SUCCESS: Tektronix P6247 1GHz DiffProbe Teardown and Repair - Page 1
Forum section: electronics / testgear

author: IDEngineer
author_rank: Super Contributor
timestamp: September 30, 2019, 04:17:47 am
content: I'm troubleshooting a P6247, went looking for internals info, and didn't find anything. So as a start I'm posting some photos.I'm running this probe through a Tek 1103 power supply since my Rigol DS4000 scope doesn't have native TekPower BNC's. Everything checks out fine on the 1103 - all the supplies are at their proper voltages, the offset voltage (when enabled) varies as you turn the knobs, etc. Both channels operate identically. Everything suggests that the 1103 is working as it should.With the probe disconnected from the 1103, the scope shows no DC nor AC levels. When I plug in the P6247 probe, the scope immediately displays a -1VDC offset, and nothing else. Doesn't matter if the input pins on the probe are shorted together, connected to a signal, or open. Moving its three switches (DC Reject, Bandwidth, and Attenuation) causes a brief glitch on the screen, as you might expect when changing settings, but then we're back to the -1VDC level. Remove the P6247 and the scope goes back to a null trace. The behavior is identical on all four channels of the scope and both channels of the 1103.The probe itself gets warm (but not hot) after power has been applied for a while, again as you might expect, which suggests it's getting power from the 1103.To the extent possible, I checked the cable at the probe end since I've been told that can see some wear and tear. Gently pulling back the rubber cover, the cable heat shrink looks fine. I'm loathe to cut the heat shrink, or desolder and open the fully-shielded probe itself, unless absolutely necessary. We know at least some of the wires are connected because of the heat buildup.The next step was to open the box at the BNC end, which resulted in the following photos. The PCB's are dated 1998, while many of the chips have datecodes of 1997.Removal of the top cover yields this view. Note the row of trimpots at the top, which are mounted to the lower PCB. These are used to calibrate the probe as explained in the service manual.Here is the switch PCB separated from the probe PCB.This is the underside of the switch PCB. The amber flex cable connects to the power pads on the TekPower BNC connector. The tan connector in the upper right corner mates to another connector on the probe PCB. The white connector at the bottom goes to the cable which exits the box and heads toward the probe.This is the top side of the probe PCB. The coax coming from the probe cable enters on the left and uses a sort of a modified Peltola connector (better view in the next photo). Of interest on this board is the large black DC power connector in the upper left corner; this connector is totally inaccessible when the unit is assembled. You might think it is used during testing/calibration but since the device gets four supply rails from the 1103, it's hard to imagine that a single DC supply rail would be sufficient to test/calibrate it. I'd love to know why that connector is present.This is the other side of the probe PCB. You can see more detail of the incoming coax connection from the probe; a sort of Peltola connector is crimped onto the shield while the center conductor is soldered directly to the PCB. The calibration trimpots are more visible here, as is the tan connector that ties to the switch PCB. This board also sports another mystery component: The LED in its lower left corner, silkscreened "DS1". There is absolutely no mention of this LED in the P6247 user manual or service manual, and it doesn't illuminate when power is applied. Nevertheless, the top cover and label each have a hole for it to shine through. I have no idea when this LED is supposed to turn on, or if its lack of operation indicates what is wrong with this unit. Tektronix obviously meant for it to be visible with the device fully assembled.Here is a closeup of how the TekProbe connector ties into the probe PCB. There are three sockets mounted to the PCB, and three mating pins on the BNC itself - one is the center conductor, the other two are the shield. When everything is assembled into the box, the pins are held snugly in the sockets. I'd like to see the assembly jig they used to hold those three sockets in place while (undoubtedly) hand soldering them to the PCB.If anyone has schematics for these two PCB's I'd be very grateful to have them. If anyone has any troubleshooting advice, I'd appreciate that too. I'd really like to get this probe working.Thanks!

