| Products > Test Equipment |
| Keithley 6485/6487 teardown |
| << < (5/8) > >> |
| razberik:
I replaced the AD829 and BUF634, but no change. When AD829 is removed and I short the GND/Guard and output pin on AD829 footprint, I receive some value on display. When BUF634 is removed, it reads 0.00001nA. Any idea ? |
| razberik:
Some news. plesa - Yes, I have checked all power supplies, these are all OK. I didnt reply because this was the absolute first thing to check. No ripple, no significant noise, just nominal voltages. I have tracked the voltages around U25 LMC662 and U27 LMC6081. This differs between mA+uA and nA ranges. On mA+uA ranges, I receive saturated rail-rail swing at the U26 AD829 output, on nA ranges, it saturates to negative supply. IN+ on U26 is -17mVDC all the time. I receive -11.2mVDC on fully functional Keithley 6485 I lent from my colleague. I believe this is OK. I believe that saturation is driven by the input at IN-. On nA ranges, the U27 LMC6081 behaves strange to me. Output is -30mV all the time. IN- is at zero. When I touch the DMM probe IN+, the DMM reads some miliVolts ramping up. Starting value is random, but it ramps towards few miliVolts. No change at the output. I used HP 3478A for reading voltages. (When I read e.g. 10VAC, it corresponds to +/14V rail-rail swing) All is referenced to GND/Guard. The instrument has shorted Analog Output GND with chassis. I attach the input area schematic with voltages and waveform of the rail-rail swing at the U26 output. |
| razberik:
As I indirectly promised, here is the rearranged schematic. I also found out the relay switching matrix, noted in schematic for ZCHK OFF and nanoAmps range. K9 is operated during startup and switch off only. What is interesting is the LMC662 circuitry. It seems that only the input FETs are used to compensate input current of LMC6081. Thus these two opamps are matched ? :o |
| TiN:
razberik doing great job on schematics reversing, my respect! :-+ Keep it up. :-/O To contribute at least something, I took few ugly photos of my latest toy, KI 6485. As expected, very very almost same :) Obviously no point of having triaxial on 6487, as guard is not driven actively. |
| lukier:
--- Quote from: TiN on February 02, 2017, 06:57:14 pm ---To contribute at least something, I took few ugly photos of my latest toy, KI 6485. As expected, very very almost same :) Obviously no point of having triaxial on 6487, as guard is not driven actively. --- End quote --- Hmm is that the right front panel? ;D I have the old 485 from the ugly-brown era and it also has just BNC, no triax. I wonder how similar is the front end circuitry, maybe I'll take some photos and compare (AFAIR no LM399 in 485). |
| Navigation |
| Message Index |
| Next page |
| Previous page |