Tektronix P6042 Current Probe Repair

[omerd]

Hi!
I'm new here but always been a fan of Dave's videos, so I look forward to being active on this forum as well

I have got a Tektronix P6042 DC-50MHz current probe with some issues and decided to repair it.
This unit was designed in the mid 60's and it's very well constructed, almost entirely discrete-transistor based with documentation available from Tek. It is still a very useful piece of test gear as it can go from DC to 50 meg and supports currents as low as 1mA.
Forum user PA4TIM had repaired one and has very useful info on his website - http://www.pa4tim.nl/?p=3135
Original Tek manual is here - http://www.atecorp.com/ATECorp/media/pdfs/data-sheets/Tektronix-P6042_Manual.pdf

When I got my unit I noticed two things:
1) Probe cable was broken near the front panel strain relief rubber, this was an easy repair just shortening the cable and soldering the wires inside.
2) R11 missing, instead there was just a piece of wire there, and it was disconnected at one of the little studs, i.e. open circuit in place of R11.
R11 is a factory-selected resistor that has something to do with the temperature compensation for the hall effect probe. I tried using the same value as R13, a short and open... this has some effect on the drift of the output but doesn't seem to be related to the main problem I have.

The custom diff. amp looks OK, and now the whole system kind of works. I can adjust the cur/div balance circuits and play with the calibration trim pots inside while holding the tongue at the right angle 
But there is a little problem that just drives me crazy... the P6042 output is designed to be 50-ohm terminated. This is what I do - I either use my internal TDS540's termination or an external terminator, but the reading I get from the probe is about half of what it should be! and playing with the gain calibration pot has very little effect on this issue (not much dynamic range).
When not terminating the probe's output the reading seems to be correct... or at least I can somewhat calibrate the probe to give a correct reading without being 50-Ohm loaded.
I thought maybe someone hacked a 50 ohm resistor inside for some reason, and nope.
This problem exists on both low and high frequency amps inside, I get the same "half voltage" output regardless of if it's DC or say 20 MHz input (at 50 Mhz I should expect a 3db drop).
Anyway, I'm pretty sure when it says "0.1A/Div" (when scope is set to 50mV/Div), it should be one graticule line in amplitude for 0.1A of current... not for 0.2A!!

I have measured the voltage rails and tested some points along the signal path. Also inspected the attenuator and it was NOT modified in any way.
I really can't track the problem down as I don't know what signal levels to expect along the path. All I know is that the final output is no good.
The manual doesn't mention any test points along the signal path for some given reference input. Only some basic performance checks and calibration procedures, all based on final output stage.

I welcome any ideas. Thanks in advance!
I will be glad to share more info, upload photos etc...
Omer

[Alex Eisenhut]

In any case, be extremely careful with the probe itself. The slightest mechanical shock will *destroy* the probe beyond repair.

Don't drop it on the desk, not even from 1 atom up. Gently put it back into the unit. Treat it like a Fabergé Egg.

The R11/R13 resistors are part of some esoteric probe matching system that I don't know how it works.

The best place to ask for P6042 stuff is in the Yahoo Tekscopes group. Search for "x-ray" if you wanna find out how I know how fragile and unrepairable the probe is.

Have you tried degaussing the probe?

[tautech]

You will likely find much of the info you need in "Tek Probe Circuits" linked in this post:
https://www.eevblog.com/forum/testgear/dc-resistance-of-a-1x-oscilloscope-probe/msg538880/#msg538880

[edavid]

First, can you fix the topic title?  It's very confusing since the P6240 is a FET probe.

Second, half gain usually means that half the differential path isn't working.  So, I would look at the M18 output and Q22/Q24/Q29.

Or, maybe you still have a broken wire in the probe cable.

[omerd]

Hi,
Thanks for the quick replies and oops... I did get the model number wrong, thanks for correcting me on this one!
I do try to be very gentle with the probe itself, thanks for reminding me how important this is.
I always degauss the probe.
"half gain usually means that half the differential path isn't working" - shouldn't I expect to see some artifacts in the signal as well?
Now when I think about it, I do get some overshoots and undershoots when feeding a square wave, but I assumed it is due to miscalibration.
I will check some transistor gains and biases when I come home.
I appreciate your help!
Omer

EDIT: thank you tautech for the excellent reading material, lots of useful info on differential push-pull current probe amps. Great stuff!

