Electronics > Repair

(FIXED) Keithley 225 (current source) repair

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motocoder:
Ok! I figured out why Q104 was heating up so much. It is because of R104. I believe this resistor is just there to pull the base of Q105 down so that the transistor is not on when Q104 is off. However, 100 ohms is a very low value when you consider the voltage across it can be ~130V. The current this was generating through the collector of Q104 was around 15mA. When you combine that with the 5mA needed to drive Q105 at full output current, that's 20mA flowing through Q104. At 130V drop across this transistor, that's about 2.5W, way too much.

This is why the first transistor I used for Q104 failed. I see no reason why R104 needs to be 100 ohms. I have replaced it with a 12k, everything still appears to be working, and Q104 is nice and cool.  :)

TiN:
Added to Keithley section, many thanks!  :-DMM

Keep up good work.

motocoder:

--- Quote from: TiN on June 10, 2015, 04:57:40 am ---Added to Keithley section, many thanks!  :-DMM

Keep up good work.

--- End quote ---

Thank you. You should add a link to your web site to your EEVBlog profile.

motocoder:
Here's a final pic of the whole thing buttoned up and working.

cncjerry:
Nice job, I'm sure you are very satisfied with the results.

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