Products > Test Equipment
Keithley 6485/6487 teardown
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TiN:
Thanks for photos, I might have to add some soon  ;).
razberik:
My boss agreed with trying the repair. I replaced the LM393, but no result.
It seems to me that instrument is switching ranges correctly. I mean - if I set manually some nano/micro range and then I press to AUTO, it switches to the highest range 20mA.
If I connect DMM to analog output, I receive som random voltage with nothing on input. If I short the input, the analog output changes, but I dont receive correct number either.

Any tip what to do next ?

I attach little bit more detailed photo. There are some DG411 switches. Shall they be wrong ?

TiN - I have some hi-res photos, but not very good angles and ambient lighting.
TiN:
Feel free to upload all or any pics to my ftp, I'll reshare. There are not many photos of there online. Also if you have more photos of bottom voltage source board, would be cool to see. Access info is in my sub link ;).

Did you check obvious, input protection parts?
If I be you, I'd check voltage source operation. Set meter to get small output, like 1V or 5V and test if it's present.
Or opposite, source small stable signal to source and check where it disappears on circuit, assuming problem is somewhere between input connector and ADC chip ;)
Check input voltages on front end opamps (DIP8 fella's). Be careful not to contaminate things, to prevent leakage (using unpowdered gloves helps, also protect you a bit from high voltage shock from FE supply).
razberik:
I was wrong. There is no DC voltage at the Analog Output. There is 2VAC for odd ranges and 6VAC for even ranges. It is 100Hz frequency, looks like two combined 50Hz. WTF ?
Everything is disconnected. When Zerocheck is active, zero at output.
I was wandering about the input and I simply dont understand the purpose of the LMC662. :o Is this the input ? And what is the LMC6081 for ?

AD) I am getting the exact AC signal from AD829 video amplifier output.
plesa:
OK, so it seems that other components has been damaged. Another components in the chain is LMC6081. This will also explain the weird analog output readout.
During zerocheck is input shorted, so you can check input impedance. Analog output is direct output from input stage.
Are you sure that on input was only voltage 500V?
I have few 6485/6487 for service and all of them has damaged comparator only. Never input stage blown. On their input were voltage between 1-5kV. This meter do not have any protection on input. And all input protection needs to be external.

For theory check chapter 4 in manual
http://www.tunl.duke.edu/documents/public/electronics/Keithley/keithley_485_picoammeter_manual.pdf
There is also schematic but 6485 is similar, but different.


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