DIY Vintage Janszen Diagnostics and Repair

The Z-N00 series:

Other than the number of tweeters, the Z-600 schematic should match up.

You won't be able to get an accurate read on the bias voltage without an electrostatic field meter, though. Regular meters load it down, but a regular meter will at least give you a sense of whether it is working at all, and generally they either work well or hardly at all.

The supply diode is a selenium stack, so it can't be checked with a meter's diode checker. It is the usual failure point, and you can replace it with a string of silicon rectifiers. Three 1N4007 or 1N4006 will do the trick, available at Radio Shack. Polarity is important, because this dictates whether the sound is in phase or 180° out. Amusingly, the schematics are all drawn with the diode drawn backward, although the negative is supposed to go to the membrane as labeled. In those days, tube designers weren't commonly aware of the notion of positive current that came with the advent of solid state components, and got anode and cathode mixed up in their schematics. Anyway, the supply will then draw less current than with the selenium diode, so the 5kΩ rheostat in series with the AC (if it has one) should be replaced with a 50kΩ pot, and whether or not there is a pot, a 5kΩ/0.5W resistor placed in series with the AC.

Checking continuity on the transformer windings is worthwhile. The secondaries are the second most common failure.

The panel should be mounted from the outside and come off by removing screws. If it's glued/stapled on from the inside, that would be unusual, although early versions of some models were made this way.

I can't be sure about the signal step up transformers until I test them, but usually they are okay if the speakers have been kept away from high humidity. If they have been stored in a damp area for a long time, though, they will not be working. If the screw heads are corroded/rusty, then the transformers are probably not working.