Short story: Loose bolts securing heater to bed cause mysterious “not heating as expected” errors. Easy fix once the cause was finally identified.
Long version: I started having trouble with my SV08 Max bed heater cutting out while pre-heating causing Klipper to shut down, reporting that the bed was not heating as expected. After a lot of experimentation, I noticed that the Mainsail temperature graph showed the bed was actually cooling shortly before each shut-down. I confirmed this by watching the temperature rise then suddenly start falling for no apparent reason. For example, the temperature might rise about 1°C per 12 seconds from 65° to 72.5°C. then suddenly start dropping although Mainsail was still reporting 100% power to the heater. Eventually I couldn’t even get it up to 65°C.
Opening up the bottom end and taking readings with a scope and meter, I was able to confirm the power board was working properly – the SSRs were still supplying power to the heater. However, the heater resistance was going open (IE: infinite resistance) at the moment the bed started cooling instead of heating.
It took a couple of e-mails to get past Sovol’s 1st tier CSR and get this bumped up to a tech who understood how the system works, but once I got there the tech had some useful steps to take. Long story short: (s)he had me unbolt the bed and check the underside. There is a self-resetting thermal protection switch (independent of the thermistor that talks to Klipper) under there. Also, the heater itself is bolted to the underside of the aluminium bed with an array of short M3 bolts.
If those bolts are not securely tightened, the heat is not transferred to the aluminium efficiently, causing the heater to overheat even though the bed is not up to temperature.
As soon as I took the bed off, I found 4 of these bolts lying loose under the bed. One more was about to fall out, many more were loose. Two more were MIA (probably went up the vacuum cleaner hose when cleaning up stray bits of plastic some time in the past). Discolouration and flaking of the lacquer coating on the heater underside shows it was getting pretty warm, especially in the area of the thermal protection switch, which was surrounded by empty bolt holes.
With all bolts in place and snugged down, the bed works properly again.
One final note: The bed and heater rest upon 8 plastic standoffs. The legs have screwed-in brass thread inserts (not heat set inserts). I had a heck of a time reinstalling the bed – it just didn’t want to sit down properly. I took it off again and discovered two of the brass inserts had partially wound out of the standoffs when I initially removed the bed and were now sitting proud, preventing the bed from touching their respective standoffs. Threadlocker often makes plastic brittle, so I used a drop of CA glue when re-securing the inserts into the plastic.