author: DaJMasta
author_rank: Super Contributor
timestamp: September 30, 2019, 05:15:33 am
content: I don't know how this probe specifically is constructed, but typically with high bandwidth active probes, there is an active part right at the tip - in this case, in the molded hand piece at the other end of the cable coming out of this board. Most active probe faults that I know of are caused by damage to this circuitry, and it's basically always a proprietary element that can't really be fixed.That said, I don't know if this is the case for your probe - often since this part is the most sensitive and is prone to damage the manufacturer will sell replacements, which doesn't seem to be the case here, so it may be that there isn't an active element in it. If you want to try and troubleshoot it, I would put it on your scope like you were going to use it, PCB exposed, and use it to measure a lower frequency signal from a generator. Then use the scope's other channels with passive probes to probe the input signals on the board, and see if you can trace the signals through the board. If you can find them when they come from the tip, there's a good chance you can figure out where they get to before they go dead, and that will point you towards your issue.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 30, 2019, 06:59:00 am
content: check all wires continuity including the core coax, running from probe connector body to the tip. i also have a few bunch of non working tek active probes waiting to be repaired. dont make mistake like me, i broke a perfectly working probe tip ceramics pcb into 2 pieces because i pushed hard to make contact with metal tip end without knowing there should be conductive rubber in between that is missing.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 30, 2019, 07:17:23 am
content: Quote from: DaJMasta on September 30, 2019, 05:15:33 amThere's definitely active circuitry in the probe itself. As I mentioned, the rubber cover can be pulled back, and doing so reveals this:...which is a very tightly shielded enclosure that, in addition to the locking halves, has a wire soldered across the enclosure seam (on the side not visible in this photo) to insure good continuity. As mentioned, this probe gets warm after being powered on for a while which is likely the active circuitry therein. Also, there is a bundle of wires in the cable in addition to the coax itself. Finally, it's a differential probe yet there's only a single coax coming back to the box, which means the differential to single ended conversion happens in the tip. This is definitely not a passive probe tip.Yes, I can probably separate the metal shielding halves in the tip to access the components within, but I strongly suspect they are not serviceable... and it's possible that opening it at all will damage it. That's why I'm hoping to get a schematic of the enclosure at the other end, and start debugging there.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 30, 2019, 07:19:58 am
content: Quote from: Mechatrommer on September 30, 2019, 06:59:00 amThat's exactly why I don't want to open the probe's metal shielding unless absolutely necessary. I'm concerned that it may be a "sealed" unit, where merely opening it can cause damage. But I can't run continuity tests without breaking open the probe shielding. Chicken-egg problem.I'm cheered that the tip warms up after being powered for a while. That suggests the active circuitry is getting power. What I will probably do first is apply a signal and see what comes back on the coax in the enclosure. If there's any sort of signal there, it would strongly suggest the problem is in the enclosure and not in the tip.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 30, 2019, 08:54:14 am
content: Quote from: IDEngineer on September 30, 2019, 07:19:58 amyes, try to probe some signal, if you see something on the screen, your coax and psu wiring probably ok. your problem seems to be bias issue. some trimpot should fix it, if not, maybe some bias circuitry broken. the active probe tip enclosure with me is clamped, so we have to peel the clamp a little bit carefully to slide it open, yes we have to do as a last resort if nothing else works.

author: CDaniel
author_rank: Frequent Contributor
timestamp: September 30, 2019, 01:09:26 pm
content: For sure you have to see if the a signal is present after the tip , at the board input .I see that it has 2 serial E2PROM X24C02 , could be some comunication/software issue . One E2PROM could comunicate with the tip itself , if I see corectly where traces go ...

author: IDEngineer
author_rank: Super Contributor
timestamp: September 30, 2019, 08:32:42 pm
content: Update: I scoped the end of the coax coming into the module and connecting to the probe PCB. Bad news, it has the exact same -1VDC level on it as is emitted by the module itself. This points the finger at the probe tip and its carefully shielded housing.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 30, 2019, 08:59:17 pm
content: Here's the opened probe:The sealed module is to the left. Careful inspection strongly suggests trying to open that would cause irreparable damage. EDIT: A bit more inspection reveals it MIGHT be possible to desolder the ground pin and then slide the contents out of the shield. No idea how serviceable the contents might be.The center portion is just a shield extension. It keys into the sealed module to make sure that the connector to the right is properly oriented to the pins on the module (the two connectors are otherwise symmetrical, so it would be easy to reverse them).On the right you can make out how the little wires from the cable connect to the 8-pin female connector.There's not a lot to go wrong here that can be fixed. I'm going to examine the wire connections, but otherwise this may be as far as we can go.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 30, 2019, 11:16:59 pm
content: SUCCESS!!!After removing the secondary shield and the connector insulator, I checked the microscopic wires andthreeof them had broken away from the back of the Cinch connector. I suspect someone yanked hard on the probe cable and the stress was transferred to the connector. The probe cable is not well strain relieved, so stress on the cable goes right through the "strain relief" and yanks on the back of the connector.I need to tear it apart again after my quick test and do a proper job of sealing it up, but application of sine waves at various frequencies to the differential inputs yielded a very nice display on the scope. The 1103's offset controls now shift the waveform +/-1VDC per spec, and both the coarse and fine controls do their jobs.Barring any last-minute screwups on my part, I think we have a functional P6247. And I'm going to improve on the mechanical shortcomings during final assembly so I hopefully don't have to revisit this again.EDIT: Once everything was back together clean and proper, I put a ring of UV-curable cyanoacrylate around the base of the "strain relief" so that it would actually live up to its name.Hope this helps someone in the future... thanks!