(FIXED) Keithley 225 (current source) repair

[cncjerry]

Good job with q104.  Maybe if you had the correct parts it would work now.  The transistor shouldn't get too hot unless it has a heatsink then I think you should be able to hold your finger on it. I went back a looked at your pictures.  Are those paper caps still good? Did you hang a scope or meter on the PS?  I've replaced those in most of my old radio gear as they might show the right capacitance but leak like hell.

I can make a coupler, new shaft, ect for that switch coupled pot if you get in a jam.  Cost of shipping only.

[motocoder]

Good job with q104.  Maybe if you had the correct parts it would work now.  The transistor shouldn't get too hot unless it has a heatsink then I think you should be able to hold your finger on it. I went back a looked at your pictures.  Are those paper caps still good? Did you hang a scope or meter on the PS?  I've replaced those in most of my old radio gear as they might show the right capacitance but leak like hell.

I can make a coupler, new shaft, ect for that switch coupled pot if you get in a jam.  Cost of shipping only.

I think the caps are good, but I may replace them anyway once I have the other issues resolved.

I spent about an hour troubleshooting it early this morning before I had to head out. I think I understand in part why it's not 100% working, but I don't understand the root cause  yet. The negative polarity side of the amp is not shutting off when the input selector is set to a positive voltage. The positive side polarity working fine. This is why Q104 is heating, why the bias current is out of range and you can't adjust it, and why it isn't able to hit the full positive output voltage (the negative side is dragging it down). I want to retake some measurements and I'll post those later tonight.

Regarding the coupler, thank you very much. I might ask your advice on some related mechanical issues (I am just awful with  mechanical stuff), but the existing shaft and coupler seemed to work with the potentiometer I lucked into at the local electronics shop. The only loss over the OEM equipment seems to be that this pot doesn't have a built-in switch like the original.

[motocoder]

To determine why Q105 is not shutting off, I measured the current going into Q105's base by measuring the voltage across R105 and dividing by it's value (180 ohms). With the output current set to a large negative value, it's 1mA. With the output set to a large positive value, it's as high as 0.47mA!  The transistor I selected for Q105 had a beta of around 48, so that's 48mA flowing when this thing is off - unacceptable!

Where is this current coming from? Q102, which I've replaced with an NTE154, has a typical value for emitter cutoff current of 1nA. Q104, which has been replaced with an NTE397, if the NTE "data shit" is to be believed, has a value for collector cutoff current of around 50uA. We're clearly well above that. Where is the current coming from? Is Q104 not performing up to snuff? It  definitely should be cutoff hard; Veb on this PNP transistor when output is set to the positive polarity is -0.67.

I've removed C101, so it's not leakage from that. The schematic shows a cap across the base-collector of Q104. I can't read the designator for the cap on the schematic (C110?), and I can't find any caps in that area of the PCB that might be this.

It's a mystery.

[motocoder]

Ok, I think I finally have it sorted out. The amplifier stage is really sensitive to the characteristics of Q104.

Now when I had it working great for a few minutes the other day - before the big flame-out of R105 - I was using a 2N5416S transistor that I thought was going to be a really good match. Those transistors are quite pricey, so after the flame-out, I replaced it with one of those fell off the boat" (NTE) parts, an NTE397. The NTE transistor apparently has really bad Icbo leakage at these high voltages. So once I was pretty confident this was the source of my over-current problems in the negative channel, I replaced it with the second 2N5416S that I had on hand. I've put some heatsink compound between it and its heatsink, and I'm carefully watching its temperature while I run at max output. I'm hoping the first burnout of this part was just a fluke, or a side effect of the resistor problem (see below).

The second thing I did was to replace the output driver transistors with ones with lower Hfe. I'm using the same NTE part, an NTE124, just two I selected that have Hfe of 17 and 23. Based on the specs for the OEM transistor, I think this will be a bit closer, and I am hoping to get the amp gain back down to closer to the original amount, so that these oscillations will go away. If not, we'll have to look at some additional caps to frequency compensate it.

Oh, also forgot to mention earlier that R104 was bad as well. It's tucked away behind the big yellow filter caps, so I didn't initially check it. When I did, I noticed it was reading about 30% high, so I went to replace it. As soon as I touched it with the hemostats, it crumbled.

