I got green LEDs as replacement for my vintage Marklin 7188 signals.
https://www.ebay.com/itm/294482539108?var=593044434873

They work fine in analog AC mode with classic 4-button switch box.
The LEDs do not work when signal is powered from a k83 decoder in my digital layout. Surprisingly, lights work when switching yellow current and signal ground connections. However, then the solenoid does not get power, so no pulse switch.

I did some testing and found the answer: this particular LED

The green LED is polarity sensitive when powered from DC source, with current to bottom of LED.
The green LED works with standard AC power (yellow, brown) from white transformer, regardless of polarity (switching wires).
The green LED works with digital AC power (red and brown) from CS3, regardless of polarity (switching wires).
The green LED connected to k83 decoder (yellow current wire and brown ground) only works when brown connects to bottom of led (current), instead of yellow.  So now it is polarity sensitive, just like DC transformer source, but with ground as lead.  A yellow-brown wire switch does not activate the solenoid, obviously, which requires normal yellow current and brown ground.

I have white LEDs from another supplier and they work fine in all setups (incl normal yellow-brown from k83), and are also not polarity sensitive in DC.

Thus, the green LEDs are polarity sensitive from k83, like they are from a DC source, except that the wires are now switched.  The white LEDs are not DC polarity sensitive, so yellow-brown wire to LED does not matter, which provides a quick test for suitable replacement bulbs.

Now contacting vendor and ask what they suggest, since these are advertised as Marklin bulb replacements.

(sorry voor het engels)
Ben

Beste Ben,

Om de lichten te laten branden van een 7188 sein heb je naast de "gele" draad (die ook voor het schakelen van het relais gebruikt wordt) ook een massa-verbinding naar de baan-massa nodig.
Deze levert een K83/M83 niet via de 3 aansluitingen (rood, geel,groen) van de K83/M83. Je moet dus vanaf het sein een extra massa-verbinding maken met de "0" van het digitale systeem. (dus na de centrale). Op de 7188 is dit de middelste bus.

Uiteraard kunnen daarbij polariteits problemen én flikkeren van de LED's optreden omdat deze niet met een zuivere DC-spanning gevoed worden.

In English:
To make the lights work on an 7188 you will need besides the "yellow" cable (which is used for switching the relay) also a ground-connection to the layout-ground.
Your K83/M83 do not provide this through the three connections (red, yellow, green) on the K83/M83. You will need to connect your signal to ground of your system (beyond the central station). On the 7188 this is the middle lower bus.

John (VT601)

Bedankt, John.  Je hebt gelijk. Ik gebruik een ground kabel voor al mijn signalen, in plaats van de base plate. Het probleem blinkt dit type led dat dc polarity sensitive is. De yellow van k83 is dc, maar andere polarity. Een rectifier issue.