[omerd]

Update:
I tested the probe wiring again, everything is OK.
If there was a problem with the differential amp or anything in the low frequency amp, I would have expected that the final output will still be OK when testing high frequency signals.
The signal just looks too good along the way, it really looks like the problem is in the final stages of the circuit.
The manual mentions that:

When the probe is unlocked (SW3 grounded) D128 conducts, bringing the base of Q123 up and turning the transistor on. This latches up the output of the high-frequency amplifier and prevents any display.

When I don't lock the probe, the "probe unlocked" light does go on, but I still get the output signal displayed. So something is wrong here.
I tested Q113, Q121 and Q123 (which does the "blanking" of the display), all OK. Replacing them didn't help as well.
I found that R111 was measuring ~60 Ohm instead of 47, so I replaced it.
Another 47 Ohm resistor, R123 (BTW there is a mistake in the manual, it is double-marked as R116 in the layout) was measuring 52 Ohm, replaced it as well.
D128 also tested and looks OK, but actually will never conduct!
If you look at the schematic, "probe unlocked" SW3 grounds the poor diode using R5. R5 in series with the light bulb forms a voltage divider, giving around -10 volts. On the other side of the diode I get around -7 volts, caused by R125 and R126 divider (BTW, for some reason they are around 1K and not 2K as mentioned in the manual... Arrrr).
The only thing I can think of is that the bulb isn't original, and is much lower in impedance.
That's it for now. I will measure some other components, I have a feeling some resistors have "drifted" a little bit.
Omer

[Alex Eisenhut]

How is the DC reading BTW? Is it also off?

[omerd]

DC reading is off just like AC.
I'm actually tempted to reassemble everything and keep using it without 50 Ohm termination, its accuracy isn't bad this way.
I just can't explain why the 2K resistors measure 1K, two different (also in physical form) 47 Ohm resistors measure 60 and 52 Ohm, and why the blanking circuit is grounded the way it is. Come on, it's Tektronix, there's gotta be a reason why they've designed it this way, having this incandescent-light-bulb-resistor voltage divider 
What on earth can cause resistor values to drift this bad I don't know.
If anyone has a functional unit and is willing to take a few measurements, this would be great!! hey, it's only one screw and you're in 

Omer

[Alex Eisenhut]

DC reading is off just like AC.
I'm actually tempted to reassemble everything and keep using it without 50 Ohm termination, its accuracy isn't bad this way.
I just can't explain why the 2K resistors measure 1K, two different (also in physical form) 47 Ohm resistors measure 60 and 52 Ohm, and why the blanking circuit is grounded the way it is. Come on, it's Tektronix, there's gotta be a reason why they've designed it this way, having this incandescent-light-bulb-resistor voltage divider 
What on earth can cause resistor values to drift this bad I don't know.
If anyone has a functional unit and is willing to take a few measurements, this would be great!! hey, it's only one screw and you're in 

Omer

Well it seems pretty clear that whoever had the unit before modified it because they didn't like the 50 ohm impedance. That's the thing when you buy old test gear, who knows what the previous owner did to it.

Carbon composition resistors do drift up, a 47 ohm measuring 60 ohms is something I'd expect. But a 1K won't drift to 2K unless it's burnt or filed down.

[omerd]

Could be, although usually I can tell by the solder joints and the looks of the PCB. I don't see any modern resistors or semiconductors, all look original with some of them having Tek P/Ns on them.
And what about the "probe unlocked" circuit?
Anyway I'm gonna test all components starting at the final stages against the specs, this is the best I can do for now.
Thanks for your help!
Omer

Edit: I wish I could at least trust the Tek documentation. I keep finding mistakes there. C106 should be in parallel with (R107 & 108). It is NOT. The traces just don't match the wiring net on the schematic. I have confirmed the notation is OK (unlike with R123 for example).

There is another P6042 at some junkyard not far away. Maybe I'll grab it (this one I got for free) and make one functional unit.
I have repaired lots of old (and even vintage) gear, this is the first time I encounter such problems in what looks to be a fairly simple circuit.

