Mimaki JV33 not start after changing printhead. (Motherbord without fuse)

[B3RCIK]

Hi, my Mimaki JV33 won't start after I replaced the print head. When I turn the power on, the display only flashes once (very briefly) and then the machine immediately shuts down / goes into protected mode. I wanted to check/replace the fuse, but my mainboard is the version without a user-replaceable fuse. Any ideas what could be wrong? Thanks!
I've already checked all the fuses/protection elements on the mainboard with a multimeter – no open circuits, everything shows continuity.

The issue is with the voltages:

• When the machine is powered on (with the mainboard connected), the 5V rail drops to only ~2V.
• When the mainboard is disconnected and I test the power supply directly, it gives a stable 5V as it should.

So the PSU itself is fine, but as soon as the mainboard is plugged in and the printer is turned on, the 5V line collapses to 2V. Any ideas what could be causing this massive voltage drop?

 

[Neil]

Have you tried plugging the old head back in? To eliminate that it's the new head causing problems...

 

[B3RCIK]

Have you tried plugging the old head back in? To eliminate that it's the new head causing problems...

Nothing happens after installing the head, I'm curious if Printer can be started without connecting the head so that by not connecting it, the error can be eliminated for now

 

[Smoke_Jaguar]

Not uncommon for heads to fail in such a way where they short out. That said, might need to grab ye olde voltmeter and start probing fusing to see if any have gone open circuit.

 

[B3RCIK]

Not uncommon for heads to fail in such a way where they short out. That said, might need to grab ye olde voltmeter and start probing fusing to see if any have gone open circuit.

I have already checked all the fuses on the entire motherboard and none of them are blown. I will check the power supply on the table because I don't like the way the relays look.

 

[kehall]

Those polyfuses are likely doing their job, going high resistance due to short somewhere (they're basically PTCs so will get hot when outside their rated current range and hot = higher resistance - they don't go open circuit like a normal fuse - think of them as resettable fuses), so it's a matter of isolating things and suggest starting with the head/connection/cabling and anything else that may have been touched when swapping. When powered OFF, you can look at measuring resistance from 5v to ground and see what it says before and after connecting the head cable.