So, I've been using old Brother laser printers for around 7 years now, and when they are working, they are great. The toners with their impressive 7000 pages rating really last so long that you forget about them ever running out (unlike the HP inkjet I had before
) and at least with original toners, the printing image is very sharp, despite this printer being 18 years old since release date.
However, every now and then, something doesn't work quite perfectly. So far, the issues with it were fairly minor (just had to replace the toner once, and the drum unit, both of which were affordable from "second hand" listings (new / nearly unused).
For a while now, the printer sometimes would register when I opened the paper tray to put new paper inside, but would not consistently register me closing the paper tray again. Sometimes if I pressed on it a bit more strongly, it would recognize that it was closed, but more and more often, it would keep saying "no paper tray".
Since yesterday, this error has been present almost the entire time. When putting the paper tray in and out repeatedly, occasionally it will register it as closed for a few seconds, but most of the time it would insist that it was still open.
This is clearly an issue with the sensor that detects whether the paper tray is closed, so my idea was to identify the relevant contacts on the mainboard or the respective daughterboard and wire an external switch from there so that I can force the sensor to register as closed when needed. (Finding out the problem with the sensor's contact would be more elegant in theory, however with the way printers are assembled, I would likely have to disassemble dozens of gear wheels and the chances that I manage to put them back all in the correct arrangement, and in a reasonable amount of time spent, are near zero.)
Unfortunately, even though I spent around 5 hours on this project in the last 24 hours, I did not succeed in identifying the pin that carries the paper tray sensor signal.
I've been working with the official service manual found here:
https://www.laserexpressinc.com/manuals/Brother/brother-dcp-8060-8065dn-mfc-8460n-8860dn-8870dw-parts-manual.pdfSection 8.4.9 mentions the maintenance menu's sensor operational check, which includes "Tray1 sensor", identified by the moniker C1 here. I'm quite certain that this is the sensor that I'm after, especially since in the few times that I managed to get the "no paper tray" error to go away (felt more and more random and disconnected from the positioning of the paper tray), the sensing status in the maintenance menu sensor operational check indeed had the "C1" show up, and it would disappear as soon as I'd pull out the paper tray (or it randomly disconnected without any physical movement).
Unfortunately, the service manual either does not provide clear information on where exactly this Tray1 sensor is located, or I'm just unable to extract this information from it. The sensor overview section includes this nice illustration:
Strangely, none of the sensors here are called "tray1". The closest thing seems to be "DX tray sensor" (pretty sure DX stands for "duplex", as in the small tray that pulls out at the back and is used for duplex printing). Through disassembling the printer (to just before I'd have to take out any gear wheels), I confirmed that this one gets actuated when the duplex tray is pulled out towards the back, not by the main paper tray.
I went through the entire manual searching for keywords such as "paper tray", "tray1", "sensor", but did not find the exact location of the tray1 sensor anywhere. I also took off the metal shield in front of the PCB and used a multimeter in voltage mode to find any pins that showed different voltages when the tray was open vs when it was closed. (This was made much more difficult by the fact that the sensor working very inconsistently meant that most of the time, there would not be a voltage different anyway due to the fault I was trying to fix, but I fortunately have a similar more basic printer (HL-5250DN with different problems that I only kept for its spare parts) of the same series with almost the same PCB, so I checked the voltages there while putting the tray in and out, but could not find any pins that changed in voltages with tray in vs tray out.
I also checked on Youtube for this sort of issue, however, the way the paper tray sensing works on Brother printers seems to have changed in the last 2 decades, and since my printer is from 2006, the mechanism errors that are showed in the videos from other people on newer models seem to be unrelated. I suspect that with the MFC-8860DN, the tray1 sensor is rather deep inside and difficult to identify.
It's very unlikely that anyone here has encountered this exact same issue before and managed to fix it, but I figured I'd ask just in case. If nothing else, in the future people with the same issue will find this thread and quickly realize that it's difficult to fix and likely not worth the time.
On the off chance that anyone has an idea where the sensor or its contacts are, I'd be super grateful.
Here's also a photo of the mainboard that I took:
I'm thinking it's fairly likely to be on CN10, CN12 or CN18, but so far none of these have had the corresponding expected voltage changes, so I'm quite at the end of my wits.