[Alex Eisenhut]

I took my probe apart again just to look at the little PCB that the transformer plugs to.
Then a wire popped off, I figured it broke off. But it looks like it was never properly soldered in the first place.
The solder joint on the board just looks like a grainy crater and the wire looks like in the photo.
This isn't the first time I encounter poor soldering from 1960s Tek gear.
It might pay to do a close optical inspection of your unit's solder joints.

[Alex Eisenhut]

Edit: I wish I could at least trust the Tek documentation. I keep finding mistakes there. C106 should be in parallel with (R107 & 108). It is NOT. The traces just don't match the wiring net on the schematic. I have confirmed the notation is OK (unlike with R123 for example).

It's important to match the serial number of the unit to the schematic. There's also change notices at the back of service manuals that need to be looked at.

[omerd]

Thank you Alex again for your help!
I always go over and resolder any joint that doesn't look shiny enough or if I see oxidation on component leads, although I did not take the main PCB out this time (lots of wires). Removing the bottom plate with the two screws gives access to most of the circuit.
My probe itself looks good. I have confirmed continuity all the way while twisting and playing with the cable to assure nothing is loose inside.
The only visible damage is a tiny chipped piece on the sliding half of the loop


I have re-seated the probe in its socket (as well as all transistors in the unit).
Regarding the manual - this is the only one I found and the only correction there is some typo. I will have to reverse engineer the unit a little bit.
I would love to confirm that my problem is in the final stage as I suspect. If someone can feed in a known signal, set the knob to some fixed position, and report the output right after the attenuator (on J80 with coax connected of course), that would be really really helpful. If it's about the same on my unit then the problem is with the final stage for sure.
I wish the manual could give some test points along the signal path for a known reference input signal. Unfortunately it doesn't.

Omer

[Alex Eisenhut]

There's a little known piece of info that's only included when you get a spare transformer, you should only use water to clean the surfaces of the transformer assembly.
Or Freon...

[omerd]

Just a sec, I'm getting my Freon 11 bottle to clean the probe...

Playing with the current/div balance knob I have noticed strange behavior of the signal which worsen when the unit got warm. It turned out Q54 was slowly dying. At first it measured OK with DMM on diode mode, now it's completely gone 
Replaced with PH2907 as I don't have 2N2905 right now. I will anyway have to curve-trace some transistors and find a "match" for best results. I don't care about thermal drift right now... hate this old fashioned can package.
So it seems both differential paths on low frequency amp are OK now, high-frequency amp is OK, attenuator is OK... but I still get half voltage when terminating the line.
I have replaced a few resistors that were more than 10% off.
It's probably something so stupid that when I'll find it I'll bang my head on the wall 
Omer

[omerd]

I've been replacing a few more passives, cleaning up the board and going over some solder joints.
Then I tried a full calibration. While playing with the low frequency loop gain pot, Q44 and Q45 failed. I replaced them and now things look much better:
1) High frequency amp is working very well. When feeding a high frequency signal (say, >100KHz) the output is perfectly fine, while the gain pot is around half way turned. Couldn't ask for more!
2) Low frequency response is still a little off. This is best seen when feeding s square wave - it "overshoots" and "undershoots" to the correct amplitude, but then decays to a lower amplitude signal.




(Note: the scope isn't 50 Ohm terminated internally, I use external terminator this time)
Playing with the pots on the final stage does very little effect regarding this issue.
I would still very much appreciate some reference data from a known-working unit!
Omer

[tautech]

Are you aware Tek used a link between Probe Cal and Gnd for checking their current probes.
Check your scope manual to see if your model supports this.
Sure it will be only at 1 KHz. but there should be a specified current for that setup.
That will help you check amplitude is correct.

[omerd]

I know my output amplitude is NOT correct, why check?
I use a signal gen terminated with 50 Ohm resistor as my wire-under-test current loop. CH1 is showing the voltage across the resistor.
Problem is now only with DC and low frequencies. There are various components along the signal path to compensate and "combine" the high and low frequency signals to a smooth output.
Something is still wrong but I'm on the right track, I feel.
Omer

[tautech]

I know my output amplitude is NOT correct, why check?

Because the Tek current probe check setup is a calibrated accurate pulse (assuming your scope supports it) and expected gain can then be verified along the signal path.
Might help you pinpoint the remaining faults ?

