[volt-nuts] Keithley 2001 Multimeter Fault

[Ed Palmer]

Do you have another meter with enough resolution that you could trace 
the voltage drops to find the bad component?  i.e.  lowest voltage 
reading would be right next to the shorted component or highest voltage 
drop would be between the regulator output and the shorted component.

Ed

Alan Scrimgeour wrote:
> I just  had another look at the Keithley 2001:
> At power on the display shows "Model 2001" etc., followed by 
> "Autoranging DCV", with a row of dashes above it where the digits 
> should be. Perhaps this is all just the display processor at work.
>
> I can see that the upper PCB and the lower PCB each have an 
> LM2940CT-5. The overheating one is on the upper PCB which has all the 
> analogue circuitry on it. I can't immediately see how to get the 
> analogue PCB out without major surgery and prefer not to risk 
> contaminating it yet so I've just probed around measuring resistance 
> from the LM2940CT-5 output to accessible capacitors positive 
> terminals. I get 0.2 Ohms to pin 16 of a few digital looking IC's, but 
> nothing to any of the capacitors so I guess I'm going to have to get 
> the PCB out...
>
> Alan
>
>
>
> ----- Original Message ----- From: "Laurence Motteram" 
> <LMotteram at scientific-devices.com.au>
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Sent: Friday, March 05, 2010 1:03 AM
> 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