A few days ago I was called out to work on a HP M4555 MFP machine that didn’t have any power—no lights on the control panel or formatter board, no fans, no anything. The call was through a National Service Provider (NSP) so a customer service technician had diagnosed the machine over the phone and a low voltage power supply (LVPS) was sent to the customer’s office to be replaced.
When I arrived onsite I tried a few quick troubleshooting steps and decided to go ahead and take a look at the LVPS and noticed that one of the two fuses on it had blown. Since the NSP had sent out a replacement power supply, and I already had this one out, I decided to install the new LVPS.
Typically, when you have a blown fuse, simply replacing the fuse or the board fixes the problem. Unfortunately, once I replaced the LVPS, put the machine back together, and turned it on I heard a loud popping noise and the nice smell of electrical components burning. I pulled the LVPS out again and saw that the same fuse had blown. Obviously, the problem started somewhere else and the fuse was being blown to protect other parts from receiving the same punishment.
To further investigate, I took the fuser apart to see if it had shorted out. I also took the high voltage power supply (HVPS), also known as the ECU board*, out and noticed a big burn mark located on the board under the transport assembly. So, I had the NSP send out another HVPS and LVPS, to be on the safe side, and I replaced both parts at the same time. On a side note, I might have run down to a hardware store and bought a fuse if it was one of my customers, but with this being under contract and through a NSP I opted for the safest, not cheapest, route and replaced both.
The bottom line is, most of the time replacing the LVPS will fix a no power situation, but not always. On this HP M4555 MFP machine there are two fuses on the LVPS board. One is large and seemed to be handling the power going into the unit. The second fuse was smaller and seemed to be more of an internal safety fuse, which was the fuse that was blown on the machine. If you’re working on a M4555 you might want to take a few extra minutes and pull out the HVPS and inspect other parts to make sure there isn’t something else going on.
*This board is located in the middle of the machine. The fuser plugs into the back of this board and the cartridge contacts are located in the front section. The transport assembly sits on top of the unit.
Kevin Gumpp is a certified printer technician and freelance writer for Market Point. TKO Electronics/Market Point is a HP PartsOne Partner selling HP printer parts. If you have a question regarding this topic or have any other printer repair related questions or topics for which you would like more information, please send an email to email@example.com.