Hi all, thanks for all the help, tips and feedback: I am happy to say my 3457A is back in business!!
My latest actions:
I thoroughly cleaned the A2 board and the U101 Hybrid with IPA, no change.
I checked the input resistance of the U101 Hybrid (10 MOhm), spot on, so all OK.
I checked the Amp- and DC Voltage ranges with my HP 3468 and a Keithley 2000, all OK.
I checked the Ohms-currentsource again, this time by using my Keithley175A as Amp-meter.
This one has a 200,00 uA full scale setting and with this I could confirm all currents -even the very low ones- were there.
In the lowest Ohm range (30 Ohm) the current at the HI-LO front connectors was 1,0156 mA, stepping down in factor 10 steps to 0,10 uA in the 30 MOhm and 300MOhm ranges, so this was good.
Still, the display would be about 5% off (high) when using the 2 highest ranges......
My conclusion: it had to be the gain-calibration of these ranges!
As far as I could find, this meter has all separate calibration values stored for each individual setting and range, so I decided to "calibrate" the 2 highest 2-Wire Ohm-ranges, this leaving the rest of the meter´s calibration untouched.
I first did an Offset-calibration on all the 2-Wire Ohm ranges, since this is easy and only needs a copper wire bridge.
After that I made myself a 30 MOhm "standard" resistor by combining several metal-film resistors to a 30,0 MOhm resistor, I used my Keithley 175A as "the reference" for this.
Well, it turned out there is only one High range to calibrate, that would be the 30 MOhm range. It looks like the 300 MOhm range is derived from the 30 MOhm range calibration.
So I manually selected the 30 MOhm range, connected my "standard resistor" and went for "Blue", followed by "C, Offset Comp", and the "CAL" appeared in the display.
Now I entered 30000000,3457 followed by <ENT>. (P.S.: I had to reset the security code by changing the jumper on the A1 board)
It said "Calibrating" and after that it read 30,0120 MOhm.
I tried a calibration with other values as well, but it looks like calibration is only possible when using Full-Scale values, plus- or minus a small deviation.
So, when I tried to set calibration in the 30M range with a 10M resistor, it would display "Calibrating", but nothing would change.
Also, when I used my 30M resistor and entered 30800000,3457 <ENT> it would not change anything.
Entering 30200000,3457 does work.
Anyway, after the calibration with my 30M resistor using 30000000,3457 I performed an "ACAL" and hurray, it finished without any error-message or indicator.
Also the simple test with my 100 kOhm resistor is good now.
So, here is my resume of this (successful) repair:
- 2x OpAmp in the currentsource U211, U212
- Q212 in the Ohms Overvoltage protection
- U402 Analog Switch, replaced by a MAXIM DG390 (Mouser)
- U123 MC14094, serial to parallel shift register
- 2x reed relay K301, K302, temporarily replaced by simple, small enclosed 5V relays, will be replaced by 1A05 DIP4 reed relay 5V reed relays. (Don´t think the metal magnetic shielding is really necessary here)
All these parts are in the input-circuitry, so this definitely suggest that the input was exposed to a huge overvoltage.
Again, many, many thanks for all the help!!
Un saludo,
Satbeginner (Leo)