Electronics > Repair
(FIXED) Keithley 225 (current source) repair
TiN:
Great job.
Some photos of internals would juice up this thread even more :-+
motocoder:
--- Quote from: TiN on May 31, 2015, 12:16:02 pm ---Great job.
Some photos of internals would juice up this thread even more :-+
--- End quote ---
Ok, I'll add some photos to this post.
I am starting to get a bit frustrated with this thing. I keep finding more bad parts. The latest is variable resistor R121. The manual says this pot is the adjustment for the bias current through transistors Q101 and Q102. However, after replacing the pot, I measured the voltage across the three diodes that are part of that circuit (D108, D109, and D110). The voltage across these is so low, that it's hard to imagine that this circuit is doing anything at all currently, and this voltage, nor that across the pull-up resistor (R120) does not change at all when I adjust R121. I don't really understand this part of the circuit, so hard to say what is going on there.
I spent a good deal of time looking at the very first differential stage, which uses a matched JFET pair (Q118A and Q118B). I took some measurements of D/G/S voltages for these JFETs as I adjusted the current value and range values. It appears that this circuit is completely biased over to one side, with the JFETs so saturated that they don't do much. This would explain why the output of the unit is always a positive voltage, regardless of which polarity I set the output selector to. One of the pics below shows the voltage measurements I took on that JFET - all voltages were made with respect to the 12V ground (this ground effectively "floats" on top of the voltage output by the series driver transistors, before the output current sensing resistors that connect to the output). I tried adjusting the "Amplifier Zero", which is a pot connected to the source of Q118A/B, but it has no effect at all. This isn't too surprising, because I think something else in the high gain stage is badly skewed to one side. I didn't capture it in the initial measurements, but I also took some measurements with the output polarity reversed. I could see the polarity of the gate voltage of Q118B reverse, but because of the aforementioned problem, this has no effect on the output voltage.
Finally, I have some questions for anyone who owns one of these. The manual has a diagram (Figure 16) with some points to use to measure various voltages during calibration. However, it's really hard to tell were the attachment points are for the "Ref or Zero" measurements. In Figure 16, it just points at the potentiometer, and at some unlabeled point on the range selector switch. Can someone clarify exactly where these attachment points are?
Range selector resistors burned in half (bottom left side of photo, just to the right of the black wires)
Top view of main circuit board
Measurements I took on Q118A/Q118B - first differential amplifier stage
motocoder:
Ok, looks like something is wrong in the "transient absorber" section (Q401, Q402, R402 - R405). I measured the voltages on the transistors and the voltage on the emitter of Q402 is not what it should be (-3.30V instead of the expected -1.04V). This circuit is connected to the gate of Q118A, and so might be the source of the imbalance problem.
motocoder:
Bad idea to remove the "Transient Absorber" transistors and power on. Now I'm going to need a new JFET pair...
motocoder:
Big breakthrough this evening. I replaced the part I burned out with my dumb idea to remove the "transient absorber" transistors. I also replaced those two transistors since it wasn't possible to salvage them (leads too short).
Then I started walking through each transistor in the high gain stage. I discovered that the base of Q115 was pegged out at -12V, clearly not correct. The problem turned out to be Q116, which was shorted between collector and emitter. I replaced this and Q117 with a pair of ZTX550's, which I selected to have fairly close beta. Not sure those are a perfect match, but they can sustain more current than the original part, which seemed to be the primary reason it was selected.
Now I can see the output voltage change as I turn the selection dial. I've got more to check to confirm whether or not it's all working, and eventually need to get my questions about the calibration test points answered so I can do a calibration, but this is good progress.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version