Naim Nait 2 cb - value of zener in current source

[_webwasher]

Another question related to a Nait 2 chrome bumper: the front led panel is driven by a current source (my best guess) consisting of a transistor, resistor and zener.
The transistor is defective, so might be the zener. Unfortunately the voltage value is not visible without desoldering.

Does someone know what zener to replace here? Many thanks for sharing.

 

[_webwasher]

It just says “PH BZX” … but cannot read the voltage value. Maybe someone knows what kind of zener is the like by like replacement?

 

[MJS]

The CB Nait2 used a constant voltage power supply for the front panel. Current limiting was done by a single resistor on the LED board. Two strings of LEDs were wired in parallel so current sharing was terrible and they fail quite regularly.
So we've got a common collector buffer referenced off the zener voltage. The Nait2's rails are about 22v and it sits between them so we have max. 44v, minus a bit of voltage drop under load, a base-emitter junction, and some headroom. I'm afraid I haven't seen one for years, but based on the above I'm going with a 33 or 36v zener. You might want to tack a current limiting resistor in series with the display when trying it out.

Happy to be corrected.

 

[_webwasher]

That helps a lot, thanks very much for clarification and z value

 

[_webwasher]

36v zener

Seems like the led panel board has two led blocks with using 4 of 8, the third 8 of 8 led’s. That counts 16 led in series with typical 2.2volt = 35.2 volt. Plus voltage drop at the 2(?) ohm resistor on the led pcb at If 20mA. So zener with 36v sounds very reasonable!

 

[neonixcool]

Good day
Dear gents the original current source is two resistors and two transistors for led panel
I think the ztx transistors can be replace to 2n508*

Sorry for low quality photo from Chinese diy sites

 

[MJS]

That's the Nait2/3 variant for the Olive series. They have a 2 transistor (ZTX652 IIRC) constant current driver for the front panel LEDs. As stated, the CB Nait2 used a constant voltage source.