Hi Tombi, Jerry,
After one afternoon of fooling around with this 3457A, Tombi, I think you are right about the drifting, it is temperature related.
Every time when I connect eg a 100kOhm resistor, I do this by holding the resistor, causing it to warm-up, and therefore lowering its value. (Neg.Temp.Coef.)
When I then start the resistance-measurement, the resistor cools down because I do not touch it, so I see the drifting-up.
However, what not has changed is after reaching a ´final´ value (=stabile temperature), the value shown on the display keeps going up- and down.
This behaviour seems the same with different values of resistors, eg. 100Ohm, 1kOhm, 100kOhm.
And when I perform a reset (blue, reset) followed by a selftest (blue, test) it still gave a hardware error, code 96 (=32 and 64).
I measured the following values on U211pin6: 8,1251(6/7)V and 12,203(7/8)V, so I consider this very stable, since only the last digit is fluctuating.
So the output of U211 is stable with the correct values.
About the output of U212, these values should not be the same, because the FET is used as a voltage follower, so the Source acts as the actual output of the OpAmp U212 based 1x amplifier.
The circuit-explanation in the service manual says about this, the FET is added to improve linearity of the OpAmp circuit.
By having the FET behind the output, the output of the OpAmp does not have to work at 12V, that would be too close to the 15V powersupply to maintain optimal linearity.
The good news: at the source of the FET I see the same stable values as at the input of U212pin3.
I changed location of U211 and U212 without any change to the symptoms.
Since the voltage at the output (FET source) is stable, I wanted to check the result of this stable voltage, that should be a stable current through the DUT (the resistor I want to measure, connected to High/red and Low/Black).
Instead of connecting a resistor, I connected a second mA meter to measure the current the 3457A is sending through the external resistor. (I used my Keithley 2000 for this)
Depending on the resistor range, I see the following current change from
01,01542mA
00,10146mA
00,01004mA
00,00091mA
00,00002mA
All these values are stable, only minor fluctuation in the last digit.
If I repeat this measurement but with a 1kOhm resistor in series with the external Ampmeter, the values change a bit, but remain stable.
But, the displayed value on the 3457A keeps going up- and down.
I repeated the test, but now measuring the external voltage across a resistor under test, that voltage should be similar with the 3457A reading.
The external voltage is fluctuating along with the 3457A reading on the display.
I know, these two tests are conflicting..... :-(
So, based on this, I have to assume there is something wrong with the current into the external resistor under test.
Something else: If I perform a selftest after power-on, it fails with a errorcode 96.
If I, after power-on, first perform an "AutoZero", and then directly perform a seltest, the errorcode is 64.....??
To see if there is something with the Offset circuit around Q111/Q112/U11b, I performed Diagnostic-4.
This allows for setting the Vos Adjust.
Although the diagnostic #4 already gave "Passed", it responded well to adjustments of R113, it does show "Failed Lo" or "Failed Hi" when I turned it up and down, so I put it back to "Passed".
I did look for cold solder joints around the circuit, resoldered them, mostly everything around U211, U212, Q111, Q212, Q213 - Q216.
So far for a Tuesday afternoon.
Final thought for tomorrow:
If the voltages going into U101 are OK and stable, it could be the contacts of U101, because the resistors used to determine the test-currents, are inside U101.
I read something somewhere about cleaning the contacts by pulling an IPA drenched cotton wire through the holes.
Un saludo,
Satbeginner