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| Keithley 193a multimeter leaky jfet with drain not connected |
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| Lorisermejo:
Hi everyone Months ago i bought a keithley 193a and only some days ago i discovered a problem: the meter is drifty and has an offset in almost every range (the 20v one is almost ok) and i traced down to q122 and q123(q123 most probable)jfet but i can't understand what those do on the circuit because looking at the schematic their drain is not connected (nc) Does anyone knows why are there? Can i detach the leg that is connected to the input of the dmm and don't replace the component? P.s. the photo is taken from page 171 of the schematic at https://archive.org/details/manualsplus_01600 |
| Kleinstein:
The JFETs are used as diodes, looks like as clamps to the positive side. Using JFETs (specially to the positive side) can help to get compensation of the gate leakage of the JFET switches that are off and thus with a negative gate voltage. extra low leakage diodes could work too, but may not be cheaper than the same fets already used. At least for a test (with some care to avoid ESD and higher voltages) one could remove these diode used 2 JFETs. A different low leakage diode would be possible too - at least for a test. |
| Wallace Gasiewicz:
Had the same problem with an HP 3456 which uses same sort of clamping FETs as diodes but with both drain and source tied together. Kleinstein helped me understand why these were used rather than just clamping diodes. They theoretically at least would have same leakage (but reverse) as the turned off FETS. My HP diodes were extremely low leakage. I did find some new FETs with low leakage that worked, but it was a long search. Did not find any low leakage diodes that I thought were acceptable. One FET is clamping POS and other one NEG. Disconnecting them one at a time might help narrow down the problem. Since the FET PN junction is used as a diode, perhaps tie the drain and source together as a test, maybe that might help?? I do not know the leakage of your FETs and a look at the datasheet might be helpful. I assume you tested the FETs grossly (P N Junction test) but testing them for the very small leakage is beyond my capabilities. Mine tested OK but when I replaced them, my problem was solved. |
| Kleinstein:
The semiconductor leakage is usually highly temperature dependent. At least for working parts it is usually going up about 2 fold every 10 K more. Often this also applies to the extra leakage of somewhat bad parts. So there is a good chance to identify leaky parts by lokal heating of the parts - the more leaky the more sensitive to temperature. For local heating tests I use a 1/4W THT resistor powered at about nominal power, hold just above the parts as a probe. The leakage specs with low leakage parts (e.g. < 1 nA) are often just test limits. It just costs extra to test to very low current and the spec limits may thus be far higher than the actual performance. Low leakage THT diodes are tricky to get, as the normal glass case is not really suitable as it reacts to light. |
| dietert1:
Some people use freezing spray for spot-cooling of electronic components during troubleshooting. Of course inside a meter you want to avoid remnants/dirt, so check what it really is. Dirt might be the issue to start with. Checking low leakage parts: We have an old HP 3478A meter that goes down so some pA. I use an external voltage source and a 1 MegOhm resistor as shunt to measure current. Since the meter resolves 0.1 uV, resolution reaches 0.1 pA. Of course the meter may produce some leakage of its own. This type of measurement needs care. One may need a good capacitor as well in order to reduce hum. Regards, Dieter |
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