[Alex Eisenhut]

Are you aware Tek used a link between Probe Cal and Gnd for checking their current probes.
Check your scope manual to see if your model supports this.
Sure it will be only at 1 KHz. but there should be a specified current for that setup.
That will help you check amplitude is correct.

Witness the power of my 547!

[omerd]

Following the manual you need a pile of vintage Tek gear to calibrate the probe
I do keep an old 465B ("Any good lab should have an analog scope", right?), I think it has a current cal loop on the front.
What I meant to say is that my probe is still way off in DC and low frequencies. There is clearly something wrong there and it's not a matter of tweaking a pot or two.
Say I use the Tek scope current calibration loop, where did you find information about signal levels other than the final output? Did I miss something..?
To pinpoint the problem: DC and LF response is poor. DC is attenuated just like before, I don't get much more than half of the expected amplitude.

[edavid]

Say I use the Tek scope current calibration loop, where did you find information about signal levels other than the final output? Did I miss something..?

No, you are right.

To pinpoint the problem: DC and LF response is poor. DC is attenuated just like before, I don't get much more than half of the expected amplitude.

Bad Hall element? 

[omerd]

Bad Hall element?

I hope not.. It does work, it is sensitive and noise free.
Maybe it's yet another drifty resistor causing all the problems, who knows.
Omer

[omerd]

Found some time to get back to this probe...
Setting the high frequency correctly is fairly easy. The final trim pots can modify the signal by orders of magnitude. When getting closer to DC this final stage does nothing, so I went back to the Hall sensor and the first stages.
Now, I feel a little stupid that I didn't notice it before, but the schematic does give some voltage test point levels around the custom differential amp. Why couldn't they put the letter "V" following the number? Anyway, the outputs should float on 8 volts, which they do, and the input leads (pins 2 and 10) should be at 0.6 volts. There are even wires going to some nice test socket (pins 1 and 3).
According to the manual, the current flowing through the Hall sensor should be about 20mA and is right about there.
The voltage on the output pins of the device that go to the custom amp is only 0.3 volts, instead of 0.6!
So it's possible that the sensor itself is faulty in some way or maybe I can just increase the current a little bit, hmmm... I don't want to kill the poor sensor. What do you guys say?
UPDATE: Increasing R14 to bias the output at 0.6v still doesn't fix the problem. This is to be expected when dealing with a differential signal I guess. I just wondered maybe I was under the working point of some transistor inside the IC and that was causing the problem. I will try measuring the Hall sensor voltage directly.

[Alex Eisenhut]

Found some time to get back to this probe...
Setting the high frequency correctly is fairly easy. The final trim pots can modify the signal by orders of magnitude. When getting closer to DC this final stage does nothing, so I went back to the Hall sensor and the first stages.
Now, I feel a little stupid that I didn't notice it before, but the schematic does give some voltage test point levels around the custom differential amp. Why couldn't they put the letter "V" following the number? Anyway, the outputs should float on 8 volts, which they do, and the input leads (pins 2 and 10) should be at 0.6 volts. There are even wires going to some nice test socket (pins 1 and 3).
According to the manual, the current flowing through the Hall sensor should be about 20mA and is right about there.
The voltage on the output pins of the device that go to the custom amp is only 0.3 volts, instead of 0.6!
So it's possible that the sensor itself is faulty in some way or maybe I can just increase the current a little bit, hmmm... I don't want to kill the poor sensor. What do you guys say?
UPDATE: Increasing R14 to bias the output at 0.6v still doesn't fix the problem. This is to be expected when dealing with a differential signal I guess. I just wondered maybe I was under the working point of some transistor inside the IC and that was causing the problem. I will try measuring the Hall sensor voltage directly.

Sensor's dead. Sorry. Maybe you can just re-purpose it as an AC probe.
I never found out exactly how the hall sensor works. It seems to be a thin-film plated right on the ferrite core. Contacts are somehow made and the two core halves glued together with something that even epoxy remover can't touch. I don't know why a shock breaks it.
It's quite baffling.

[omerd]

I can also just double the gain on the low-frequency stage, the sensor may be defective but not completely useless.
I will try to get another unit with a working probe.

