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Intermittent problems to say the least... First noticed during a print and the bed got down to 40C before the bed kicked back into life, check all the wiring after that and all seemed well.

Happened again a few weeks later however this time the bed didn't turn on straight away again checked all wiring and bed worked, I kept changing the temps and powered on a off a few times left it a few hours and again was all good. This was maybe 3 days ago.

Today it works fine then the bed turns off mid print and no amount of checking of cables and reinserting works...

I know someone is going to tell me to check the voltages and I will do tomorrow, I tactically traded my multimeter for a carpet shampooer from my father so will steal that back on the morrow.

If I've check all the cables and all seems well, no other issues with the printer, is this likely to be either a cable or PSU problem? The fuse is not blown. I'm getting a 24v PSU delivered Friday so if it is cables or PSU I'm not bothered as they are about to be demoted to... the bin, probably where it belongs.

Neither the LED on the rambo nor the bed come on this is what leads me to believe it's PSU

...or it's the fuse.

g.

The fuse appears to be fully intact... Given the bed stopped working during an active print I think unlikely to be the fuse as well. My 24v PSU will be here tomorrow (and hopefully the SSR), but the 24-12v step down won't be here till the 22nd :'( so I'll run both PSU's and report back

If it's just the bed turning off and not the entire thing power cycling, then it's probably not the PSU, unless the PSU is only powering the bed.
Much more likely to be suspect wiring or a fuse somewhere, the fact that it started working again after "checking the wiring" also implies a wiring issue.

By checking the wiring the first time I literally took the wires out of the terminals and replaced them. The second time I recut them and replace them to the terminals.

I get that it's probably a wiring issue just hope is PSU side :p. Although the bed wire isn't exactly hard to replace just as long as the actual bed is okay I'm fine . The wiring on the bed appears fine still soldered on and covered with electrical tape.

If the new PSU doesn't solve I'll replace the wiring on the bed, then the fuse, then cry

Updates, I've checked voltage coming from the PSU and its fine and there is voltage on when the multimeter is connected to the board via the Heat 2 and the LED on the board flashes. So this is likely to be an issue with the actual Onyx...

What is this going to be...? I mean as far as I can tell the bed is a coil and a resistor for the LED... so is it buggered or am I missing something or is it just a blown resistor, but visually it looks fine?

When you checked wires, did you check the power supply connector as well as the bed terminals? An intermittant connection there on the bed rail would give the same symptoms.
Another possibility is a poor solder joint at the bed.

Were you measuring and looking at leds while the bed was actually in a failure state? Testing when it isn't failing isn't likely to be too reveailing, other than to observe the normal state.

What about the LED on the bed? Does it work when things are working? Does it stop working when the bed fails?

Yes I checked the 4 wires into the Rambo and they are all 12V~. That was my first thought.

Visually the solder joint looks fine but I'll give a go at soldering it.

Yes unfortunately I didn't have the multimeter when it was in a functional state so I am going only on when it's nonfunctional.

The LED on the bed worked with the bed so heating it was on, flickering when maintaining temp and off when off. This is also true for the LED relating to the bed on the rambo.

But when I test the Heat 2 voltage with no wires and the multimeter I get Voltage and the bed light on the Rambo comes on.

So it must be bed side...? I will do a rewire using new wires from bed to Rambo as my SSR hasn't arrived and report back

Oh, from your report I was under the impression it was intermittant, only failing after working fine for a while. If it's now in a constant failure state, that should make things easier to track down.

With the wires disconnected from the heat terminals, measure the resistance between them. It should be a bit over 1 ohm...the bed is really just a giant resistor, after all. You could also measure right at the heat bed solder pads to eliminate the wires from the test. If you get either an open circuit or a much higher resistance (via the bed LED circuit), then you probably have a problem with the heat bed itself.

Okay removed the old wires and soldered and added some 16AWG into the bed, no luck same issue.

I was midway about to cry (who am I kidding I was crying) and out if the corner of my eye I notice the Rambo pin drawing power for the bed from the PSU looked grubby...

So I gave it a feel with the screwdriver I had in my hand and it felt rough and scratchy compared to the rest which were all smooth. Cleaned with some 1500 grit i had next to the printer but its still grubby and rough. Any ideas on how to clean something so small, and ideas on how this may have occurred?

I tried to upload a picture but it just timed out each time, is there file size limit...?