Brother PS-A365 Powersupply MFC-L3750CDW short

[johnbeton]

Hello,

I have a brother MFC-L3750CDW that did not power on any more.
After inspecting the Power supply, I found the Fuse F1 to be bad and the mosfet Q1 was internal short. After replacing both fuse and mosfet the printer goes on for 1 second, I hear the relay, than goes off and both parts are bad again, so Fuse burned and Q1 is short again.

Has any one ever worked on these power supplies ? Any tips on what to check next ?
I also replaced the supercap, because they go bad on these models.

[johnbeton]

I found a shorted diode DZ4.
The top marking says VB
I cannot find any information about this diode, how can I identify this diode ?

[R_J]

DZ4 is in parallel with R4A and R4B (2x 0.27Ω) approx. 0.135Ω so it will check shorted in circuit
VB is likely a 12volt zener.
Looking at a board image, ZD4 is a protection diode across the mosfet source resistors, this point is monitored via R7 by the CS (current sense) pin of the drive ic (MIP005?)
If you do not wish to unsolder DZ4 you can lift one end of both R4A and R4B
If the mosfet shorted it is a good chance that the drive ic is also damaged either in it's gate drive circuit OR its current sense input.

[johnbeton]

Ow yes, now I see it is because of the resistors.
The Fuse, Mosfet and Regulator keep shorting, I changed them twice. The printer goes on for 1 second, I hear the relay click and then it shuts off and the power parts are short again.
The mainboard tests fine. So maybe MIP005 is bad you think ? Any idea how to check this ic ?

[CapLeaker]

That’s a colour laser printer? They draw a lot of current on startup… makes me think about disconnecting the fuser? It needs to warm up before the printer can actually print. Make sure the printer gets good power straight from the wall outlet, not from a strip.
Fix the power supply and force it on manually. Why did the PSU fail first place?

[johnbeton]

yes indeed a colour laser printer. The fuser has 3 wires directly to the powersupply. 1 F and 2 N. Between each N I measure 8omhs resistance with F. I tought this was very low, but I understand this fuser works with a light, so maybe this low resistance is normal.
I tested the motherboard with my lab-psu and with 24v on the 3 pins of the connector, the printer starts and touchscreen works.
I have no idea why the PSU failed the first time. It was shutdown normally and next day no power.

[R_J]

Ow yes, now I see it is because of the resistors.
The Fuse, Mosfet and Regulator keep shorting, I changed them twice. The printer goes on for 1 second, I hear the relay click and then it shuts off and the power parts are short again.
The mainboard tests fine. So maybe MIP005 is bad you think ? Any idea how to check this ic ?

What "regulator" is shorting?

[johnbeton]

RC1. I meant rectifier, sorry for my wrong translation

[R_J]

If you mean CR1 that is a Triac and if it keeps shorting, I would check what is connected after it (white wire) what is connected to the white wire?

[johnbeton]

Hi, it is not CR1, but RC1 it is a D3SBA60 rectifier. All internal diodes are short to each pin

[R_J]

That does seem a bit odd that the main bridge rectifier shorts, usually if the mosfet shorts it would not damage RC1 I have seen the bridge get shorted if there is a short to ground somewhere in the primary
Can you connect just the power supply to the ac main and see if it operates? maybe when one of those two triacs turns on it causes the problem?

[johnbeton]

I tried that on advice of somebody I know. He gave me a variac and told me to add a light bulb on both pins of the Fuse F1. I tried this with only AC main connected, no connection to the fuser or mainboard.
With this setup I could raise the voltage to 230v without any component going short. After this I installed the powersupply and with everything connected, it failed after 1 second.

[R_J]

So, the fault is likely somewhere else in the printer, there are two triacs that control something, it may be whatever they control is shorted

[CapLeaker]

If the only connections are the fuser and the main board to that PSU, the main board works as you mentioned, then there is only the fuser left.