| Electronics > Repair |
| Keithley 2002 repair help |
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| TiN:
Piece of paper with squares and pencil will work just fine. Then scan it and I'll be happy to digitize it for community happiness. |
| Kleinstein:
For reverse engineering the first drawings are often on paper, especially if the circuit is rather simple. There are a few programs to draw schematics, like LTspice, Kicad or Eagle (e.g. free limited version). As the Ohms circuit uses extra diodes, chances are the circuit is similar to that of the 3458. Also the Keithley 2001 might be similar from the general layout, though it is different from the usual path. The current sources are usually not that different. There are a few choices, like using CMOS MUX or a set of JFETs. A second point is the question of switching the resistors at one side only or at both side - for a high quality meter I would expect switching both sides, especially because they use the extra diodes. |
| nikonoid:
Here is what I have so far as Ohms schematics is concerned. I have traced it all the way from R277 resistor to Input Lo and path to Input Hi (R322 and on). I found what is controlled by U221: Q1, Q2, Q3 and Q4. You will see that some connections are going into nowhere, that either means that I have not traced them yet, or they don't seem to be relevant for Ohms understanding. It is collector of Q212 that is at .4V on working meter and .75V on broken one. Let me know what you think. TiN, let me know if this is enough for you to start working with. The layout can definitely be improved from what I did by hand. Thanks. PS. I have also been compiling a file with all mistypes/mistakes I found in the repair manual so far. |
| Kleinstein:
There are still a few open points in the partial schematics. So it is a good start, but it needs a little more. One interesting area would be the upper end of R277 (750 K). Another part would be how the other current setting resistors (one more would be enough) are connected. There also needs to be a kind of path from the lower side of R277 to an OP ( could be U233 Pin 2) There seems to be a connection missing from the emitter of Q210 to something like the source of Q213. I am a little confused there is no diode in series with Q213, though is might be somewhere following R322. The voltage at the collector of Q212 depends on a few non critical parts (e.g. the diode mentioned above) and also on the input connection. So I am not sure this would be a significant difference. |
| nikonoid:
--- Quote from: Kleinstein on September 03, 2017, 11:34:31 am ---There are still a few open points in the partial schematics. So it is a good start, but it needs a little more. One interesting area would be the upper end of R277 (750 K). Another part would be how the other current setting resistors (one more would be enough) are connected. There also needs to be a kind of path from the lower side of R277 to an OP ( could be U233 Pin 2) There seems to be a connection missing from the emitter of Q210 to something like the source of Q213. I am a little confused there is no diode in series with Q213, though is might be somewhere following R322. --- End quote --- Thank you Kleinstein. I am working on version 2.0, so feedback like this is very helpful. Sent from my iPhone using Tapatalk |
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