[Rupunzell]

The current transformer/hall sensor assembly is lapped to optical level tolerances (millions of an inch) after potting. Tampering with it will result in near zero chance of getting the entire assembly back together with proper alignment. If the double mu-metal housing shield become bent in any way, it's magnetic shielding properties will be compromised.

This is why cleaning with Freon 11 or water was recommended as any residue between the current sensing assembly and cap results in a larger air gap between the sensing assembly and cap altering it's magnetic circuit and overall frequency response. High purity iso-alchol can be used to clean the core. Keep in mind both upper and lower parts of the current sensing assembly must meet with great precision and no air gap.

The hall sensor is embedded within the ferrite core assembly with epoxy. Early production of these units, the epoxy was found to be excessively brittle. This resulted in cracking of the epoxy within the core damaging the hall sensor. Tek later went to a different epoxy which helped to reduce this mode of failure. This is why these tek current probers must be treated with great care or a dead probe are often the results.

The later Tek AM6302 current sensing assembly can be used in the P6042. p/n 120-0494-00 (P6042) or 120-0494-02 (A orP 6302).

These current probes were not in much demand before the rise of switching power supplies. It was not too long ago when they were moderately common and not too expensive on the used market. These days, folks have discovered just how good they are and how useful they are and the current offerings are not better and often do not work nearly as well. There are few current probes like this to this day offering this level of performance. As for it's 50Mhz rating, a properly operating P6042 will easily exceed this and it has low noise and DC stable.

The thermal compensation R value should be written on the current sensing assembly. If not, it has been rubbed off or removed at some point of it's life.


Bernice

Sensor's dead. Sorry. Maybe you can just re-purpose it as an AC probe.
I never found out exactly how the hall sensor works. It seems to be a thin-film plated right on the ferrite core. Contacts are somehow made and the two core halves glued together with something that even epoxy remover can't touch. I don't know why a shock breaks it.
It's quite baffling.

[joeqsmith]

I had posted the following back in 2003 about my trials with my P6042.  Bob Pease was still with us and my main analyzer was an HP141T!    Maybe something in this mess will be of help to you if you are still trying to repair yours. 

****************************************************************************************
 
My unit came non-working. This is the normal trend. After taking the unit apart I noticed that the tant. caps. used for the main filters in the +/-16V supplies were shot. The more I dug into it, the more I found wrong. It turned out that almost every transistor had failed. The cable going to the probe also was open on three wires and the bulb had failed. It was a question of what was left that was good. If it was not for the wide BW, I would have pitched the unit.

To repair the cable I ended up cutting off about 1/2 a foot from both ends and then testing it. It turns out that the opens were in that area, somewhere. Who cares. Reattachment was difficult work.

With the cable repaired I ended up just doing some basic parts swapping to get the unit running enought to test the hall sensor. I was amazed to find it, and the diff. amp. front end to be in working order. All of the switches tested out fine as well so I decided to go ahead with the repairs.

The following is a list of standard transistors that may be used to replace the original parts.

2N2219; TO-39; Q45,Q51,Q53

2N2905; TO-39; Q42,Q44,Q54

2N3053; TO-39; Q73,Q79

2N3572; TO-18; Q113,Q121,Q123

2N3767; TO-92; Q163

2N3904; TO-92; Q175,Q161

2N3906; TO-92; Q22,Q24,Q156

MPS6515; TO-39; Q167,Q183

MPS6521; TO-92; Q29

All of these parts may be purchased, still, from Mouser (800-346-6873).

Q45 & Q44 are a matched pair and also Q53 & Q54. You may have to buy a few of these to try and get two matched sets.

More transistor problems ................

The P6042 was built back in the late 60's. The common practice at the time was to match transistors and then heat sink them together in order to null out any temperature drift. HP was putting transistors into a single 6 lead package to avoid the problem. The P6042 uses two transistors that were screened by Tek. They only have a house number and are no longer available. Tek's service could not tell me what the original part was which was a surprise. These parts are Q87 and Q96. Both parts are also in a TO-18 package. I plan to try and sort some 2N2222 and 2N2907s and use these for a replacement.

These parts are configured as a voltage follower. The emitter of the NPN is connected to the base of the PNP. So, Vbe must match on both parts.

