I never experienced a failure with either type and never had a unit in for repair at any of my stores so all in all the panels were really reliable. Sadly this was not the same with their offshoots from RTR, Koss and others.
Brian,
Perhaps, in their earlier years, the Janszen panels were very reliable. However, in the 60 or so panels I have acquired within the last few years, approximately 70% have failed exactly as described by ztatic above. They have lost conductance from the center (membrane) connection post to the membrane. On all of the failed units, with the exception of 1, I have been able to restore performance of a weak or dead panel by bypassing this post somehow.. (I have tried various methods) In fact, the tweeter panels in the KLH Nines you gave me suffered the same problem.
Having played with several generation II panels, I have yet to see one unit have a failure. Further, the type II panel is capable of higher output. However, when working correctly, I prefer the sound of the type I panel despite its lower output.
It should be noted that measuring the voltage of a electrostatic bias supply has it's own serious difficulties as it has to measure the voltage without creating any load on the supply.
One of these days, I intend to take a day to go visit David Janszen (Janszenloudspeakers.com), son of Art Janszen, who is now building speakers in Columbus Ohio. He has been very gracious in helping me diagnose some of the problems I have encountered with my KLH Nines. Further, I hope that David decides to participate at the next AKFest in Detroit next year. I would love to hear his speakers. If they sound half as good as their lineage might suggest, they will be the hit of the Fest and perhaps the industry.
Further yet, as Ztatic mentioned, Russ Knotts (of Justrealmusic.com or eslrepair.com) is a great repair facility for repairing any electrostatic speaker. On his website, you can even find a testimonial written by me regarding his work. I have no affiliation, other than that of a customer, with Russ and I have great respect for his knowledge.
Either of those two organizations should be able to help resolve the Janszen Z-600 problem.
Clydeselsor, if I can be of help to you, please PM me.