Tektronix A6302 current probe hall effect sensor diagnosis

[chrisLF]

Hello everyone.

After lots of browsing and eevblog videos this is my first post - please be gentle :-)

I've been amazed by how many experienced and clever people there are on here, so a big thank you to everyone who's posted all the comments I've been reading for the last while.

So here goes...

I'm trying to diagnose a Tektronix A6302 current probe.
I bought it off eBay as a non-working / untested but complete system comprising the following.


TM502A frame : All voltages and the transistors look fine so hopefully can rule this out
AM503            : had a blown fuse and I traced the cause to a shorted bridge rectifier - all supply voltages now as per manual
A6302 probe   : well worn but no visual mechanical damage

I also have another working set, so have swapped out the parts and the fault always follows the probe.

The overload LED comes on whenever the probe is connected
It disappears during a de-gauss but comes back afterwards

Looking when I first opened the probe, I found a missing wire and also that one of the common mode inductor wires was unsoldered / broken off - I replaced it

With the inductor disconnected so there is no AC sensor, I can see the Hall effect react to DC via the amplifier and a 50R scope input.  It only works up to about 200mA without setting off the overload LED though, and looked pretty linear over that range.

With the inductor connected, the LED lights permanently.

I took some DC resistance measurements across the disconnected sensor head
the coil measures 1R27
Hall sense to hall supply between 100 and 110R
Hall(-ve) to hall (-ve) = 20R (Datasheet shows 10+10R so that looks good

I also took some voltage readings at all the probe connections within the AM503 and are in the table below
I started with no probe connected, then my good probe, then the bad probe with no inductor connection, then with the inductor, then with the top of the core mounted, and finally fully re-assembled and locked jaws.


I calculated the differential voltage across the two hall outputs in one of the lines below.
I can see that the output amplifier seems to hit either rail whereas with the good probe its around 0V
Also noted the good probe has both hall outputs around 600mV - is this related to the bias resistor?

When the probe is un-assembled but with the top of the core in place, I can feel the magnetic pull when powered.
During turn on and de-gauss, I can hear the probe head vibrate so that looks good.

It's quite a complex system to get my head around with a full feed back system, so hoping for a few pointers from someone with experience in this system.

I'm obviously hoping the head is not damaged, and I've read that Overload LED = fried hall,  but as they are expensive to replace I want to be sure either way.


Thanks in advance


 

[chrisLF]

Here's a few pics.

In the posts I've read, I've always found the pictures really handy to help understand what's going on.