Tektronix P6042 Current Probe Manual Errata

[RaymondMack]

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.

[RaymondMack]

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.

[coppercone2]

I think its a cool one, I have it for a while

joe q smith (IIRC) modified his to have a higher bandwidth at the cost of some pass band ripple, its on his youtube channel I think

not 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

[chronos42]

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.

[coppercone2]

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

[chronos42]

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.

[coppercone2]

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)

[chronos42]

Hi coppercone2,
Slightly changes in reading is normal, but when the probe is damaged the readings will not change slightly, but heavy.

[0culus]

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.

[RaymondMack]

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.

[Hamelec]

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.

[RaymondMack]

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.

Good 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.

[Hamelec]

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..

[RaymondMack]

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..

Range 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.

[Hamelec]

no further idea from my side - i think you should have a look into the instruction manual and check the calibration
(Gain=R344)

[0culus]

Interesting. Definitely time to go through that AM503 setup.

[Hamelec]

please report...
thanks

[RaymondMack]

no further idea from my side - i think you should have a look into the instruction manual and check the calibration
(Gain=R344)

I'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.

[Per Hansson]

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.pdf

And on eBay (no affiliation)
https://www.ebay.com/itm/184941794042

[Hamelec]

No need for extender.

only remove the cover of the TM501.

But be careful of open voltages!

[nfmax]

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.

[coppercone2]

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).

[RaymondMack]

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.

That 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.

[RaymondMack]

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.pdf

And on eBay (no affiliation)
https://www.ebay.com/itm/184941794042

Thank 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.

[RaymondMack]

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.