Inside Tektronix P6205

[siggi]

On a lark, I picked up a lot of 5 "for parts" P6205 probes. These are 750MHz active probes, with a really nice form factor. On receipt, they all behave the same way, railed high with not the least hint of a signal coming through. So, for my money, I guess I got some TekProbe connection hardware and a bit of coax .

Out of curiosity I disassembled one of them to see whether I'd perhaps be able to repair it.
The construction of those is pretty neat. The scope end contains a small PCB with a smattering of chips on it. My understanding is that this has an I2C ROM, which declares the probe type to the scope. The coax from the business end passes straight through to the BNC connector of the scope, using a connector that looks very much like the Peltola connectors used in older Tek scopes.

The business end of the probe is a little more interesting. All the electronics are contained inside a shielding tube, which is crimped around the coax. The probe tip and the plastic retaining center both screw off, exposing a small PCB inside the tube. The PCB is soldered to a gold or gold-plated retaining bracket, which in turn is soldered to the shielding tube. With a bit of solder braid, this is easily undone, and with a bit of force to relax the crimp, the assembly can be coaxed out of the shielding tube.

The construction of the PCB looks pretty interesting, as it contains a network of laser-trimmed resistors and a capacitor. The tip resistor measures around 800K, and this connects to the laser-trimmed capacitor. This is then followed by an approximately 24Ohm series resistance, and then a 200K resistor to ground, so this is a 5X attenuation network at the tip, which fits with the +-10V range of the probe and the +-5V supplies it uses.

Here's a closeup of the tip resistor and the network it connects to.


The real magic happens in the bare-die IC, which is wire-bonded to the PCB.


The backside of the PCB has this network, which I've neglected to measure so far.


Once I saw the construction of the probe end, I gave up hope on repairing these. However, if you look at the die, I believe these are repairable, once, though you'll need a steady hand and a wedge bonding machine. It looks to me that the IC contains a second, duplicate amplifier, presumably for the/a differential version of the probe.


Now to find someone with a wedge bonder and time on their hands...

Please forgive the image quality, this is shot with one of those $20 eBay microscopes, and it's really hard to get angles that don't have glare from the LED ring light in the tip of the microscope.

[VintageNut]

Very cool teardown. I have a P6045 that I bought many years ago including power supply. It even has the foam lined carry case.

[siggi]

Here's an "exploded" view of the business end of the probe. The coax is double-shielded, with the red and black power wires in between the two shields.

And here's a picture of the scope end.

[snoopy]

Hi

I've got a P6205 probe that is kind of half working but only responds to a 50MHz LVTTL clock signal without showing the DC component and of reduced amplitude to what it should be. However it appears to be blind to the 1KHz test signal showing only spikes at the transitions ie no low frequency response !!

Sometimes with the 50MHz clock signal the DC offset intermittently comes back up with the right p-p amplitude but the square wave is severely rounded. It's as though there is a bad ground connection between the probe and the Tek-probe scope connection.

The other P6205 probe that I have works fine !

Regards

[robert_]

I've got a P6205 probe that is kind of half working but only responds to a 50MHz LVTTL clock signal without showing the DC component and of reduced amplitude to what it should be. However it appears to be blind to the 1KHz test signal showing only spikes at the transitions ie no low frequency response !!

I had a similar Tek FET Probe just like that (got it for free for this reason). Wiggling the cable at the probe end would sometimes restore proper operation.
The inner conductor of the coax cable was broken around the end of the strain relief, so i shortened the cable (no problem since it is 50Ohm Coax, not the special kind of cable used on passive probes) and re-attached it to the ceramic pcb and reassembled the whole probe. Works fine. Just need to get around to finish converting it to ProBus to actually use it on my main scope.
Just be really careful not to damage any wirebond or lose the tiny conductive rubber cylinder that is inside the transparent probe tip insulator and makes contact to the probe PCB. That would be another possible location of an intermittent contact, although this one would have a longer time constant than a broken coax.

[tautech]

I've got a P6205 probe that is kind of half working but only responds to a 50MHz LVTTL clock signal without showing the DC component and of reduced amplitude to what it should be. However it appears to be blind to the 1KHz test signal showing only spikes at the transitions ie no low frequency response !!

I had a similar Tek FET Probe just like that (got it for free for this reason). Wiggling the cable at the probe end would sometimes restore proper operation.
The inner conductor of the coax cable was broken around the end of the strain relief, so i shortened the cable (no problem since it is 50Ohm Coax, not the special kind of cable used on passive probes) and re-attached it to the ceramic pcb and reassembled the whole probe. Works fine. Just need to get around to finish converting it to ProBus to actually use it on my main scope.
Just be really careful not to damage any wirebond or lose the tiny conductive rubber cylinder that is inside the transparent probe tip insulator and makes contact to the probe PCB. That would be another possible location of an intermittent contact, although this one would have a longer time constant than a broken coax.

+1
It's sometimes worth taking a punt on a non-working probe as just as you say it could be just broken coax.

This was the case in a Tek P6021 current probe I got years ago but rather than broken at the probe end this one was just outside the strain relief boot on the BNC end.
Either way, easy fix.

[snoopy]

I've got a P6205 probe that is kind of half working but only responds to a 50MHz LVTTL clock signal without showing the DC component and of reduced amplitude to what it should be. However it appears to be blind to the 1KHz test signal showing only spikes at the transitions ie no low frequency response !!

I had a similar Tek FET Probe just like that (got it for free for this reason). Wiggling the cable at the probe end would sometimes restore proper operation.
The inner conductor of the coax cable was broken around the end of the strain relief, so i shortened the cable (no problem since it is 50Ohm Coax, not the special kind of cable used on passive probes) and re-attached it to the ceramic pcb and reassembled the whole probe. Works fine. Just need to get around to finish converting it to ProBus to actually use it on my main scope.
Just be really careful not to damage any wirebond or lose the tiny conductive rubber cylinder that is inside the transparent probe tip insulator and makes contact to the probe PCB. That would be another possible location of an intermittent contact, although this one would have a longer time constant than a broken coax.

Yes I have some later probes P6245 with broken cables at the probe end so it is a common problem with them. I will have to try and fix those as well.

Is it difficult to take the probe apart ? Any tricks to it ?

regards
david

[robert_]

For disassembly, you need to remove the tip (as said, dont lose that tiny rubber contact piece, could be stuck to the ceramic board) and plastic cover, then remove the crimp on the cable shield (you might be able to do so without breaking the sleeve, i did not and had to turn a replacement part) and then carefully unsolder the ground/thermal connection to pull the board out.

[Neomys Sapiens]

BTW, I'm looking for a TekProbeII contact head with all positions fitted. So if anyone does not succeed with a repair of such a probe, please let me know!

[robert_]

I think i still have the TekProbe connector left over from my P5205 differential probe somewhere...

[Neomys Sapiens]

Nice! If you find it, send me a PM with your expectations. That would enable me to build a TekProbe breakout box!

[ozkarah]

 may be you would like to write the updates on the topic. Were you able to fix the probes?