The first step was to make a test fixture to screen the parts. I used Vishay's TC parts. These are very low temp. coef. and are 1%. I screened these using an HP34401A meter to get down to 0.1% I picked the current based on the normal operating current the parts see in the probe.

The following shows the basic test fixture (with the 2904/6s).

Testing so far has turned out very poor, and I have been unable to get under a mV of offset. I see some major changes when trying different manufactures parts, but running from the same manufacturer, same lot, there is not enough varience to get this close. I am now thinking about changing to a different type part and see if I can get something a bit closer.

I wrote Mr. Robert Pease on the subject who suggested trying some different parts. I procured 40pcs of a 2N3904 and 2N3906s from Motorola and ended up getting some very good results.

A wise comment from Mr. Pease:

"You could ask Fairchild if they sell the 2N3906 and 2N3906 chips in TO-18. I bet if the military wanted to buy 100 of these for $30 each, they could get them just fine,but, how much AUTHENTICITY do you want to pay for?

Best wishes. / rap"

I gave this some thought and decided to stay with the matched set TO92s I came up with. To get around the TO-18 heatsink problem, I ended up just filling the sink with grease and flipping it upside down. It seems stay very stable when I tested the unit with my heat gun.

After changing 3 caps, 13 transistors, 1 fuse, 2 resistors and fixing the cable, the work paid off. This unit must have really had one great SNAFU done to it! From the damage that was done, it appears it had been hit with a VERY LARGE line transient. Or, maybe someone plugged it into 220 without changing the switch from 110. What ever it was, I'm sure some EE said #$^%$ when it happened. Shown is the rebuilt unit with a 50MHz waveform. To get around using the TO-18s, I ended up just filling the sink with grease and flipping it upside down.

Shown is the unit after compensation has been performed. Scale is 10MHz/div, 2bB LOG. My guess is with some very simple modifications you could push this probes 3dB point out to 70MHz.

[tautech]

I had posted the following back in 2003 about my trials with my P6042.  Bob Pease was still with us and my main analyzer was an HP141T!    Maybe something in this mess will be of help to you if you are still trying to repair yours. 

****************************************************************************************
 
My unit came non-working. This is the normal trend......

Valuable info, thanks for sharing. 

[joeqsmith]

Took a few pictures of my 6042 apart.   I used a 50 ohm resistor connected to BNC for a test probe. 

[joeqsmith]

Plots showing P6042 connected the the network analyzer.    DC up to 30MHz is under 1dB.    Fuzzy picture shows 10dB/div up to 100MHz.  There is 3dB of gain at 69.7MHz. 

[joeqsmith]

Connected the test resistor to a square wave generator to show the edge rate.   For a current probe made in the 60's, it does a good job.   

[joeqsmith]

Was attempting to increase the BW of the P6042. 

For the current source, I had started out with axial lead on BNC.   Even at 50 MHz it was poor.

50MHz - 53.1 ohms
100MHz - 64.7 ohms
200MHz - 102.8 ohms
400MHz - 277.5 ohms

I built the small PC mounted loop next with 2 X 1206 100 ohm.  This works much better and should be fine for 50MHz.

50MHz - 50.6 ohms
100MHz - 54.6 ohms
200MHz - 68.3 ohms
400MHz - 134.2 ohms

Next I built something more like a standard calibration jig which appears to work even better. 
 
50MHz - 50.2 ohms
100MHz - 50.2 ohms
200MHz - 50.2 ohms
400MHz - 50.1 ohms

I had noticed in the common on-line manual and the one I have, page 6-12 section 14 is not the same.  The schematics are also different.   In the common on-line manual, C62 is shown as a fixed 15pf.    My manual shows this as a 5-25pf and it is adjusted in section 14.   

[joeqsmith]

With the high end boosted, I made an attempt to tame it.    Note that previous 100MHz sweep was 10dB/div compared to 5 in the attached plot.   Still not the best but may be the highest performing P6042 out there.   

Also shown is rise time improvement.   

[joeqsmith]

Made a short video showing my P6042 in operation after increasing the BW to over 90MHz.  I am not sure yet how much current it can handle at the higher frequencies. 

https://www.youtube.com/watch?v=6f8zoyBxizs&feature=youtu.be

[Alex Eisenhut]

I guess I have to go through the thread again, but was the sensor damaged or not?

