While recently troubleshooting a piece of non-Tek, tube-type test equipment, I
discovered a condition that might be of interest to this group. A misconnected
selenium bridge rectifier caused the polarity of a 12V DC filament supply for a
critical stage to be reversed. This forced the 3000 mfd/15 V filter capacitor
(Sprague TVL-1170 clearly marked "can negative") to operate with reversed polarity.
The instrument has apparently functioned this way since leaving the factory in 1965.
Surprisingly, the capacitor measured 2420 mfd and exhibited low leakage current in
this reversed polarity operation. When I experimentaly tried applying the proper
polarity to the capacitor, the leakage current soared. It appears that at the factory
the relatively high internal resistance of the selenium bridge rectifier might have
limited the leakage current sufficiently until the capacitor could reform itself for a life
of reverse polarity operation.
This raises the question of whether such aluminum electrolytic capacitors are
electrically symmetrical until initially polarized at the factory. If so, perhaps
if one wanted a "can-positive" electrolytic, it might be possible to carefully
reform a standard one.