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| Tek p5205 HV differential probe teardown. BTW, what are the red and brown wires? |
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| walter76:
Very useful post, it helped a lot in fixing a P5205 that was scrapped. The probe was marked as no-good and its showed a huge offest (around -2V) that could not be recovered by acting on the adjusting trimmer. It took some time to discover that the switches of the DG444 (U150) were no longer isolated each other: acting on R160 & R165 was partially successful in recovering the offset, until the scope suddenly switched to another scale! I removed U150 (obsolete) and I used a small 12V relais I found in my junk box: one switch to short R145, the other one to select between R160 & R165 according to the x50/x500 scale. Just move R155 to be in series with the wipe of R165, then most of the lands for U150's pins can be conveniently used to connect the relais. I hope this could help somebody else having the same issue. Walter |
| Dbldutch:
Just for kicks, while trying to understand how the various differential probes work, I used LTspice to model the input attenuation using the schematic provided by abulafia in an earlier post. Interesting to see that the higher frequency response is probably used or can be used to extend the bandwidth. I'm wondering why Tek used two identical 18R21/10-90pF in parallel??? Enjoy! |
| Hydron:
At anything higher than a few MHz the model would need to include input cable parasitics (L&C) and source impedance to get an accurate model of the frequency response. As the leads can move around (thus changing the parasitics) the response will be all over the place well before 100MHz, let alone 1GHz. |
| Dbldutch:
Thank you Hydron for the comments, and I agree, but that was not my intention. I was interested only in the rather unusual front-end section and wanted to get a feel for the three things that stood out for me in this design. Tek engineers generally do that with good reason, and I wanted to try to understand why. First the relatively low parallel HF capacitors of 2.2pF, (only 1.1pF in total) which makes the front-end much more sensitive to parasitic or stray capacitance. In all the other designs I have found a minimum of 4pF in total, which is about a factor 4 more and so is 40x the typical 0.1pF for parasitic or stray's instead of only about 10x more. Second, the attenuation factor due to the 4M vs 6K81, which drives the rather odd LF compensation circuit with the strange, for me at least, dual parallel trimming section with R/C networks. In simulating the circuit, I found that the value of the LF 60.5pF I arrived at is extremely sensitive (I also used MC analysis), and the whole front-end is not flat. All the other attenuation designs I looked at can be easily calculated, and when simulated are flat. Third, the interesting circuit to adjust the CMR (2x 100K and a 25K trimmer). In all the other designs I found, the engineers use a 100-500 Ohm trimmer between ground combined with the two attenuation resistors (here the 6K81), which not only adjusts the CMR, but also the attenuation factor. I like the Tek method better, so I'll use that for my own design. I'm looking forward to get more comments about of this interesting circuit Enjoy! Paul |
| Tantratron:
This thread is quite old but maybe since then some of you do have a document explaining how to service manual or calibrate the P5205 which would be similar to the P5210 which I'm discussing in this thread https://www.eevblog.com/forum/testgear/repair-tektronix-p5210-high-voltage-differential-probe/msg3220840/#msg3220840 Thank you in advance, Albert |
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