[volt-nuts] Keithley 2001 Multimeter Fault

[Alan Scrimgeour]

Yes, it might come to that.
I just measured the resistance across the 0V & 5V lines with the power off, 
expecting it to be very low, but got 500 Ohms. Perhaps the LM2940CT-5 is 
faulty after all. Either that or the short only develops after power is 
switched on.
I've also noticed that if I push and pull gently on its heatsink it clicks. 
None of the other several mechanically similar regulator/heatsinks do that.
I'm tempted to cut the LM2940CT-5 output leg and check it's voltage and 
dissipation unloaded and also what current is actually being drawn from it 
in use.

Alan






----- Original Message ----- 
From: "Ed Palmer" <ed_palmer at sasktel.net>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Friday, March 05, 2010 8:37 PM
Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault


> 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
>
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