Anet A6 3D printer

Further, thermistor is ok, works when connected to my test meter that has a temperature input. Because of the errant insulation one of the thermistor wires shorted to the, freshly cleaned, heater block. A new main board is on order (a Big Tree Tech SKR V1.4 Turbo) the blown one was a V1.3 which I now know inside out and backwards. A wise man once said, 'learning by experience is fine, just don't lose the receipt!'

Just been checking on Github, it seems I'm not alone. It seems that a number of people have have similar problems, reading between the lines it's when messing in the hot end area with the power on, certainly that's what happened in my case.

Ok, the heater cartridge has an internal element contact to the case, it's possible I over tightened the securing screw while clamping it, in defense I did only 'nip' it up. A contact to the heater block from the 'hot' side of the thermistor puts 12volts down the 3.3volt side which is carried to ground via LV5, a tiny surface mount inductor. Once that blows the 12volts finds its way into the CPU which fries it.

My new board arrived, eventually, a BigTreeTech SKR V1.4 not Turbo, but there's so little difference. At the same time I ordered Power fail, Power resume board. Fitting is easy, finding what to change in the Firmware not so much!
 
The Capacitors hold the power up long enough for the Z axis to rise 2mm and save the stop point to the flash memory. Next time you power up you can continue the print.
That's the theory. I did notice while testing that the voltage decay to the board took several seconds. Knowing that electronics dislikes part voltages I was a bit concerned. Sure enough after about 5 or 6 trials the new firmware failed to load. Then shortly afterwards I got a TMC connection error.
Another new board was ordered, my recommendation to anyone reading this, don't try using the UPS option, it now resides in a drawer gathering dust.

To move on, the new motherboard arrived, I ordered from a different supplier a new Hot End and a spare heater cartridge. Being forewarned I checked a few things 
 
The hot end so badly over tightened that one leg of the heater cartridge was already short circuit to the block, the thermistor retainer so badly over tightened that both wires were shorted to the block, I complained and the vendor has given me a rebate, somehow I don't think I'll be buying from him again.
I did get up and running to run into my next problem!
 

Having fitted the new hot end I switched on, and immediately realised what the problem had been with my old one! The new fan is twice a noisy as the old one and blows much more air! I ran for about a day before my next problem which has taken me 2 weeks to solve. While printing tyres in TPU for my Savage Bobber the printer would stop feeding filament, or push the filament out of the side of the extruder block about 3 hours into a 3 1/2 hour print! Ultimately I discovered that the hinge bolt (it should be a fitted bolt but isn't) for the pinch roller had come loose, allowing the filament release lever to move off line. I should make a proper fitted bolt but in the end I took the Neanderthal option and Lock tighted the bolt in its final position. As of yesterday I am now printing again. I've bought some locally produced LW-PLA, I have very little left of the Colorfab which performs so nicely.

I seem to be copying your tails of woe Klippy. My hot end failed on my Creality CR10. So bought new thermistor and heater cartridge. Fitted them, no difference. So bought a complete new hot end which worked fine for about an hour then clogged up just above the heater block. Keeps doing it now so completely lost on a fix. Any ideas  before I open the window and conduct a maiden test flight with the printer?? 

Dont know anything about printers but my hot end packed up years ago 

There's blue pills for that BB!   I wish a blue pill would sort my printer! 

Hi Ralph, clogging above the heater block would appear to indicate it's a bit too hot there, the filament is melting before it gets to the block, is the cooling fan blowing hard enough? Or do you not have enough clearance between the block and the carrier. A picture would be nice.

The head is set up as per the original. The only thing I can think of is heat creep. But everything is as per the original head. I can only think this is a cheap copy so doesn't have the correct tolerances.

Hot end without cooling fans

 

With cooling Fang

 

I think I may have found the issue Klippy. Explaining to you in the forum triggered a thought. Just doing a test print now. I dont think the bowden tube feeding the filament was going into the hot head far enough. I did some measuring and found it 6 mm short of the nozzle. Which correlates to the size of the blockage I was getting. I thinned the wall of the bowden tube so it goes all the way down to the nozzle. The test print will tell me if i've fixed it. Fingers crossed.

Well 2 hours into a print and all is going well. So that confirms my theory. 

Great, it always helps to talk about the problem.

Despite the obvious advantage of having the filament feed motor remote from the head I was aware there can be issues with a bowden cable feed.
It was the main reason I chose a direct feed high temperature hot end which hopefully avoids blockages or at least makes them simpler to clear. 
Time will tell.