I've had the whole thing assembled, semi-calibrated, and running now for about an hour, with no issues. I think next up will be the electro-mechanical clean-up - clean the PCB, clean the switch contacts with De-Oxit, etc.

[motocoder]

Looks like you are correct about those caps (Edit: caps replaced). Tons of rectified 60 Hz ripple bleeding through. (Edit: This was mostly common mode noise being picked up by my o-scope differential probe)

[cncjerry]

Are you not able to get the actual parts?  For my keithley parts I found a place called something like westfloridasupply.  They shipped the old transistors I needed pretty quickly.

[motocoder]

Are you not able to get the actual parts?  For my keithley parts I found a place called something like westfloridasupply.  They shipped the old transistors I needed pretty quickly.

I emailed Keithley before I started the repair - still no reply from them. If you know of a place that stocks the parts, can you give me a link or phone number? I searched for "westfloridasupply", but all I found was some company selling trash bags.

BTW - every thing is working now, except for the power supply ripple and some oscillation. Swapping the big yellow caps out for some Nichicon 220uF helped some, but there's still what I would describe as a ridiculous amount of 60/120Hz noise, and a lower level oscillation.

[motocoder]

There is more noise on the output then I would like, but it's not the dire situation I posted earlier. A lot of that was noise I was picking up on my differential probe.

[motocoder]

After the clean and De-Oxit on the switch contacts, it's pretty well. Output noise is still around 60mV. I'll poke around with it and see if I can figure out the source there, but in general I am pretty happy with it.

I'll prepare some notes and get a scan of my annotated schematic, so that hopefully someone else that buys one of these turkeys doesn't have to go through as much effort on the repair.

(Late breaking update on noise)
I soldered a 1uF film cap on the underside of the board on the +/- 130V transformer secondary. That dropped the mains noise way down. Measuring on my scope, into a 50 ohm terminator (not the one inside the scope - too scary) the peak-to-peak noise is around 14mV. RMS is 240uV.  Spec in the manual says to measure it using a meter into a 10 ohm load. I can't even measure anything using that method.

[cncjerry]

https://www.westfloridacomponents.

They had the mosfets I needed for my 228.  I only dealt with them that one time.

I'm thinking I'll just buy another 228 though Keithley made a lot of current and voltage sources.

Glad tee someone other than me stick with a difficult restoration.

[Vgkid]

Good job 

[motocoder]

https://www.westfloridacomponents.

They had the mosfets I needed for my 228.  I only dealt with them that one time.

I'm thinking I'll just buy another 228 though Keithley made a lot of current and voltage sources.

Glad tee someone other than me stick with a difficult restoration.

They do have some interesting, older, parts, but I don't see any match for the challenging parts in that 225. I did find that Central Semiconductor has a part, 2N3585, which is pretty much an exact match for the output transistors. Mouser carries those if you need them, although I think the NTE parts I'm using are fine. It's really just Q104 that is a challenge. Even now that I have it working, it is the one part that heats up quite a bit under operation - up to 80C when I was watching it, and that was with the case open.

I don't think I'll be buying anything else from Keithley. This thing is a somewhat fragile design. Compared to the HP precision and Power Designs power supplies, it doesn't stack up very well.

[motocoder]

I've scanned in the main schematic page with my annotations. I also added some notes and a list of replacement parts that I found to work. Hopefully this will save someone some time.

I have a higher res version in Photoshop format, with the annotations on separate layers. However, it is too big to upload here. PM me if you want a copy.

[TiN]

I would be happy to rehost it on my site in Keithley section, if you may upload here.
 

[motocoder]

I would be happy to rehost it on my site in Keithley section, if you may upload here.
 

Thanks. They are in a folder called Keithley_225.

[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! 

Keep up good work.

[motocoder]

Added to Keithley section, many thanks! 

Keep up good work.

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.

[motocoder]

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

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]

they are likely matched hfe npn/pnp pairs

replacement should not require an exact match, but ymmv.

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]

they are likely matched hfe npn/pnp pairs

replacement should not require an exact match, but ymmv.

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.

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.