Electronics > Repair

Keithley 2306 Dual PSU / Battery simulator repair

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SparkyBruce:
Ah - I replied too soon.

Channel 1 started to get a negative offset after about 1 hr of use set to 3.000V
It went down to 2.780 volts on the display (and on my external DVM) , despite being set to 3.000V

Channel 2 is still happy.

So the monitoring side of things (voltage out and current) are still correct , there is just this difference between demanded voltage and output voltage.

I have quickly tried a calibration run at home but despite being better than before there is still an offset between demanded voltage and the output.

I am going to swap out the two elec caps (10uF 35V and 220uF 50V) tomorrow just in case it is one of them.

The channel 1 side of the heatsink does feel warmer than the other side so it could even be an issue with the power transistor driver.

Thanks

Bruce

picburner:
Hi,

A few days ago, during a full load test on both channels, it has  suddenly restarted without giving error messages.
Perhaps it is the case of making a ESR control of all electrolytic capacitors.
In normal conditions, apart the previous case, my instrument has no problems of overheating in the analog power section and the voltage will not drop under load.

Giampiero

SparkyBruce:
Sorry to hear your troubles.

I have been checking the zenners on my unit as it appears that channel 1 is running far warmer than the other channel , despite channel 1 mostly working.

The output voltage is a few hundred millivolts low from the demand voltage. So if I set 1V I get 700mV ish which does wander a bit , but the output current sits at about 0.

When I load it up it does drive the output and measure the current correctly so it is acting like the bias current for Ch1 is too high (hence why I am checking the zenners in the circuit).

I will be borrowing a friends thermal camera to see if that can shed any light on what is going on, but I guess I will have to map out the circuit.

The big fets are IRFP240 & IRFP940's.

Bruce

SparkyBruce:
Ah fixed.

In the end after borrowing a friends thermal camera (thanks Steve) it was tracked down to one of the op-amps in the output voltage sense cct (an AD818). the chip was getting over 100C and after a false start with the main voltage control op-amp (which was also very hot) it proved to be the positive sense cct that was messing everything up.

After a quick re-cal all looks happy so I will run it over the weekend to see what it does.

Thanks

Bruce

Prehistoricman:
Big thanks to those in this thread who found this solution and shared it with us all. I also had a Keithley 2306 display INITIALIZING forever and I ordered a replacement RAM on eBay that fixed the issue.

My unit also had a faulty fan. It works at 6V but higher voltages cause the fan to run slower. Perhaps one of the windings is open.

Thank you!

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