Hello Everybody,
Sorry for resurrecting this quiet old topic.
I have a Keithley 2000-20 with quiet strange behaviour.
Most times it works normal but I don't have another 6.5 digit DMM (or better) to check accurecy of the last digits, so it could be slightly off.
Sometimes it is measuring complete bullshit and selftest 101.2 and following will also fail than. This is absolute random and sometimes it fixes itself, somtimes after powering on and off or switching ranges.
I already replaced the relais (and the tweeter by the way) but this haven't made any difference.
After several weeks of short trubelshooting sessions (the error is wenting away when you hunt it) I'm pretty sure that my TF-245 is gone.
The 27.42K resistor between pin 10 and 12 goes open when I heat it up.
I can measure it in circuit when powered off (Q127 and Q131 are non conducting by default). In non failure state the value is 27.468K (in circuit) not bang on but close. When I heat it up the resistance rises slowly until it suddely changes to infinite. This corresponds to the voltage measured on pin 10 when stopping after test 101.2, if it passes, I can measure 6.507 V as result of the voltage divider of R300 and the resistor inside the TF-245. When the tests 101.2 fails the voltage of pin 10 is at 7.015 V, exactly the reference voltage which indicates that no current flows through R300.
Maybe this description helps other people with similar problems.
Could someone please help me out with a source of a spare part? I'm located in Germany and haven't contacted Keithley/Tektronix yet.
Best regards and a hot weekend
Robert