[volt-nuts] Keithley 2001 Multimeter Fault

[Alan Scrimgeour]

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
>
>
>
>
> ----- 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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