TEK AM503 / A6302 Current probe - works but not as described.

[Jester]

I have one of these old Tek current probe setups. It works fine in ac mode, and it works in dc mode however not if I follow the setup procedure in the manual, only if I use my "Special" calibration method.

I'm guessing something is not quite as it should be. Perhaps someone familiar with these units has some words of wisdom?


If I follow the procedure:
Close the clamp (no I), in 1ma scale and Cal DC mode and then center the scope at 0V using the DC level control, I find that the balance adjustment has little effect almost as if it's ac coupled instead of dc coupled, if I move the pot, the offset moves but then resumes it's original offset. If I then switch to dc mode,  there is a large dc offset ~400mV.

IF I compensate out the offset using the  DC level control so it's zero again, I can measure DC current down to 1mA and its reasonably accurate. It does drift over time, however it's quite usable.

[Jester]

It sounds like the DC balancing loop is not functioning.  Do you have a different AM503 you can try?

No I don't

[woodchips]

One thing I found with these current probes is that you have to fully close the sliding part of the transformer, the front must be in line and definite click at the end of the movement. If it is only a fraction short of fully home then the DC measurements don't work.

[Jester]

One thing I found with these current probes is that you have to fully close the sliding part of the transformer, the front must be in line and definite click at the end of the movement. If it is only a fraction short of fully home then the DC measurements don't work.

I did notice that the probe closing is a bit finicky, that being said it's definitely fully closed when I'm using.

[duak]

Just to rule simple things out, is there a possibility of an intermittant in either the probe cable especially near the strain reliefs or in the probe closing mechanism and switch?  I recall that the cables can become less flexible over time and then stress their conductors.

Cheers,

[Jester]

Just to rule simple things out, is there a possibility of an intermittant in either the probe cable especially near the strain reliefs or in the probe closing mechanism and switch?  I recall that the cables can become less flexible over time and then stress their conductors.

I did try flexing the cables and I did not see a big change. That being said after looking at the schematic (have to love the Tek manuals from this era) the DC component from the hall effect sensor is sent to the DC amplifier via pin "H", so I might open up the probe to verify that conductor.

Can I just take apart the probe (carefully) or am I going to damage the probe in the process?

Cheers,

[Hydron]

I'm not sure you have any problem at all, and would suggest not opening up the probe!

The DC level control and Balance are for different things. The DC level control is for shifting the trace on the scope, while the Balance control is for zeroing out the probe offset. The probe is completely disconnected in "CAL DC LEVEL" mode, and balance will have no effect here.

Zero the trace using the DC level in "CAL DC LEVEL" mode, then switch to DC mode with no conductor in the probe, then degauss, then use balance to re-zero. Ideally there should be no trace shift between "DC" and "CAL DC LEVEL" mode with no conductor present in the probe. Do all this after a substantial warm-up period, there's a reasonable amount of drift!

If balance has no effect in "DC" mode then yes, you may have an issue.

These probe amplifiers are a bit strange by today's standards - while they are not a true scope plug-in module, they do a lot of the vertical scale/offset stuff normally found in the scope. Ideally you'd just have 2-3 ranges and no DC level control, and do all that stuff with the scope controls rather than the amplifier (this is how the newer amplifiers work I think). I may try and build my own amp one day to go with my probe along these lines (there is an old Jim Williams linear tech app note about doing some of this).

[Jester]

I'm not sure you have any problem at all, and would suggest not opening up the probe!

The DC level control and Balance are for different things. The DC level control is for shifting the trace on the scope, while the Balance control is for zeroing out the probe offset. The probe is completely disconnected in "CAL DC LEVEL" mode, and balance will have no effect here.

Zero the trace using the DC level in "CAL DC LEVEL" mode, then switch to DC mode with no conductor in the probe, then degauss, then use balance to re-zero. Ideally there should be no trace shift between "DC" and "CAL DC LEVEL" mode with no conductor present in the probe. Do all this after a substantial warm-up period, there's a reasonable amount of drift!

If balance has no effect in "DC" mode then yes, you may have an issue.

These probe amplifiers are a bit strange by today's standards - while they are not a true scope plug-in module, they do a lot of the vertical scale/offset stuff normally found in the scope. Ideally you'd just have 2-3 ranges and no DC level control, and do all that stuff with the scope controls rather than the amplifier (this is how the newer amplifiers work I think). I may try and build my own amp one day to go with my probe along these lines (there is an old Jim Williams linear tech app note about doing some of this).

Thanks to everyone that responded. I did not end up opening the probe, but I did find the problem (it was an easy fix).

It turns out that the ac/dc coupling switch was a tad loose on the shaft causing the contacts to be out of sync sometimes. Quick adjustment, tighten the set screw. Then use the simplified steps (than Tek instructions) in bold above and works like a charm.