I’ve acquired a Zenith ZVM-122-T monochrome composite video monitor that I would like to use with an Apple IIe. There is no RCA-type jack on the input plate but instead there are 4 cables coming out of a cut-out in the lower left rear corner.I have seen pictures of the exact same monitor configuration on various websites, so it is possible this could be a factory modification? The second photo shows the ends of the 4 cables. There is continuity between the AC ground pin, the shell of the BNC, the green wire, and the braided strap. Can’t I just connect the BNC for the video input and the AC connector for power and cut off the green wire and braided strap?
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There is a service manual for the ZVM-122 and ZVM-123 monitors. These were used with Zenith Z-100 micros, among other things. Here is a brochure.
On the first page, it warns not to perform servicing unless the monitor is powered through an isolation transformer. This usually indicates a "live-chassis" unit, which is not grounded internally. Until about 1980, live-chassis televisions and monitors were the norm, and they can be extremely dangerous if incorrect assumptions are made about their grounding arrangements.
But the schematics in the service manual show that the circuitry is all transformer-isolated. If there is no continuity between the BNC shield and either Live or Neutral on the power plug, it should be safe to use with any computer.
I would speculate that the additional grounding wires were to ensure that the computer and monitor are referenced to the same ground, in case the BNC shield was not connected to the computer's ground or if it became loose. Unlike some monitors that used a DC Restorer circuit, this unit is DC coupled and so any offset between the computer's ground reference and the monitor will cause problems.
One other thing: don't move the "40/80" switch while the monitor is powered on. The schematics show it connected directly to the horizontal coil.
Thanks for the great information @robespierre! Especially the caution about moving the "40/80" swtich with the power on.
There is absolutely no problem moving the 40/80 switch while the monitor is on, otherwise they would not have designed it with a switch at the back. See this video.
Also Adrian has a great video on the previous model, where he explains exactly how this switch works.