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| Tektronix TCP202 current probe repair - Schematic and suggestions needed |
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| Mrnt:
I picked up a used TCP202 and I am not sure if this is me being thick, but it only seems to measure positive DC current ie it seems to accurately measure dc when flowing in the direction of the arrow on the probe but if I reverse the current direction, the probe only measures approx -700mA no matter what current is measured. Is this normal? I have not tried AC yet and nothing appears scorched inside the head. |
| MarkL:
The probe should have a symmetric output for AC and DC signals. I don't think it's likely to be something in the probe head. Maybe one side of the Hall driver is not working. You could start by checking the power rails in the comp box. A convenient place is on the little white ribbon that connects the TekProbe connector to the main circuit board. The ribbon pads on the circuit board are numbered starting from the end where the probe cable enters: 1: GND 2: -15V 3: -5V 4: +15V 5: +5V Taking a look at a high frequency AC signal (like >1MHz sine) could also be enlightening, since the Hall driver should be out of the picture at that point. EDIT: Fixed description for pin 1. |
| Mrnt:
Thanks Mark for your prey response! It look like the voltage while not exactly a you listed are within the +\- 2% pc for the Tekprobe connections: At the white ribbon 0 -14.9 -5.16 14.82 4.96 Trying at 1.5Mhz AC with approx 10v pp into 50 ohm the current waveform was fairly smooth, correct amplitude and symmetric about 0 mA. Unfortunately I do not have a higher current source. I notice that there is a different opamp on this board from the other pictures I have seen - an OP275. Any thought as what to look at next? |
| MarkL:
Those voltages are fine. It was worth checking to make sure one of the rails wasn't completely missing. My older probe has OP27 and MC1458, but the newer one has OP27 and OP275 like yours. Have you done the coarse balance procedure on the underside of the probe? What kind of a range do you get for the coarse adjustment? Does it also bottom out at -700mA? For comparison, my newer TCP202 can be adjusted from -7.9A to +6.1A (or in volts: -0.79V to +0.61 into 50R). My older probe has a much lower range of -1.6A to +1.1A. If it does bottom out, I think a general approach would be to start working backwards from the Hall power amplifier which includes 1/2 of OP275 and the BCP68/69 SOT-223 driver pair (see vtp's earlier post). One approach would be to vary the coarse adjustment back and forth to find who's responsible for the amplifier falling out of operating range. Possible probe points would be the emitter and base of the BCP68/69 pair, and examining the OP275 output and inverting and non-inverting inputs. A few other simple voltage checks might also be in order. On the white connector on the main board, they are numbered 1 to 7 (same order as the ribbon connector): 1 - 0V (Black to bottom board) 2 - Varies with DC signal/offset (White to bottom board) 3 - Hall power -2V 4 - Hall power +2V 5 - Hall 1 0.206V 6 - Hall 2 0.259V 7 - Center tap on course adj. pot, varies +/-2V 5 and 6 will vary depending on the specific Hall device, but should be close to each other. I don't have any specific suspicions in mind related to this connector; just gathering data. Without a schematic and without resorting to shotgun parts replacement, you're probably going to need to do some reverse engineering. I haven't done any of that for the TCP202, but I can help with voltage comparisons from a working probe. |
| Tantratron:
--- Quote from: Mrnt on December 06, 2020, 07:16:53 am ---I picked up a used TCP202 and I am not sure if this is me being thick, but it only seems to measure positive DC current ie it seems to accurately measure dc when flowing in the direction of the arrow on the probe but if I reverse the current direction, the probe only measures approx -700mA no matter what current is measured. Is this normal? I have not tried AC yet and nothing appears scorched inside the head. --- End quote --- Have you press the degauss button as a start ? |
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