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Yes it surprised me too. Has a sense of "its not right" doesn't it ? It was in circuit, 'scope is new and I'm new to this too so I could easily have missed something. I will get you the voltage measurements no problem, but It will likely be Thursday as I am away just now. I'll also try that battery idea see if I can test the Zeners. Your theory that the two in series are the same could be the case as if one goes short the second will still clamp the voltage down, could just be a move by the designers to halve the chance of the failure we are seeing.
2 zeners can dissipate twice as much power as a single one of the same size. Mind you, one can get a higher wattage zener to replace smaller ones.
Their design may have tried to handle the heat dissipation of the zeners, or they used two common values to get to an uncommon value, who knows...
Well I bought some more PP3's... connected 17 in series to give and actually measured the "pack" at around 160v DC. In reverse bias SO6 drops around 140 to 144 volts across it... SO11 did the same. It does seem high and I didnt see a 140v zener in a glass package for sale anywhere. However if the 39V zener you tried gave you 15v then perhaps a higher value zener would get you closer to the expected voltage.... For me unfortunately its time to solder it all back together and put it back onto the shelf... good luck and let me know if you know anyone who needs a lot of PP3s