[joeqsmith]

Using the modified P6042 to look at output signals from a small power amplifier at 80MHz. 

https://www.youtube.com/watch?v=RVY9q5JhHHU&feature=youtu.be

[MattisLind]

(I know that this is an old thread)

I just repaired my Tek P6042. The cable was broken in both ends, the black lead in one end and the green lead in the probe end. While taking the probe apart I was very careful both to not knock the probe and then to not lose the metal ball. But the metal ball was already lost.

What kind of diameter is the metal ball? The Tektronix part number is 214-0997-00.

Is it possible to buy the appropriate cable somewhere?

[tautech]

(I know that this is an old thread)

I just repaired my Tek P6042. The cable was broken in both ends, the black lead in one end and the green lead in the probe end. While taking the probe apart I was very careful both to not knock the probe and then to not lose the metal ball. But the metal ball was already lost.

What kind of diameter is the metal ball? The Tektronix part number is 214-0997-00.

Is it possible to buy the appropriate cable somewhere?

Welcome to the forum.

It is not uncommon for the cables to break near the end of the strain relief and shortening them is the normal fix.
Tek current probes have quite long leads 5Ft IIRC.

For hobbyist work this shouldn't affect operation, risetimes or calibration significantly.

I have P6021 probes that are quite similar and IIRC the ball is ~ 1/8 inch (3mm).

[Alex Eisenhut]

(I know that this is an old thread)

I just repaired my Tek P6042. The cable was broken in both ends, the black lead in one end and the green lead in the probe end. While taking the probe apart I was very careful both to not knock the probe and then to not lose the metal ball. But the metal ball was already lost.

What kind of diameter is the metal ball? The Tektronix part number is 214-0997-00.

Is it possible to buy the appropriate cable somewhere?

I've been down that road myself. It's a 3/32" ball bearing. The cable is unobtainium AFAIK, it's two 50 ohm coaxes with 4 wires and an overall braid. I couldn't find any new.

I've tried to make my own with heat-shrink fabric but I couldn't find small enough diameter.

Anyhow, I can mail you a few ball bearings from my personal stash of Tektronix hoarding... But they're not that exotic if you have a big enough hardware store nearby.

[icpart]

Hi guys I found in eBay auction that untested P6042 probe. Also from pictures seems the current probe is broken because there is missing plastic switch for open the probe.
http://www.ebay.co.uk/itm/Tektronix-P6042-Dc-Current-Probe-/271979183225?hash=item3f5338fc79


Do you think is that will be repairable because I read from that threat the current probe seems to be very fragile itself. Do you think it is worth bidding on this item in this condition also the condition of current probe amplifier is also unknown and  may also need repairs?

[omerd]

Hi everyone,
I know it has been a long time, but since I started this thread my P6042 was laying on the broken test gear bench and I almost forgot about it
Joe - It's really nice to see your results at higher frequencies! The potential of this vintage probe is amazing. I'm really eager to get mine fixed.
I decided to give it another try today. Unfortunately I'm still stuck at the same point: The AC part of the probe works fine and I can easily calibrate it and use it as an AC only probe, but at DC and lower frequencies where the Hall effect sensor needs to do its magic - the output I get is attenuated.
I would be really happy to know if it's the sensor itself or some stage along the signal path that's causing the problem.
If someone with a working unit can take a quick measurement to help me verify, that would be awesome!!
When I hook the probe on a wire with 1A DC I get about 0.5V just before the attenuator (and the same on J59 as the transformer in the probe does nothing at DC). Changing the Loop Gain (R24) does not change the reading by much, and when I turn it more than 2/3 clockwise I start getting all kinds of ugly oscillations.
Of course I set the bias to 0V using the balance knob and only then take my measurement.
On the collector of Q29 I get around 0.68V for 1A input, and again, changing the Loop Gain pot does very little effect.
I suspect something is wrong with the differential stages or where the signal is converted to single-ended. Either this or I have a bad sensor.
If the sensor got broken in a way it would output lower values but still work, maybe I'll just tweak the circuit to boost the gain a little, but I'm curious to know if the voltages I get at the end of the low frequency stages are correct.
The Tek documentation is really nice but I wish they would give me some numbers for testing the signal path for a given input.
Anyone else having this probe or trying to repair one - I will be happy to take measurements for comparison too.

