Hot bed heating problem

[Soadyheid]

Rats! My printer has developed a hot bed problem (No longer heats up! ) The display shows the target temperature; 65 degrees but actual temperature of 0 degrees. The Repetier-host temperature graph shown no hot bed input. I've moved the bed front to back to flex the cabling but the heating LED doesn't come on though it did at one point and the bed felt warm. Checked wire connections to the board, AOK, thermister plug reseated a couple of times, still nothing. Looks like I'm going to have to dig out the multimeter and start checking components.

My last good print showed the bed temperature spiking up and down on Repetier-Host where it should obviously cycle around the target temperature, this morning it started to heat up then dropped to zero. Even without powering the hot end and bed both ambient temperatures register, the hot bed now registers zero on the display so I'm hoping its something to do with the actual bed or thermistor. Getting spares from China during a World wide lock down could prove difficult!

My only major change recently is I've just started printing face shield frames in PETG @ 230 degrees but haven't changed the hot bed setting from my normal PLA, 65 degrees so I don't think this change is related.

Can anybody advise that sort of resistance I should be measuring for both the bed and thermistor?

Anyone had a similar problem? Thanks for any help and advice.

Soadyheid

[William]

Hi，you can refer to this：viewtopic.php?f=23&t=17030
Hope can help you.

[Soadyheid]

Hi William,
Thanks very much for the link, some good information here. This is what I've done so far:

On power up I got hot end: 17/200 degrees, bed: 17/65 degrees on the display. Moving the bed back and forward had the bed temperature change to 0/65 degrees intermittently. (No power to hotend or bed, just ambient temp registering)
Measuring the resistance of the hot bed thermistor I got Open Circuit or approx 130 KOhms but it was very difficult to hold the meter probes on the plug contacts.
I then swapped the hot end and hot bed thermistor connectors round. The display then changed to 0/200 and 17/65 degrees so the board electronics looks sound, the problem is therefore with the hot bed thermistor assembly.
After unwrapping the Y Axis cabling, it rather looks like there is a break in one of the thermistor cables so I reckon I'm stuck with a major strip down and overhaul but no spare parts should be needed. Here's hoping!

What I think happened to cause the problem...

During the coronavirus lockdown I've been trying to print off frames for face shields, The print isn't too complicated but it has a foot print which takes up most of the hot bed. (See below) I had to modify the .stl file to have the initial pre-print brim moved to 2mm from the print rather than 9mm as the filament was dribbling over the edge of the glass plate!

Frame_1.jpg (148.68 KiB) Viewed 9131 times

Way back I made a cable strain clamp to prevent flexing of the Y Axis cabling (red arrow) but with the bed to the rear the cabling's been a bit tight. Most of my prints during the past couple of years have had a lot smaller footprints so it hasn't been a problem. I now need to strip the cabling out, find and repair the break and hopefully free off some spare cable length from the circuit board to relieve the strain. Then, with a bit of luck, we should be back in business!

Thanks again,

Soadyheid.

[William]

[Soadyheid]

Update!

I had to disassemble a lot of my cabling to free off some slack in the hotbed heater and thermistor wires to give me better movement of the hot bed with less cable flexing. I found a break in the black thermistor wire which I ended up splicing, the resistance is now constant over the full bed travel. Re-bound the cables with the Spiwrap to include my strain clamp. (Now longer, less flexing with the spare length. - Happy!)

Thanks again for your help! Back in business!

Play Bonny!

Soadyheid.