| Electronics > Repair |
| Tektronix TCP202 current probe repair - Schematic and suggestions needed |
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| vtp:
Just digging the notes again, TCP202 head HALL-element operating voltage comes from +-5V through 220R resistors in series with both voltages. |
| lukier:
--- Quote from: vtp on July 11, 2018, 06:16:32 pm ---My notes are four pages written and hand drawn schematics and I do not have scans of those. Not that they would do much good as the whole thing is a bit scattered around those pages. But the operating principle is similar to other current probes. There is the head with its HALL-element and coil in the same magnetic circuit. HALL-element signal is differential input to the OP27 with 1k60s as inputs and 20k0s to GND/output. Then it goes trough 475R to one JFET that can disconnect the OP27 during degauss. If the sensed signal is allowed to continue it goes to input of 1/2 1458 which has PCB68/69 discrete output stage driving the coil in the probe head. This is used to compensate the field sensed with HALL-element. The other end of the head coil is filtered and brought out then directly to the probe output BNC-connector (at the bottom board). So the coil is in series with the PA and probe output. Gain of the PA looks to be set with 10k0/2k00 resistor divider and it is non-inverting. OP27 output is brought to the 1458 inverting input trough the said 475R and the JFET. The rest of the probe electronics are degaussing oscillator and power supplies. It uses +-15V and +-5V from the tekprobe interface. TCP202 head differs from others regarding few parameters, the coil DC-resistance is higher at about 6.5 ohms to make driving it with a smaller amplifier easier. A6302 and TCP312 coil DC-resistances seem to be around 2 ohms. These are not very accurate measurements, they were taken with a Fluke 87 from the element. --- End quote --- Thanks! This is very appreciated. Now I get a better idea what's inside TCP202. It seems radically simplified from AM503A schematics (the most modern amplifier schematics available). There hall signal feeds MC34081P via 475 Ohm resistors, with non-inverting input grounded with 47.5K and inverting having 47.5K in the feedback loop + some RC filtering. Output from that goes via JFET (degauss disconnect) and feeds another MC34081P that drives multi stage totem-pole amplifier, with darlingtons doing the final push into the probe transformer. On both opamps there are input signals to adjust the offset, HALL_PREAMP_DC_OFS and PWR_AMP_DC_OFS, while TCP202 has only one knob. Degauss signal comes from amplified DAC output. In AM503A the second transformer output goes to the attenuatior and the rest of these duplicated scope frontend stuff. I'm glad to hear that in TCP202 it goes straight to the scope, without any termination inside the probe or similar stuff. What confused me in AM503A is that there is this FEEDBACK_LOOP signal that comes from Tek's custom made attenuator module into the line bettwen Hall OpAmp and PA OpAmp, adding something there for some reason. Now I might try some DIY, but still very careful, especially with this power amplifier output and fragile transformer :) |
| vtp:
--- Quote from: lukier on July 11, 2018, 06:49:59 pm ---In AM503A the second transformer output goes to the attenuatior and the rest of these duplicated scope frontend stuff. I'm glad to hear that in TCP202 it goes straight to the scope, without any termination inside the probe or similar stuff. --- End quote --- I was not very clear about this. There is some filtering and termination and even one trimmer in the probe box between coil output and BNC connector. So the coil comes to the probe box in a coax cable, goes through some filtering and four parallel termination resistors one of which has a trimmer in series probably to adjust the output amplitude/gain. Let me try to draw: --- Code: --- ---R--- | | ------L----------L---------L------------------------------ output to BNC/scope | | | | | | | | R R R R R R R R+trimmer | | | | | | | | ------ ------ -----------------gnd | | C C | | - gnd - --- End code --- I do not have component values for those except that the last L in chain is 10uH and has two 68R1 resistors in parallel to it (only one drawn above). Left is input from the coil. |
| lukier:
Maybe it is similar to the termination used in Tek's AC probes, like shown here: http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf (pdf page 69 and 70) |
| AndersJ:
Hi all, I have replaced the visually obviously damaged parts. After that the probe behaviour improved, but it was not fully functional. The "undamaged" OP27 was uncomfortably warm so I replaced it too. After that, the probe seems to work. --- Quote ---To the OP, I do not quite understand how someone has managed to blow up those two JFETs and the 1458. --- End quote --- During the repair i noticed that the grey collar that mechanically locks the probe in place, was rotated 180 degrees. This means the probe will connect "backwards", possibly reversing power inputs, thus blowing the probe. This is normally not mechanically doable but with some ingenuity even the impossible might have happened. I don't know if this is the right explanation, but efter repairing it, I do not want to try that. Thanks for all help! /Anders J |
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