Tektronix A6302 and A6312 Current Probes

[Gandalf_Sr]

I'm trying to get set up with an AC/DC current probe capability that's around 20A and 20-50MHz but I'm on a budget.  I actually have an Agilent MSO7104B so I could work with their Autoprobe 1147A/B or N2893A probes but they are hard to come by and cost an arm and a leg and an eye.

I recently bought a pretty beat up A6302 with an AM503 Amp in a TM502A housing, the eBay listing which was 'used' included pictures of it 'working' i.e. showing a trace on a scope http://www.aplussurplus.com/mirror/63306a.htm and I can replicate something similar but the indication I get is all or nothing meaning I can set the trace at zero when there's no DC current and see the trace jump up to a steady line when there's 2A flowing through the probe but there's no linearity - e.g. I'd expect half the Y deflection at 1A but that's not the case.  Maybe it's operator error, maybe the probe is junk, maybe the AM503 is junk, maybe a combination of all 3!

I cleaned up the probe and it looks like it might well be serviceable.  Given that I paid less than $200 for the lot, I'm keen to see if the probe works so I bought a clean 'used' AM503B from eBay that's on its way to me; if that works with my probe, I'm happy.  I may try to repair the AM503 if I get time.

Any advice on tests I might perform or known points of failure for the Tektronix probes and Amp would be appreciated.

[EDIT1]  I changed from using a switched mode PSU (Extech 382260) to an old linear PSU to generate the DC current and now I get uniformly linear change on the scope as I ramp the DC current up, looks like it's working after all   I think the bezel behind the knob is supposed to light up but it doesn't - in fact nothing at all lights up on the AM503;  I think it's RTFM time.

[EDIT2] Opened up the probe and found that a white wire with black tracer was broken, it was the signal from the switch to indicate that the probe was open, some work under the microscope put that back as it should be and now I get the 'open' indicator LED lighting up on the AM503