Brother MFC-9140CDN – laser printing issue on black only

Sep 28, 2020
Hello everyone,

This is my first post here and I’m a newbie to laser printers. I am looking for help to pinpoint the troublesome part to my problem. I have received the Brother MFC-9140CDN printer from someone who said that there is streaking issue with the printer. I needed a printer and took a chance with a second hand one, it cost next to nothing after all!

All cartridges within the printer are remanufactured, and all have above 40% toner left, albeit cyan (how is toner remaining capacity calculated?!). Anyway, the first test page I printed came out absolutely fine (see PDF number 1). However, after printing a normal text document and a subsequent test page it is apparent that the black printing appears streaky, botchy and smudgy (see PDF number 2).

I then did some maintenance. I cleaned all the corona wires, checked and cleaned all the drums, LED heads, and cleaned the high voltage contacts with isopropanol 99%. Unfortunately, maintenance did not remedy the situation. Please see PDF number 3 for a cartridge test print. This shows the extent of the issue with the black printing. All other colours are fine.

To rule out a bad black unit’s drum and cartridge, I swapped the black unit with the magenta. This resulted in the black being printed as normal (where the magenta should be) however no magenta was printed where the black should be (please see PDF number 4).

This tells me the following:

1. The black toner and drum unit is fine (as it produced a good print in PDF 4)
2. The transfer belt (on the outside anyway) is fine as there is no vertical streaks or other issues to the whole printed document.
3. The fuse unit is fine as it is fusing all the colours (PDF 3 and 4).

My suspicion is the black unit’s LED head or power to the corona wires. My thinking is that either the LED head has gone bad, or the power supply to the LED heads is not sufficient – is that the low voltage board?? Or the power supply to the black unit’s corona wires is not sufficient – is that high voltage board?! What seems strange is that the first print comes out okay, but after time the black print degrades – is this related to the board’s capacitors? Or is this a detac corona wire issue (positive charge wire for the black unit in the transfer belt if that exists)?

Can any shed any light to my problem please?


  • PDF No 1.pdf
    416.5 KB · Views: 53
  • PDF No 2.pdf
    387 KB · Views: 50
  • PDF No 3.pdf
    654.9 KB · Views: 45
  • PDF No 4.pdf
    519.7 KB · Views: 53

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