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@sasktel.net> To: "Discussion of precise voltage measurement" <volt-nuts@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@scientific-devices.com.au> >> To: "Discussion of precise voltage measurement" <volt-nuts@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@febo.com [mailto:volt-nuts-bounces@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 > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there.

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@pacific.net> To: "Discussion of precise voltage measurement" <volt-nuts@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@lsr.com> >> To: "Discussion of precise voltage measurement" <volt-nuts@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@febo.com [mailto:volt-nuts-bounces@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@febo.com [mailto:volt-nuts-bounces@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@scientific-devices.com.au> >>> To: "Discussion of precise voltage measurement" <volt-nuts@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@febo.com [mailto:volt-nuts-bounces@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 >>>> _______________________________________________ >>>> volt-nuts mailing list -- volt-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>>> and follow the instructions there. >>>> >>>> _______________________________________________ >>>> volt-nuts mailing list -- volt-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>>> and follow the instructions there. >>> >>> >>> >>> _______________________________________________ >>> volt-nuts mailing list -- volt-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>> and follow the instructions there. >>> >>> _______________________________________________ >>> volt-nuts mailing list -- volt-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>> and follow the instructions there. >>> >>> _______________________________________________ >>> volt-nuts mailing list -- volt-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>> and follow the instructions there. >> >> >> >> _______________________________________________ >> volt-nuts mailing list -- volt-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >> and follow the instructions there. >> >> > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there.