I built a TM500 mainframe tester, and updated the design. Someone might find this useful? GROUNDING ISSUE

Mac Perkins

Many thanks to Jared for his elegant design, I've been meaning to build one of these for some time.

After building the test unit and starting to test frames, I encountered a problem in the beta test positions of the switch.

The BNC connector is not isolated and is wired across the meter HI and LO posts, The toothed lug washer shorts the LO terminal to ground through the front sub-panel and the frame of the module.

Fortunately the 5K1 resistors seemed to limit the current available and prevent damage.

My solution was to connect the BNC ground lug to the GND terminal with a short wire rather than the pad provided which connects to the meter LO terminal . It is still a good idea to disconnect the oscilloscope in the beta measuring positions as even the 1 meg scope input influences the reading significantly.


Shaun M


Thanks for posting this. I encountered exactly the problem you described and I had just started to troubleshoot it: I thought perhaps I had a bridged solder connection somewhere. Problem solved!

Shaun Merrigan

Jared Cabot

Oh wow, nice catch. I think a fiber or nylon/etc washer or the like would be a workable fix here.