| Electronics > Repair |
| (FIXED) Keithley 225 (current source) repair |
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| 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:
--- Quote from: cncjerry on June 06, 2015, 10:04:16 am ---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. --- End quote --- 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) |
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