Tektronix P6042 Current Probe Manual Errata - Page 1
Forum section: electronics / repair

author: RaymondMack
author_rank: Regular Contributor
timestamp: July 29, 2021, 02:04:17 am
content: I just finished replacing the electrolytic capacitors and front panel potentiometers in a Tektronix P6042 current probe and found a typo in the user's manual while reassembling the unit.The wire colors listed in the manual are reversed for the O (blk, grn on wht) and Q (blk on wht) pins.Thankfully I took photos and drew a schematic of the wiring harness before disassembly. While double checking everything I found the discrepancy.Figured I would make a thread to point this out for anyone who has downloaded the same manual as I did. I'm not sure what, if any, damage might occur using the incorrect wiring, but it no doubt would take a while to figure out.

author: RaymondMack
author_rank: Regular Contributor
timestamp: July 29, 2021, 05:17:44 am
content: I finished testing the fully repaired current probe/amplifier and man am I impressed! This thing was a drifty bastard, but now it's rock stable. The trace on the oscilloscope hasn't visibly budged in over 30 minutes now. I didn't even need to adjust the R12 temperature compensation trimmer. Crazy.The old 100ohm balance pot had failed and not only made zeroing the output a pain the ass, it was the primary source of the drift. Since zeroing was such a pain, I had to put a 10-turn pot in for the Output DC Level control to make it even remotely usable. I'm just happy that it wasn't one of the transistors that was causing it. What a nightmare that would be to solve. I'll still go through the calibration routine when I get time, but this guy is already looking good to go.It's amazing to see such old equipment hold up and remain relevant. While ugly by today's standards, these older probes are an amazing value and offer comparable performance to current commercial offerings. I managed to pick this guy up for 140 USD, which is the same price I paid for just a TCP312A probe (both of which were a steal). But I still need to get the amplifier for the TCP312A, which sells for more than double the P6042 alone! The best part of the P6042 is that the power module/mainframe, probe and amplifier are all integrated. This saves some serious $$$ and time if you are looking for a used Tektronix current probe.

author: coppercone2
author_rank: Super Contributor
timestamp: July 30, 2021, 06:06:43 pm
content: I think its a cool one, I have it for a whilejoe q smith (IIRC) modified his to have a higher bandwidth at the cost of some pass band ripple, its on his youtube channel I thinknot very popular bit of kit though because of the price so you don't have too many people integrating them into design approaches or troubleshooting techniques etc. I am sure they are capable of ALOT of interesting things in the lab but its evolving slowly compared to scope knowledge

author: chronos42
author_rank: Regular Contributor
timestamp: July 31, 2021, 07:20:17 am
content: I also have a P6042, it is now 50 years old and still in very good condition.The only weak parts are the probe-cable and the dc-level potentiometer, what I will be replacing soon. The probe cable had already a wire breake, which was a PITA to fix. Unfortunately this cable is very special and difficult to substitute.All other parts are fully functional, even the 50-years old electrolytes are in specification in terms of esr and capacity, no replacement is required at the moment.Very important for all P6042 owners: Handle the probe with extremly care!I have two other P6042 with broken probes (damaged by the pre-owner) with tiny cracks in the ferrite core, both of which are beyond repair.

author: coppercone2
author_rank: Super Contributor
timestamp: July 31, 2021, 06:11:19 pm
content: hmm how do you find this crack in the core, or detect it?what performance test is it going to show up on the best?I dropped one of my p6042 on the tile floor a while back, I never saw problems but I suppose I should take a look. Last I looked at it was audio AC frequency

author: chronos42
author_rank: Regular Contributor
timestamp: July 31, 2021, 09:23:19 pm
content: Hi coppercone2,you can see the damage under a microscope. But there is a clear evidence of mechanical damage of the ferrite core. The offset is very instable, especially when the probe is moving. The slightest mechanical movement or preasure on the probehead leads to a enormus offset movement. In this case the probe is unusable and there is no way to fix it without original Tektronix replacement parts.

