| Electronics > Repair |
| Keithley 2002 repair help |
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| nikonoid:
This sounds right. Thank you. I mixed up the sequence of diodes. It should be the opposite sequence. The max326 chip is in the mail. The surgery is scheduled for Wednesday. :) Sent from my iPhone using Tapatalk |
| nikonoid:
Today I replaced the Max326 (U223) with a new one. The situation did not get worse.... and also did not get better. The board underneath U223 looked clean. I cleaned it more anyways. I am still getting same errors: 306.2, 500.6, 500.7, 600.2. I guess next step is to desolder diodes. Any other ideas? Sent from my iPhone using Tapatalk |
| Kleinstein:
The direction of the leakage current is in a way that it can't flow through the diodes (higher voltage at the other side), unless there is massive photocurrent. So unless there are other ports connected, what I somewhat doubt, the current must go through the max326 or maybe leakage on the board. As the current is through the turned off 75 K resistor, it can't be the OP following the max326. It is slightly strange that the 20 K range (using the 75 K resistor active) also has a current more on the low side - so leakage is not between the two sides of the switch in the max326, but more like towards the negative supply. One might want to check the supply of the max326 - a wrong supply might cause trouble. One might need to use a scope here to check for AC trouble. A wrong DC supply level should give different effects. Also a borderline / oscillating / out of bound (e.g. to negative) signal at the control input could be a cause. Oscillation in the current regulation part is another possibility. It would make sense that the 2 ref voltages for the ohms circuit are generated from the charge pumps (LTC1043) - so AC ripple there might be a possible source of errors too. To understand which voltages are needed for the supply the value of the two ref level would be interesting too (could be 14 V and 7 V or 21 V and 14 V). |
| nikonoid:
Thank you, Kleinstein. I will recheck power on the chips this weekend. I am now looking to get an insulation transformer, so I can safely do oscilloscope measurements. |
| Kleinstein:
Measuring on the analog side of the DMM does not need an isolation transformer. The analog part is isolated from ground anyway. Even with the digital part one can get around the transformer, accepting slight possible noise from a ground loop. |
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