Keithley 225 Current Source Problem - Page 1
Forum section: electronics / testgear

author: OLD EE
author_rank: Newbie
timestamp: September 24, 2014, 03:42:29 am
content: I recently picked up a Keithley 225 current source on E-bay for a decent price. It was a bit rough looking but was said to be 'working' so I figured I'd give it a shot.So far I've checked out most of it and it does, indeed, appear to be working. I have discovered a couple of problems though and I know a number of subscribers (including Dave) have experience with these units and can perhaps assist me with solving them.The first problem is that the 130v supply is running at about 150 v. The mains input is 120v +/-1 volt(rms) so I don't think that is the problem (even though the 225 is spec'd at 117v). The circuit is very simple; transformer, bridge rectifier and filter capacitor the output of which goes to the regulator which controls the compliance voltage for the output current.I performed a TINA simulation of the transformer/bridge/filter cap with a 10K load. I used the measured transformer turns ratio, N = 1.85, for the value of the transformer. With the input voltage a nominal 120v rms I got 148v DC across the capacitor. This correlates well with what I am measuring on the circuit board. With N=1.55 I get about 131 volts from the simulation. Sort of looks like perhaps too many turns on the transformer (for which I have no information).The main problem here is not so much the 20 volt difference ( the transistors in the regulator are rated at 400 volts) but the 140uF caps are rated for 150 volts (they appear to the originals) . Makes me nervous to be running these 50 year old caps close to their rated voltage.Where I could use some help is for someone who has one of these things to measure the '130 volts' across the filter caps (C205 and C206) to ground. I have no way of knowing whether or not what I'm seeing is sort of normal with tolerances or do I need to deal with this problem. Also it would be helpful if you could check the turns ratio value of your transformer.The second issue is the pot which sets the maximum compliance voltage for the output. It appears to function as expected until the ends of the rotation are approached. Here the output voltage jumps to 106 volts. I doubt that this is normal and it may just be the pot (wire wound) needs cleaning or replacement but I'd be curious to know if yours does anything like this.I would appreciate any help any of you other Keithley 225 owners can give me with these questions.Keithley manual for unit is athttp://exodus.poly.edu/~kurt/manuals/manuals/Keithley/KEI%20225%20Instruction.pdfThanks, Old EE

author: JuiceKing
author_rank: Regular Contributor
timestamp: September 28, 2014, 01:49:15 am
content: Well, my 225 is working fine and I get 154.6V and 154.7V across C205 and C206 to ground respectively.

author: OLD EE
author_rank: Newbie
timestamp: September 28, 2014, 03:20:06 am
content: Thank you for the reply and for taking the time to check the voltages. Mine appears to be working well also and I just wanted to be sure it wasn't a problem unique to my unit that needed repair. Looks like what ever it is that is causing the voltages to be too high doesn't seem to affect operation of the circuit.I enjoy the old equipment. Takes me back to my first days out of school. Back when things were done well and made to last.Again, thanks.Old EE