author: coppercone2
author_rank: Super Contributor
timestamp: July 31, 2021, 09:31:25 pm
content: ah, so the cracks form a pressure to signal transducer that you can detect easily, I will try this test.I noticed with all these probes sometimes you press on the jaws and the reading changes slightly,. but thats every magnetic probe I have seen (9 diff ones maybe)

author: chronos42
author_rank: Regular Contributor
timestamp: July 31, 2021, 09:51:49 pm
content: Hi coppercone2,Slightly changes in reading is normal, but when the probe is damaged the readings will not change slightly, but heavy.

author: 0culus
author_rank: Super Contributor
timestamp: August 01, 2021, 02:09:49 am
content: These are great probes, and if you are a reader of Jim Williams, a favorite of his for it's excellent performance. I have four of them, one of which was pre-worked on by the prior owner. He found a way to replace the cable with some sort of multistranded cable and a pair of RG-174 coaxes, all heat shrinked together every few inches. Seems to work very well. Somewhere I have the extras he sent me so I can do another one.The probe itself should be handled as a Fabergé egg...or you will get the problems the person above is mentioning with the cracks.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 05, 2021, 01:14:07 am
content: Well, I couldn't help myself from buying another current probe... This one came with all the gibblets: TM502A power module, P6302 probe and AM503 amplifier.However, it seems not all is well in current measurement land. The AM503/P6302 combo is noisier than the P6042 and I believe the amplifier has some issues since the gain is roughly three times too high and the rise time is way too slow for a 50MHz probe.Despite the above issues, I think this example highlights the value of the P6042, as it is both smaller, simpler to use/setup and seems to perform better than the later current probe offerings from Tektronix (assuming the front panel pots and cable are good!).Does anyone have experience with the AM503 amplifiers? I'm curious what might be wrong with it. Though, I might just get an AM503B to avoid all the mechanical switches that are in dire need of cleaning.As for cable replacements, couldn't the cable off of a newer, but scrap probe (e.g. A6302 or similar) be used instead? The black jacketed cables appear to be the most flexible, so if you could find a broken probe or cut cable from one of those that would be an easy solution to the problem.Since we are on the topic of current probes, I attached a photo of my Tek P6021 and HP 1110A AC probes. The P6021's termination has oxidized switch contacts which I am planning on cleaning here soon, just need to unsolder the switch then pull it a part. While broadband DC current probes with hall sensors are nice, a good passive AC probe can also be quite handy to have, and are inherently very low noise. They also extend the use of sensitive AC voltmeters like the HP 3400A/B and Racal-Dana 5002 as well as DSAs, spectrum analyzers and so on.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 05, 2021, 06:50:11 am
content: are the 50 Ohms input resistance of the scope channels are ok? sometimes one is burned...try to interchange the cables/channels and check if you get the same result.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 05, 2021, 09:15:57 am
content: Quote from: Hamelec on August 05, 2021, 06:50:11 amGood call, sanity checks are always important to make. I've attached a picture with the inputs swapped, which shows the same issue as before. So the scope inputs and the internal 50ohm terminations appear to be fine.My guess is that the AM503 amplifier is faulty. I don't think a bad probe would have that slow of rise time and the gain couldn't possibly be that far off by the probe alone. We are talking about 3x more gain and about half the rated bandwidth. Usually a bad probe would show large shifts in the DC offset while being moved about, which this one doesn't exhibit.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 05, 2021, 10:10:49 am
content: is the range switch properly working?- electrical, 1-2-5 steps ?- mechanical - knob turned on the axle?also check if the bandwith switch is working..

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 05, 2021, 08:10:19 pm
content: Quote from: Hamelec on August 05, 2021, 10:10:49 amRange switch seems to be working and set correctly on the shaft. Going up in range to 2mA per division reduces the gain by 2--as expected.The full / 5MHz bandwidth switch also seems to work fine. The observed peak-to-peak noise decreases slightly when engaged (as shown in the first image) and the rise time gives about 5MHz bandwidth (as shown in the last image). So no problems with the front panel controls. It has to be a faulty part inside the amplifier, possibly even a failed calibration by the previous owner.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 05, 2021, 08:48:27 pm
content: no further idea from my side - i think you should have a look into the instruction manual and check the calibration(Gain=R344)

author: 0culus
author_rank: Super Contributor
timestamp: August 06, 2021, 04:26:03 am
content: Interesting. Definitely time to go through that AM503 setup.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 06, 2021, 06:42:05 am
content: please report...thanks