[omerd]

OK, I think I've got it repaired!
I decided to disassemble the probe tip once again, being careful not to loose the little metal ball or damage the sensor. When putting things back together I noticed that the sliding half of the sensor isn't free to move and touch the fixed half, and is kind'a stuck in the plastic housing. Maybe the plastic (ABS?) shrieked over time or I don't know what.
Also the springy metal ground behind it was a little weak, so I bent it to apply more pressure on the sensor.
I used some emery cloth to very gently polish the side walls of the housing until the sensor could fit without much pressure.
Now, when both parts of the sensor are touching the readings I get seem to be correct!
Rupunzell was right about that:

The current transformer/hall sensor assembly is lapped to optical level tolerances (millions of an inch) after potting

I performed a quick calibration and everything is spot on. The voltage before the attenuator is around 1V per Amp at DC as expected.
I still need to get two 2N2905 transistors that I blew when I started the repair months ago... I used 2n2907 for now but they are not thermally coupled with the NPN ones so I get some thermal drift when powering up the unit.
When I'll get everything the way it was I will perform full calibration, check the bandwidth and maybe extend it a little
Joe - thank you for all the info, love your little calibration jig!

[joeqsmith]

 

This site gets so many posts, things I am interested get buried quickly and I miss a lot of them.   Was looking for some pictures I had uploaded and noticed you had solved your problem with the P6042.   Good job!   

I would be interested in knowing how well yours works once you have it finished.   



 

[omerd]

I got a bunch of the transistors I needed and matched two pairs where the little heat sinks are.
Performed full calibration again. The performance was well within spec! Unfortunately I didn't take any photos or recorded any data.
It still takes a minute or two to warm up and stay stable on the lower ranges, I guess it's normal.
The most important thing with the probe in order to get accurate results is to make sure it is properly locked and verify that the wire under test does not push the floating part upwards. Also taking care to degauss the probe from time to time to get rid of residual magnetism does only good.
As to the bandwidth I have decided not to modify anything right now, I use it mostly around 10-100 kHz where the performance is excellent. Maybe as a future project when I will have time to take it apart again
This is a really nice piece of gear, probably the oldest on my bench and yet a very useful instrument with no modern alternative at a reasonable price.
Once it has warmed up and the line is nice and centered I  know I can trust it.
Hope others find this thread useful 
Omer

[joeqsmith]

I spent a few days attempting to match transistors.   Getting a tight enough set took some time but worth it.   

[omerd]

I don't have a good old curve tracer, so either I do it manually or set up a little LabView app and do some automation.
I'm happy with the performance the way it is now, I did try to match V-I curves but did not go through full performance tests taking thermal drift into account etc. I could have done better I guess

[ehobby]

In the interest in not starting yet another thread on the P6042 I will add to this one.

I have just fixed my P6042 which was an auction parts only unit. It was also one with the cable broken at both ends about 1.5 inches from the amplifier and about 2 inches from the probe. Once the broken sections were cut out, it was not hard to reattach the ends, but time consuming. I would suggest you not try to attach the two shielded wires at the probe end directly to the coil on the socket. Leave about half an inch length in the wires coming from the probe plug and splice the wires to the new cable and shrink-rap. The coil at the socket is delicate and probably hard to find if the little wires break. The other four wires are easy to attach to the socket in the probe.

I am impressed with how well it works now, though like others, it has some drift. Accuracy seems to be very good for AC and DC.

I have not seen the cable described correctly so will pass along what it is made of in case you want to try to find a replacement.
It has seven #26 stranded wires. Two have their own braided shields (for the 50 turn high frequency AC pickup) and then all seven are in an overall braided shield.

There is no coax used in the probe interconnect cable. It might be that four shielded twisted pairs might work for the cable but the wires must be stranded for flexibility. It would be interesting if someone tried just a standard 8 wire cable or a four pairs twisted wire to see how they perform, since the original cable seems to unobtainable.

Regards

[Hamelec]

I spent a few days attempting to match transistors. 

How did you matched them? with a Curve Tracer? according hfe? thermal behavior?