>> away
>>> rapidly. The display is fine and presumably the processor driving it,
>>> but it
>>> no longer responds to the keyboard.
>>>
>>> A smell of overheating plastic after a few minutes lead me to an
>>> LM2940CT -
>>> 5 volt regulator. It's located in the fan assisted cooling path but
>>> getting
>>> far too hot even allowing for that. It's output voltage is well below
>>> 5V.
>>> It's input voltage appeared to be reasonable given it's being
>>> overloaded, so
>>> I conclude that either the LM2940CT is faulty or more likely, something
>>> is
>>> overloading its output. A circuit diagram would show me what connects to
>>>
>>> this output so I can start searching for the culprit.
>>>
>>> Alan
>>>
>>>
>>>
>>>
>>> ----- Original Message ----- From: "Laurence Motteram" 
>>> <LMotteram at scientific-devices.com.au>
>>> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
>>> Sent: Thursday, March 04, 2010 9:34 PM
>>> Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault
>>>
>>>
>>>> IP issues aside, what seems to be the trouble with the 2001?
>>>>
>>>>
>>>> Regards,
>>>>
>>>> Laurence Motteram
>>>>
>>>> -----Original Message-----
>>>> From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com]
>>> On
>>>> Behalf Of Alan Scrimgeour
>>>> Sent: Friday, 5 March 2010 5:47 AM
>>>> To: Discussion of precise voltage measurement
>>>> Subject: [volt-nuts] Keithley 2001 Multimeter Fault
>>>>
>>>> I'm trying to repair a 7 1/2 digit Keithley Multimeter but can't get
>>> the
>>>> circuit diagram. Keithley say they don't release schematics any more
>>> due
>>>> to IP issues, whatever they are?
>>>>
>>>> Can anyone help?
>>>>
>>>> Thanks,
>>>>
>>>> Alan
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>>
>>
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