| Electronics > Repair |
| Keithley 2002 repair help |
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| Kleinstein:
The AC signals don't look very good, but also not that bad. So it is hard to tell if this could be the cause of the problem or just normal. Due to the charge pumps (LTC1043) on the K2002 board (1 should be used to generate the 14 V ref level for the ohms circuit, not sure what the second one is good for), there is a chance that some spikes are normal for the K2002. However to much could cause trouble in precision DC circuits. One should be able to see which spikes originate from there, by checking the frequency / timing. At that level, some could be a problem of where the ground is connected - there might be comparable background between the com terminal and corresponding ground on the board. It somewhat depends on the environment and scope, how much "noise" is picked up. Finding out if such small spikes are real of an artifact from the measurement is a little difficult and where experience makes a difference. However it looks like the noise spikes at the amps part are larger and thus very likely real - possibly the source of the spikes. I see no legitimate reason to have them there, except if there is a chopper amplifier used for the amps circuit. It might be worth comparing at least the measurement at the amps terminals to the working K2002. At least the large spikes look suspicious. The large spikes could cause some current in protection diodes as a bootstrapping/guard amplifier might not follow that fast. A possible cause could also be the guard/bootrapping amplifier in the amps section oscillating. Also the DC offset looks a little high, especially in the 200 µA range. 62 µV at an 1 K Shunt would be 62 nA (about what the DC reading is). Have you checked the AC reading of the "bad" meter ? No need for a throughout test, but is it at least reading near zero on something like AC amps or AC volts with a shorted input. The AC circuit could be another possible source for higher frequency spikes (e.g. the rectifier circuit switching hard between positive and negative). |
| nikonoid:
Here is an updated OHMS schematics, version 3. Changes to V2 are fairly minor and include Q226 transistor. By the way I still cannot figure out what that transistor is. It seems to be a matched pair with Q227. If someone knows, please post that info. In trying to troubleshoot AMPS functionality, I also traced most of AMPS circuitry. It is very similar to K2001 schematics with addition of an opamp. So I used 2001 schematics as a basis and just changed what needed to be changed. Let me know if these look ok. TiN, if you have a chance please incorporate changes to the nice drawings you did. Thanks. |
| nikonoid:
Back to troubleshooting. It appears that problems with AMPS and OHMS are NOT related to each other, but both seemed to be related to temperature. When starting a cold meter it would start reading -6.3uA with open inputs. It quickly comes down to -2ua, -1ua and so fourth as meter is warming up. If I blow some cold air in the general direction of the APMS components the reading goes back up. To me that sounds like a clear proof of temperature dependency. I measured voltages relative to Input LO while meter is still cold. Please see attached file. For OHMS, I used hot air soldering station at its lowest setting of 100C and medium airflow. Blowing hot air focused around U232 for about 5 seconds fixed 2MOhm range readings (+/-40ppm of perfect). Blowing for 5 seconds more did not change reading from being good. I am still not sure if one of components is temperature sensitive or contamination on the board is responsible for this behavior. While hot air fixes OHM issue, AMPS reading problem appears unaffected, so they seemed like independent issues. Klienstein, seeing this temperature dependency should I still further pursue AC analysis? |
| Kleinstein:
The Ohms section working only when warm is kind of odd. Usually leakage gets worse with higher temperature. Also the leakage current is really high (like up to 500 nA) this is a lot for leakage due to contamination. Just looking at the DC readings would strongly point to the max326 already changed. So the 2 main suspects are leakage due to contamination/burned board and some AC effects. Checking for AC trouble might be easier, at least a quick check. There should be very little AC visible around the Ohms circuit, especially at Q213 (with a 100 K or 1 M resistor at the output), output of U233, emitter of Q251, U223 pins 9 and 11. No need to measure around Q214 this is close to having the probe in the air. Instead of directly the output, one might use the low value 4 wire shunt as ground. How are the old measurements done (e.g. Probe). the old measured 25 mV_pp at Q213 might indicate a problem, but could be just an artifact from poor ground / probe. For a precision instrument I tend to prefer a quieter signal. As the leakage problem got slightly better with cleaning, there is still a chance it could be just contamination. Residue from the caps might leave behind rather thin layers that might not be so visible. For cleaning DI water (usually plenty in several steps) might be more effective than IPA or similar. The main suspect would be the area around U223. The extra transistor (Q226) seems to be part of the high ohms part. I don't think it is really related with the problem, though it might be nice to have the Ohms schematics complete. It is kind of separate from the normal ohms source, more like having the 14 V reference (or ground) connected through a large series resistor. The amps section look rather conventional, though leakage in the µA range would be a lot even for the DG411. One suspect would be U234, especially the 2 gates connected to the DCA line. There might also be leakage from the DCA side. It is likely going towards some precision OP or maybe the main mux before the gain stage. Again a lot of leakage from the board might be the culprit too. For surface leakage it would be more like relative humidity than temperature that is the culprit. So cold and really dry should be similar to warm. |
| nikonoid:
I already replaced U234, U229, U225, and other and cleaned everything under them, so I had to look elsewhere. I was just checking the board again and Q217 caught my attention. The source to drain resistance was about 4Ohm in both directions and gate to source and gait to drain measured between 52k and 54k. Not sure is this is OK. Does it make sense to you guys? The solder on leads of Q217 looked dull and pitted. Since one of the legs is sitting at -15V, I decided to clean between the legs of this transistor. Immediately after cleaning the AMPS performance improved to almost perfect state: On cold start: 200uA +000.0087uA 2mA +0.000121mA 20mA +00.00116mA 200mA +000.0183mA 2A +0.000103A After 5 minutes, readings stabilized around: 200uA +000.0037uA 2mA +0.000043mA 20mA +00.00044mA 200mA +000.0050mA (this reading was a bit strange, it would go as high as +000.0095 when turning this range on and then go back to 000.0050 fairly quickly) 2A +0.000042A These reading did not change after additional 30 minutes of warm up, so calibration might be able to take care of this offset. This seems like a proof that the contamination was the reason for trouble with AMPS. I will try to do more cleaning with DI water and IPA. Kleinstein, I will try your suggestions tomorrow. Thank you. |
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