It took MUCH longer than I hoped, but here is a way how to replace the entire probe tip and cable. Of course, the easiest way to do it was to adapt some existing probe. So I scoured my workplace and evaulated 5 or 6 different probes, mostly oscilloscope ones. Eventually, I decided to use Hantek T3100, because
-it can be disassembled and reassembled easily,
-it's designed to withstand high voltages,
-it's dirt-cheap,
-yet has suprisingly good frequency characteristics.
This is also why
I posted T3100 teardown elsewhere on this forum, I recommend checking it out before you attempt the replacement. What particularly surprised me is that its coaxial cable has resistive coating over the center insulator, along with the usual wire shielding. I've never seen anything like it, but I guess it serves to dampen signal reflections at high frequencies. T3100 also has some drawbacks, but more about that later.
In any case, you need to replace the original 99M resistor with 5M one, and it must withstand at least 600 VDC (half of the N2772A rating). At first, I tried free-hanging construction similar to what I originally found inside T3100, but I had to abandon it, because it required high-voltage miniature axial THT capacitors which are hard to get nowadays. So I decided to emulate the SMD design of the original N2772A tips. I'm attaching Eagle 7.x files, so you can easily make your own PCBs. I'm also attaching Gerber files for PCBway, but of course every manufacturer requires a bit different format. The PCB resembles the original one, but employs 0805 components and has larger pads for wires. I originally planned to use 1206 components, but the resulting PCB was too big to fit into the T3100 probe cavity. The compensation capacitors are a bit smaller than in the original tip, I settled for 8p2/5% after a few experiments. I also made a version with 1% capacitors, but there was no discernible benefit. Likewise I tried a few different values (from 220R to 1k) of the tip "anti-ringing" resistor, but again, I saw no improvement. In the end I arbitrarily chose 560R because it was between 511R (in N2772A) and 680R (in T3100). Total assembly length must be under 78 mm, otherwise it won't fit into the probe cavity. The length of the "tip wire" is 43 mm and the "clamp wire" 6 mm in the photos. I used 0.6 mm diameter wire, which is similar to the original T3100 design. Don't forget to put insulation sleeve over the PCB and "tip wire", so it wouldn't short-circuit if it touches the cavity wall. The rest should be obvious from the photos.
Important! Make absolutely sure you use 1M resistors and 8p2 capacitors rated at least to 150 V, because probe safety depends on them! Which also brings me to the main T3100 drawback - the ground shielding is easily accessible around its tip, which is against N2772A safety design.
If you need to use the probe at high voltages, it's absolutely imperative you do following:1. Permanently glue on the retractable cover with hook. This will cover the exposed ground sleeve near the tip. Be careful not to glue the internal retract mechanism. Alternatively, you could machine or 3D print some thick insulating sleeve, perhaps even with banana adapter, but I will leave that to you. In any case, heat-shrink tube will simply not do here, unless you wish to meet the Reaper prematurely (or burn your scope).
2. The grounding cable with alligator clip is not detachable. You must snip it away and then remove all traces of its grounding wire as deep into the probe's body as you can. I drilled it away with a new (sharp) 2 mm drill bit. The wire is not perpendicular to the probe's axis, so I had to carefully drill it away piece by piece. Then you need to fill the resulting cavity with some insulating material. I used Spinner Plast2000 sealing compound I had left over from some other project, but I guess pretty much anything will suffice, as long as it won't fall out over time. Be sure not to leave any air pockets!
3. T3100 comes only in gray color, which is a complication if you need to replace the positive (red) probe. The outer plastic is soft, so any paint probably won't last very long. If I ever need it, my plan is to buy red heat shrink tubing and put it over the probe.
If you've seen some probe that doesn't have these drawbacks, please let me know (we have several N2772As in my workplace and I suspect some of them will need tip replacement soon).
The most laborious part is to connect the new coaxial cable into N2772A, because it's thinner than the original one. It took me 3 tries to get everything right, so I've prepared step-by-step instructions:
1. Pull the T3100 BNC pod cable grommet on and then the pod cover. Cut the cable as close to the pod as possible.
2. Slide the cable through hole in the N2772A plastic cap.
3. Slide on the original N2772A cable grommet.
4. Slide on two or three 1 cm long pieces of heat-shrink tubing, 4 or 5 mm diameter. They will serve to increase total diameter of the cable in step 11.
5. Carefully remove about 40 mm of outer black insulation. Be sure to cleanly cut around its circumference, otherwise it will tear during step 10. Try not to cut into the shielding wires underneath.
6. Unbraid the shielding wires. I'm usually using a thick needle for this.
7. Shorten the shielding wires to about 15 mm.
8. Remove the resistive coating from the white center insulator. Leave about 10 mm near where the shielding ends.
9. Slide the original brass ferrule over the resistive coating - it's a tight fit, but it should be possible.
10. Use a hot air gun to soften the outer black insulation and then use small pliers to push the ferrule inside the cable. Don't set too high temperature, the insulation melts easily.
11. Move the shielding wires away and slide the heat-shink tube from step 4 so it would be aligned with the end of the shielding/ferrule. Then shrink it.
12. Bend the shielding wires backwards over the heat-shrink tube, like it was on the original cable.
13. Bend open the aluminium clip and slide it over the wires, then close it tightly with pliers. Try to pull on the cable, it shouldn't slip out from the clip. If it slips, go back to step 11 and apply another piece of tubing.
14. Very carefully strip about 7 mm of the white center insulator. It's rather weak, so you can cut it lightly around the circumference and then tear it away. It's absolutely critical to not even touch the center conductor with knife, otherwise it will almost immediately break.
15. The white insulator is too thin for connectors on N2772A PCB, the cable would fall out. Use about 5 mm long piece of 2 mm diameter heat-shrink tubing to thicken it.
16. Carefully bend the center conductor over the tubing and test if the connector can hold it. If it still has tendency to fall out, it will be probably better to use hot glue than to add another tubing.