Unknown EDC, EDN and EBC, EBN diodes

[Megatron]

Regards,

I have 6 diodes in laser printer, some with an EDC tag, some with an EDN tag on the casing.
When I test them with the multimeter in the test area of ​​the diode, they all show endless resistance in both directions "open".
I did not find some data on the Internet, possibly are some temperature diodes. Otherwise, they are black in cylindrical shape with a white ring for cathode designation, 2.5-3 mm in diameter and white letters EDC or EDN on casing.

These diodes are used in a laser printer that is connected to a sponge transfer roller that is powered and serves to transfer paper toner to make letters from an optical OPC cylinder over to paper. However, I found burned 20,7k ohm metalfilm resistor. Instead of 20.7 k ohm I put 22 k ohm and now I get the error "917 service transfer roller", it means something is still burned. I checked capacitors and other resistors and they are fine, but I can not check this EDC and EDN diodes with multimeter, it always shows "open" on both directions, how to test them?

The problem with printer is that the inprint is faded because there is not proper voltage on sponge transfer roller. The cartridge and OPC drum are new. Problem is in circuit board. Printer is Lexmark E342n.

Thank you

[floobydust]

They sound like high voltage rectifier diodes used in a multiplier for the corona wire.
These kind of diodes have a high forward voltage drop because inside they are a stack of many diodes. So a multimeter will not read anything.
Example 2CL69 4kV 5mA has a Vf of 20V, not like 0.7V for an ordinary diode. I use a bench power supply and LED+resistor to see if they conduct.
There might be dirt or a short on the HV power supply output, at the wiring or corona wire or drum. I'm not an expert on the drum error message.
The PSU has status signals to report back if there is a problem.

[Megatron]

Thanks for answering.

I think I found the way how to test this diodes. I'll put them on capacitor leakage tester which can go up to 2000 Volts. Still I don't know how much Voltage can I use to test and not to damage the diodes?

On the datasheet what you have put it is the wire on the splice on the main power supply board in left upper corner and the red wire leads to transfer roller as on datasheet.

Why are they building circuit board with unstandard resistors such 20,7 k ohm?! Why not standard renard chart 18k ohm or 22k ohm?!

[floobydust]

You can even try 20V and the diode should conduct. They are pretty tough and likely not what has failed.
The circuit around the burned resistor will give clues why it was overloaded.

I would measure megohms on the transfer roller and wiring, to see if there is carbon tracking or an arc or leakage current on the HV wiring. It's most likely toner got all over it and it's dirty.
This would overload the 20.7k resistor, if it is part of the HV output wiring.

20.7k is oddball there, unless the value is used at other places on the board. Then you would need this one value, not 19.2k, 22k etc. and easier to use a common part.

[Megatron]

Checked all high voltage diodes with capacitor leakage current tester up to 20V 200mA and they all behave normal: one position leads current, other position is open. Found 500k ohm trimer resistor failed, no changing resistance when turning the slider. I have to buy new maybe it solve a problem.