Tektronix AM503/P6303 troubleshooting

[ConnorGames]

Hi EEVblog forum members,
    I just recently got a Tektronix AM503 amplifier and A6303 current probe on Ebay for a brushless motor driver project I am planning to start soon. The A6303, however, seems to have a DC offset that cannot be canceled with the DC level control on the AM503 at ranges below 0.1A/div, even after adjusting the DC offset pot on the probe. I have heard on the internet that there is a resistor on the probe that allows the DC offset to be further adjusted, but before I do that, I wanted to ask if it is normal for the probe to deflect ~100mA when a light pressure is applied at the end of the jaws (further with more pressure). If this is abnormal and indicates that the hall device is defective and unfixable I'll try to return the probe, but if it is normal I'm willing to just adjust the offset with the aforementioned resistor and call it a day (I got a very good deal on the probe, so even if I return it I would be paying double what I paid for this probe for an older probe in cosmetically worse condition). Other than this the probe seems to work fairly well, detecting DC and AC current with seemingly reasonable accuracy (although I cannot really measure accuracy until I head back home where I have a scope with 50 ohm termination). Also of note is that I don't have another probe to test the AM503 with, so it is possible that the problem lies with it instead. I have also been running the AM503 without a 50 ohm termination because I am away from home and don't have access to my normal scope with switchable 50 ohm termination.

[Hydron]

I'm sorry to report that on my unit (A6303 + AM503 amp) putting pressure on the end of the jaws (when in the locked position) only changes the reading by ~5mA or so even with a fair amount of pressure - far less than what you're seeing.

First thing to do is check that you're using the locking switch at the end of the probe above the handle/trigger bit - this lowers the upper core half onto the lower and applies pressure via a spring built into the sliding part. A big offset and deflection with pressure like what you've seen would be expected in the un-locked position.
If you're using the lock mechanism then I'd next check that the sliding and fixed parts of the jaws are actually making good contact when closed&locked (should be almost zero air gap, as they are lapped to fit each-other in a matched set) - it's possible that the springy bits in the sliding jaw could potentially bind up and not press the sliding core half down as they should even with the lock engaged.
Otherwise if you can prove it's not mechanical then I'd strongly suspect a dead hall sensor, which is completely un-repairable.

The A6303 service manual available from here: http://w140.com/tekwiki/wiki/A6303 should have all the step-by-step mechanical dissassembly/assembly info (note - beware of the big spring in the handle!).

I would be interested in who you bought the potentially defective probe from (if on Ebay) - is good to know which of the surplus traders try and flog broken stuff off by hiding behind the "unable to test, as-is" bullshit.

[ConnorGames]

This probe was not sold as-is, so I don't want to take the risk of taking it apart, especially considering I can see the top part of the probe move as the lock is engaged. I'm going to try to return it to the seller. At this point I really don't think there was any foul play on their part, probably just shipping damage. They sold two probes and two amps and said they all worked, and other buyers left good feedback. I'll follow up if they don't accept the return.

[PartialDischarge]

To me it seems a problem with the am503 not with the 6303.

[ConnorGames]

Really. Even the mechanical pressure part? What should I be looking at? Hall device bias?

[PartialDischarge]

Its difficult to tell and the test with no 50ohm termination adds another layer of uncertainty. Usually when the hall sensor in the 6303 goes bad it creates and offset so high that the overload led lights up in the 503.

But you say it can measure dc, and if the internal dc offset adjustment of the probe doesnt solve it well, its either a weird fault mode of the 6303 or the am503.

It it was so cheap I’d keep it and try to fix it. I’ve got 3 A6303 probes, one of them had the hall effect broken, I did bypass it and works very well to measure AC north of 1khz


The hall effect is inside epoxy, so it is not moving with your pressure. If it is that then there is something wrong mechanically, easy to fix

[ConnorGames]

I finally received another A6303 (from another seller, for twice the price of the first) and got a refund on the first one, and the new one broken too. Unlike the previous probe, there is no DC offset on this one. But one edge of the ferrite is chipped, and the probe latch switch is mechanically broken. It measures current accurately at least at DC, but the broken latch switch makes it very difficult to operate and the chipped ferrite raises major concerns. The new probe reads DC currents very accurately and doesn't make noise when degaussing, so I can conclusively say the previous probe was bad. Now I just need to track down a working probe without going broke first...

[rx8pilot]

I have two of the same units. It took a few tries to get two complete working systems. I tried to fix one of the probes, but in the end I just kept buying until I got two the were working. Now I have some spare parts.

[ConnorGames]

The second probe I received looks like it would be mechanically fixable. Is 1mm of chipping on opposite long edges of two mating core pieces enough to keep the probe from meeting spec, or should I see if I can negotiate a discount with the seller (who is encouraging me to fix it), keep the probe, and fix the latch? A photo of the core piece is attached.

[Hydron]

Opposite long edges makes it sounds like they collided when it was closing, possible due to the faulty latch?
As for fix/return, really depends on how much you paid and whether you feel like a DIY job, and if you're leaning towards keeping it I'd make darn sure it works over it's full capability range as well and see what you can get back from the seller. If you're based in the states it shouldn't be hard to find another - I've seen a few come up around the $150 mark.
I'm guessing that missing a portion of the mating area will reduce the maximum flux it can handle at frequencies above the hall sensor region of operation (below this the flux is cancelled by the bucking current), and may make it more sensitive about conductor placement? Really just guessing here.