| Electronics > Repair |
| Keithley 2002 repair help |
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| Kleinstein:
I think TIN got a mistake around Q213 / Q210. The hand drawn circuit looks much more plausible. I also doubt that Q213 would be a VN0605 - it should be more a high voltage (e.g. 1200 V) MOSFET, as it is for protection against high voltage in ohms mode. VR216 should be either the other way around or a zener diode. |
| TiN:
Fixed a bit. I think "wrong" direction for opamps cause the perception rejection :-DD. That thought crossed my mind... Usually it takes 2-3 redraws for clarity once more blocks filled in. Anyhow still hopefully better than K2001's schematics, which are pure joy. |
| nikonoid:
TiN, Thank you for doing drawings. It is a lot of work. Q213 is a "2SK1413" not "25K1413". That is another mistype in the repair guide. I am attaching a list of errors I found so far. |
| nikonoid:
--- Quote from: Kleinstein on September 08, 2017, 08:29:27 am ---The 100 K resistors don't need to be accurate, they are just there to cope with any leakage from CMOS switches above. One should measure the voltage on both sides of CR208 - ideally it should be very close to zero voltage over that diode, unless the resistor is chosen as the active one. The voltage should also very close to the point where the 4 precision resistors join together. As there is some extra current through the resistor, the polarity of the voltage over CR208 should tell it the current flowing this way (e.g. leakage in the diode) or the other way (through the other CMOS switches). In the 2nd case is would be de-soldering the switches U223: as it would be either that chip, or maybe residue - likely under it. Likely a new chip for U223 is than needed - as there is quite some risk it is damage (either from the beginning or when removing it). If it is current through the diode, one could for a test remove CR208 - trouble is than more like with the other CMOS switches (U221-Q6). --- End quote --- Kleinstein, I measured the voltage across diodes (100k resistor on 2MOhm range) CR206: .41V CR207: 50mV CR208: .1mV CR209: -4.6mV On a good meter I got: CR206: .41V CR207: -0.26mV CR208: -0.20mV CR209: -0.13mV Broken meter seems inconsistent in terms of both magnitude and sign. I am not sure though what these numbers are telling me, though. PS. I have a spare DG411. I am ordering MAX326 now. |
| Kleinstein:
The positive (though small) voltage over CR208 tells us, the 0.5 µA leakage current is not flowing through this diode (would be the other direction). The 50 mV for CR207 indicates there might be a problem it that path too (or maybe the diodes / readings are interchanged: 50 mV across the diode at the 75 K resistor would make more sense). Anyway this would point towards a problem with U223(max326), or maybe some leakage on the board around that part. A slightly different voltage over the diodes for different meters is not a problem. A main contribution would be from the offsets of U228 and U233. Ideally the drop should be the same one all 3 non active paths. Differences would be due to leakage of U223 and less critical U221/U220. |
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