>>The device appears to be a "current regulating diode" analogous to a
>>zener.... 15mA!

>>In this instance the LED and "current regulating diode" are placed in.
>>series, and the combination can be driven with a voltage between 4V and 20V.

>>Further testing shows that below the "turn on" current level the device has
>>a fairly low impedance (approx. 200ohm), above the "turn on" current level
>>the device has a fairly high impedance of more than 100Kohm.

>>The only marking on the device is E153.

>>The symbol the vendor used in the schematic was a circle with a bar, kind
>>of like an arrow head with a bar (for a diode).

>>What is this device?  Am I missing something simple?

  I first saw this device in a Kenwood automotive audio amplifier about three
years ago, while testing the constant current source in the pre driver stage.
I thought it was defective, since it didn't measure properly..and seemed
leaky. In the Kenwood KAC-820 service manual, it's listed as--none other
than--Constant Current Diode E-153. It's used in this circuit with an NPN
transistor between emitter and base, with no current limiting resistor (of
course).