Electronics > Repair
(FIXED) Keithley 225 (current source) repair
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motocoder:

--- Quote from: cncjerry on June 19, 2015, 04:09:30 am ---Nice job, I'm sure you are very satisfied with the results.

--- End quote ---

I still have a few quibbles, mostly with the "rail splitter" circuit that I built to replace the broken one in the source, but I'm forcing myself to leave it alone. Thanks for your advice and help on the restoration.
motocoder:
I'm selling one of these 225s (not the one discussed in this thread, but a second unit I purchased to use as a reference). Details here:

https://www.eevblog.com/forum/buysellwanted/keithley-225-current-source/
dom0:

--- Quote from: johansen on May 22, 2015, 09:37:41 am ---they are likely matched hfe npn/pnp pairs

replacement should not require an exact match, but ymmv.

--- End quote ---

The electronics of the 225 are quite nice, actually.

Output stage: on the positive side you have your usual Darlington (alas - directly driven from the voltage gain stage of the error amp), on the negative it's a Sziklai pair. Both have additional instantaneous current limiting (Q103 / Q106 ... and switched with range!). So the available drive current is rather low, so I concur with the others - they are probably just selected for high current gain.

Pass element leakage doesn't play a role here, the error amp compensates it. The matching between the two is not very relevant, since they are biased to some cross-over current anyway.

I personally find the clarity and "well-thoughtness" of the schematic very nice. No surprise that the 225 is a lab classic. What surprises me a bit is that this is actually a voltage source controlled to output current. I would have expected a current-controlled current source.
motocoder:

--- Quote from: dom0 on July 13, 2015, 06:17:17 pm ---
--- Quote from: johansen on May 22, 2015, 09:37:41 am ---they are likely matched hfe npn/pnp pairs

replacement should not require an exact match, but ymmv.

--- End quote ---

The electronics of the 225 are quite nice, actually.

Output stage: on the positive side you have your usual Darlington (alas - directly driven from the voltage gain stage of the error amp), on the negative it's a Sziklai pair. Both have additional instantaneous current limiting (Q103 / Q106 ... and switched with range!). So the available drive current is rather low, so I concur with the others - they are probably just selected for high current gain.

Pass element leakage doesn't play a role here, the error amp compensates it. The matching between the two is not very relevant, since they are biased to some cross-over current anyway.

I personally find the clarity and "well-thoughtness" of the schematic very nice. No surprise that the 225 is a lab classic. What surprises me a bit is that this is actually a voltage source controlled to output current. I would have expected a current-controlled current source.

--- End quote ---

I wasn't able to measure the gain on this supply's output transistors because they were both blown. I have another 225, the one that I am selling, but I didn't want to remove it's output transistors as I was planning on selling it and wanted to leave it as untouched as possible. However, I found with repair of another, similar, current source (HP 6177C), that the output transistors were relatively high gain (hFE > 120). I wasn't able to source transistors with gain that high, so perhaps the replacements I am using here are not as high gain. Nonetheless, the performance of my modified 225 is quite good. In the noise/ripple department, it exceeds that of the unmodified version now. This is mostly attributable to the improved circuit I built for the virtual ground on the +/-12V supply.

I agree that the schematic on the Keithley is good. In fact, I would say that the whole manual is great. I love that they include a section on theory of operation.

If you're interested, take a look at the HP 6177C/6181C design. The manual for that also has a very good theory of operation and a decent schematic (you need to pay the money for a good scan, the free scans are awful). Many aspects of the design are similar. For example, there is a -1V reference voltage (HP calls it the "guard voltage") that is generated via one set of circuitry, and that is compared against a high-side voltage drop across sense resistors. All the control circuitry "floats" on top of the high side voltage right before the current sense resistors. The HP supply is unipolar, has a different circuit for the voltage limit, has an active circuit to shunt unused current, and has a meter for adjustment instead of the KV decade switches. Also, the HP surrounds the output conductor with the "guard voltage" as a way of minimizing leakage current. I am not sure how much not doing that impacts the Keithley, as I really don't have a good way to measure things at the very low currents necessary to see the impacts there.


Ckid:
I just want to add to this Keithley 225 repair thread. I am repairing a similar failure-mode Keithley 225 with similarly failed parts, but a different original cause to the failure.

In mine, like in this thread, R108, R107, R103, R102 are burned. Someone attempted repair by replacing a couple of these resistors. Further checking finds Q101 and Q105 are bad.

The cause of these failures is the plastic on the shaft insulator of the rotary switch has deteriorated and cracked. The insulator shorted the switch shaft to the case resulting in a similar set of failed parts. The insulator is a Millen 39016 which Keithley modified by drilling out the plastic center for the fine-adjust inner shaft.

Millen is out-of-business but I found the part on eBay.  I'm not finished with repairs yet, but this thread will be an excellent help completing the repair.
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