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 06, 2021, 09:04:16 am
content: Quote from: Hamelec on August 05, 2021, 08:48:27 pmI'll try and do that tomorrow when I get some free time.I removed the AM503 plugin out of the TM502A power module and was pretty impressed with the build quality. The cam switches are very nice and there aren't any obvious signs of damage to the module. I am, however, perplexed as to how one would actually make adjustments to the AM503 when it is plugged in. Do I need to pull the power module case off to gain access to the AM503 or did Tek have an extender board for this? These 500 and 5000 series plugins seem a lot more complicated to work on than standalone equipment.

author: Per Hansson
author_rank: Unknown
timestamp: August 06, 2021, 09:21:52 am
content: Have a look here for extender info:https://www.eevblog.com/forum/projects/interest-check-extender-boards-for-tm500-series-modules/https://xdevs.com/doc/Tek/Make_your_own_Tektronix_TM-500_extender.pdfAnd on eBay (no affiliation)https://www.ebay.com/itm/184941794042

author: Hamelec
author_rank: Regular Contributor
timestamp: August 06, 2021, 10:08:02 am
content: No need for extender.only remove the cover of the TM501.But be careful of open voltages!

author: nfmax
author_rank: Super Contributor
timestamp: August 06, 2021, 02:00:00 pm
content: I have a P6042 in the cupboard, in a non-functioning state for many years (since replaced with an Agilent probe). This thread is inspiring me to pull it out & have a go at fixing it. ISTR the output was stuck permanently at one rail or the other. I replaced the cable myself, many years ago, with some spare cable supplied by the late Deane Kydd (I think I got the name right) and it was working fine after that.

author: coppercone2
author_rank: Super Contributor
timestamp: August 06, 2021, 06:26:44 pm
content: I had to replace a spring in mine (taken from a pen) and fix the cable (I just shortened it a little bit to fix the broken solder joint).

