Heathkit Nixie DMM behavior

[rea5245]

Hi,

I picked up a Heathkit IM-1202 digital multimeter with nixie tube display cheap at a flea market. It appears to work: the display turns on and it gives approximately accurate readings for resistors and AA batteries.

I needed to troubleshoot an old Heathkit IO-21 oscilloscope so I figured what better way to do it than with my Heathkit nixie DMM? But when I connect the negative terminal of the DMM to a ground point in the 'scope and turn the DMM on, the DMM display stays dark. If I disconnect the ground, I see small sparks.

The 'scope is turned off and unplugged so this can't be a ground loop. The positive terminal of the DMM is not connected to anything yet, so there's no circuit between the scope and the DMM. Even if there are charged caps in the 'scope, the fact that there's only one wire between the 'scope and the DMM should mean there's no problem. Shouldn't it?

Any idea what's going on?

Thanks,
   Bob

[floobydust]

The Heathkit IM-1202 has an earth-grounded chassis/floating inputs but there might be a path for a DC rail or something internally to be (accidentally) touching the chassis ground.

I would say you have an oddball ground fault inside the DMM.
Check continuity between the multimeter's chassis GND, power cord GND and (-) input jack, I think with polarity switch "-".
Check C118 on power transformer secondary BLK wire 0.01uF 1kV.

The scope is probably still grounded, even though unplugged its case can touch other equipment or a metal workbench.

[rea5245]

Thank you. The scope and the prongs of the power cord are resting on an anti-static mat, which in turn is on a wooden table.

I'll look into your other points.

[floobydust]

A bench anti-static mat can also be a part of a ground fault. Some anti-static mats are low resistance (100's ohms) - always good to know your mat, by taking an ohmeter to it. Some people use them upside down, where the highly conductive layer is not on the bottom-side but instead on the top-side.

Usually a mat is grounded through a 1MEG resistor, but some people wire it direct to powerline (earth) ground.
Placing metal equipment on top of the mat can also give it a low resistance path to earth ground, or between equipment enclosures. Think of having a bare metal benchtop, anything touching it can ground it.

But the first link is the multimeter is not floating, so I would also chase that down.