Electronics > Repair
Tektronix TCP202 current probe repair - Schematic and suggestions needed
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vtp:

--- Quote from: Mrnt on December 06, 2020, 07:53:05 pm ---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?

--- End quote ---

Perhaps checking if the OP27 is ok? There should be about zero volts between plus and minus inputs. You can also check if it gets railed in operation. In any case since only half of the signal gets amplified properly your fault most likely resides in the amplifier circuit.

The attached may help.

Mrnt:
Thanks for all the help! I should get some more time to work on it in a day or two.

Meantime another possible symptom - since I have not used one before I am wondering what typically appears on the scope when you hit the degauss button? I have found that I need to degauss it regularly (and somewhat tweak the balance fine tune) to get it be close 0 when not measuring. However I also noticed that when I press the degauss button there is a fairly large ac like waveform briefly displayed - is that normal? I thought I read somewhere in this thread that the amplifier and hence scope input was supposed to get switched away from the sensor when degaussing happened.
Tantratron:

--- Quote from: Mrnt on December 08, 2020, 06:13:39 am ---Meantime another possible symptom - since I have not used one before I am wondering what typically appears on the scope when you hit the degauss button? I have found that I need to degauss it regularly (and somewhat tweak the balance fine tune) to get it be close 0 when not measuring. However I also noticed that when I press the degauss button there is a fairly large ac like waveform briefly displayed - is that normal? I thought I read somewhere in this thread that the amplifier and hence scope input was supposed to get switched away from the sensor when degaussing happened.

--- End quote ---
It is normal and healthy sign, pressing the degauss button will generate a low frequency decay signal... attached extract of service manual of the AM503A where the TCP202 is roughly same theory of operation

DEGAUSS_SIG Simulates a 160 Hz pseudo exponentially decaying sine wave function during the current probe degauss cycle. DEGAUSS_SIG is generated at the VA and VH outputs of U370, which are also used to coarse  and fine adjust the HALL_PREAMP_DC_OFS control line.
This stair stepped waveform is first filtered, to remove high frequency com  ponents, and then amplified. This signal is initiated as part of the probe degauss/autobalance routine.
Figure 3 10 shows a rough view of the degauss waveform before and after filtering. Each burst contains approximately 256 cycles and the amplitude of the filtered waveform (at pin 1 of U353A) is approximately 3 V. The degauss signal goes to the power amplifier.
Mrnt:
Thank you so much @vtp for the schematic and layout! It turned out to be the BCP69 transistor at the output of the amplifier board, however it took a long time to figure that out because I started with the OP27 which generated an asymmetric output for equal +ve and -ve DC input currents at the probe (which confused me greatly), and then worked my way through to the end.

If I understand correctly, the output of the amplifier board passes through the coil on the probe and then on out to the input of the scope, so the magnetic core of the scope forms a feedback loop from the coil to the hall detector and on to the input of the amplifier board. Presumably because the output of the amplifier board was only ever generating a positive output then that was why the OP27 generated the asymmetric output.

I'm attaching a schematic of the board inside the probe in case in helps someone else.
MegaVolt:
Unfortunately, I'm out of luck. I bought a TCP202 on ebay hoping to get it repair. Contacts from the output of the hall sensor have infinite resistance to power supply and to each other. :(

Has anyone tried ordering the transformer separately?
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