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 06, 2021, 07:56:23 pm
content: Quote from: nfmax on August 06, 2021, 02:00:00 pmThat sounds like my unit. Just touching the balance pot would send the output to either rail. Thankfully, I was able to tweak the internal trimmers used for the balance adjustment and use the DC Level control to work around the failed balance pot.I'd also check the three smaller electrolytic capacitors as they tend to fail and then pull the balance potentiometer. If the balance pot is bad, it's a pain in the ass to replace. I had to shorten the chassis mount to fit a new wire wound part from Vishay. This required removing everything from the front panel and using a hack saw to cut the aluminum mounting shaft and then drilling and taping a new screw hole. While you're in there, a 10-turn wire wound pot for the DC Level control is a nice addition, makes zeroing the DC offset a breeze.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 06, 2021, 08:17:30 pm
content: Quote from: Per Hansson on August 06, 2021, 09:21:52 amThank you! I'll pick up one of those cables for future repairs.For now though, I'll just pull the cover off the TM 502A power module. A bit haphazard, but it should work okay for adjusting the gain of the AM503.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 07, 2021, 07:19:08 am
content: I decided to fix the range switch on the P6021 passive termination instead of working on the AM503 today as it is much simpler to repair.Using my Hakko desoldering gun made quick work of removing the switch. The contacts were pretty dirty and had zero lubricant to prevent them from oxidizing. I noticed one of the rocker contacts was bent inward! Not sure if this was from someone dropping the termination or what, but a little bit of work with a pair of pliers smoothed that out.It was a fiddly bastard to clean and put back together, but at least it's working now. I put some "dielectric grease" (electronics grade silicone grease) on the contacts to keep them from oxidizing and to lubricate the plastics.Before soldering it back in I double checked the contact resistance with my HP 3456A and all four switch contacts were less than 40milliohms, which looked good enough for me. For low level switching applications like this, gold plated contacts would have made more sense.I need to go through the calibration routine as the rising edge has some ripple and overshoot but otherwise it compares favorably with my HP 1110A AC current probe.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 07, 2021, 07:43:48 am
content: After finishing that repair, I took a second look at the AM503 and noticed that the two custom ICs are chipped at the edges! Almost like someone ran a screwdriver across them... I'm honestly surprised that they still work.Anyways I took a few photos to share with everyone. The cam switches are incredible and are in great shape for their age. I thought that they might need to be cleaned before opening up the module, but if anything, the cam mechanism just needs to be relubricated since it's a little bit stiff to turn.I'll try to mess with the gain adjustment tomorrow.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 08, 2021, 07:38:26 am
content: I finally got back to the AM503 today.First, I pulled off the case to the TM502A and performed the power supply adjustment, which was about 40mV out from +-16 and well with in spec. However, any further progress was halted since I needed to make a "special adapter" cable that has me hesitating since so little information is given in the manual.But first let's focus on the adapter. I took some liberties with mine since1)I don't have a male connector that mates with the TM503, but I did find some old Winchester Electronics pins that work and2)I find it pointless to put a female BNC connector on the end only to connect another cable to it...So, instead, I just cut a 3ft length of coax, striped one end and soldered the pins on with a 25ohm resistor in series with the center pin. Looks good enough for me, and honestly I probably over built this thing since I'll likely only use it this once...Anyways, I'm unsure about the diagram that Tek shows in the manual (second attached image). Specifically, are they showing thefemaleconnector from the front or themaleconnector from the rear in the diagram below the male connector? And do the letter codes for the female sockets match the male pins? I really don't want to destroy my signal generator or the amplifier by connecting it up wrong. While there is a 25ohm series resistor, I can still see something going wrong if the letter codes don't actually line up.Any guidance on this would be much appreciated. After I get this sorted I'll do the gain adjustment as listed in the manual. Hopefully someone has just calibrated it incorrectly and the amplifier will adjust back into spec. I've got my fingers crossed.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 08, 2021, 02:43:56 pm
content: J is ground, M is signal input, you could check with a ohmmeter.but:it is much easier to connect direct to the AM503 Board.Disconnect J202 and connect your own adapter cable with the 25ohm series resistor.optionally solder it to R202 or L202..

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 08, 2021, 07:43:59 pm
content: Thanks Hamelec. It was late, so I didn't think about soldering directly to the board--which makes a lot more sense.But since I've already made the cable, I'll just double check the pinout of the front panel connector with my DMM and go from there. If needed I can always unsolder the Winchester pins and attach directly to the PCB as you've suggested.

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 09, 2021, 05:59:31 am
content: Looks like it was just a calibration error! I only performed the gain adjustment and that also fixed the rise time / bandwidth issue. The manual is rather confusing with regards to the calibration steps. I also had to look up the PG 506 to see that they wanted a negative going pulse with 200mV amplitude and less than 1ns rise time.It does makes sense that the bandwidth/rise-time would decrease if the gain was increased. So no real mystery here other than how the gain got so far out of wack, but at least it's not a faulty part.Attached is a trace of a 300mVpp square pulse at 1MHz. The rise time of the signal generator is about 5ns and Sqrt(5ns^2 + 7ns^2) = 8.6ns, so we're looking okay. It also looks like the peak-to-peak noise has decreased slightly too, which is a nice bonus.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 09, 2021, 09:12:08 am
content: thanks, nice to see how the AM503 performs.But now you should also check the calibration with the probe..

author: RaymondMack
author_rank: Regular Contributor
timestamp: August 09, 2021, 09:07:48 pm
content: Quote from: Hamelec on August 09, 2021, 09:12:08 amThose results are with the P6302 probe attached to the AM503. The amplifier itself is supposed to have >100MHz bandwidth, but I didn't have a pulse generator with <1ns rise-time to perform the compensation adjustment as listed in the manual. So I just performed the power supply and gain adjustments then checked the compensation and degauss for conformance, which they both passed.The compensation adjustment is very vague; it would have been nice if they specified the slope of the droop rather than just providing a trace to match. Performing the compensation adjustment with the probe honestly makes more sense, since you can get a better pulse response by fine tuning the complete system. But when you have multiple probes calibrating just the amplifier has merit.