Tek A6302 Current Probe: Offset Resistor Tweakage

A package deal of two Tektronix A6302 current probes arrived from eBay, with one probe having a small crack across its case (shown in the description and bought accordingly).

The other probe worked fine and was quite clean inside:

The cracked one couldn’t be balanced, with the twiddlepot on the AM503 amp unable to bring the signal down to 0 V from a positive offset on any of the ranges.

The current transformer might have suffered some stress on the upper-left corner of the main part (in the probe body), but it doesn’t have any obvious damage:

The small ball to the left of the transfomer lid provides the slide detent; it’s an ordinary 3/32 = 0.094 inch bearing. Which, as it happens, is a Good Thing, because there’s another one exactly like it somewhere in the litter under the Electronics Workbench.

Protip: follow the disassembly procedure in the instruction manual and do it over a towel or, at least, a shallow dish. You have been warned.

Extracting the transformer from the body revealed a numeric value I didn’t recognize at the time:

The top slide contacts looked awful, but they’re actually covered in semi-dried contact grease and cleaned up easily:

Swapping the “bad” transformer into the P6302 probe I got a while ago showed it wouldn’t balance, either, but the offset was far off into negative voltages. Putting the “good” transformer into the “bad” probe produced a similar too-positive offset. Conclusion: the transformer was probably good and Something Else was wrong.

Spending more time with the manuals produced this hint in the AM503 Amplifier circuit description:

Fortunately, the AM503 probe connector has pin labels:

Note the absence of pins G and I, probably to eliminate any confusion with “ground” and “one”, respectively.

Continuity checking reveals the left end of the 34.8 kΩ resistor connects to pin H:

Huh. Even a blind pig occasionally finds a truffle: where have we seen that value before? Apparently Tek measured each transformer / Hall sensor and wrote the appropriate offset resistor value exactly where it’d do the most good.

Although I don’t pretend to know why the transformer offset has changed, if Tek can select a resistor to correct the offset, so can I:

The 82 kΩ value roughly centers the offset twiddlepot span around 0 V; it’s the result of a binary search through the resistor drawers, rather than a complex calculation.

With the resistor in place and the probe reassembled in reverse order, everything works the way it should:

The lower trace is a square wave from the scope’s arb waveform generator into a (likely counterfeit) Fotek DC-DC solid-state relay, with the bench supply dialed to 5.7 V to put 5 V across a hulking 100 Ω power resistor, thus 50 mA through the probe. The purple trace comes from the repaired probe, with the other one turned off for pedagogic purposes:

That wasn’t easy, but seems to solve the problem.

Dang, I loves me some good Tek current probe action …