Debugging RAMPS for heated bed problem

Hi,

Earlier this week I started using my printer but the heated bed refused to work. I googled for hints what to try but couldn't figure out the cause of the problem. Except things to measure from RAMPS (1.4) and they are:
- Voltage on PSU pins: +12V (OK)
- Voltage between PSU - and D8 pins when heated bed is turned off in Pronterface: +12V (OK?)
- The same as above but heated bed turned on in Pronterface: 0V on both D8 pins (no voltage between D8 pins --> no power to heated bed)
- Resistance of the heated bed when detached from RAMPS 1.1 Ohm (OK)
- Voltage between MOSFET pins and PSU-: 0, 0, 5V (not sure if I got it measured properly)
- Fuses seem to be ok (measured resistance close to 0 Ohms)

And the LED on RAMPS doesn̈́'t turn on when heated bed is attached and turned on from Pronterface. But when the heated bed is detached and turned on in Pronteface the LED switches on (and there is voltage between D8 + and -).

Any suggestions what's wrong? The notorious MOSFET? Or the heated bed? The printer was working ok for a couple of years without problems before this happened.

Thanks in advance.

- Voltage between PSU - and D8 pins when heated bed is turned off in Pronterface: +12V (OK?)

yes, the + pin on D8 should always be 12v

You need to look at the -ve pin, its the line that is switched. When off its at 12v (ie difference between d8 + and d8 - is 0v, ie off)
When D8 - is on it will drop to 0v (ie difference between d8 + and D8 - is 12v, ie on)

- The same as above but heated bed turned on in Pronterface: 0V on both D8 pins (no voltage between D8 pins --> no power to heated bed)
"0v on both D8 pins" that is not right.

D8 + pin is directly connected to 11Amp fuse then to the 11amp power plug. It should always be 12v

Potentially you have a bad fuse. (yellow square thing) they are polyfuses, they heat up and open when to much current is pulled through them. Then reset when they cool down.
But they can fail.

Annycubic i3 mega
I have been the proud owner of an Anycubic i3 Mega for some time now.
I took over the printer in a defective condition.

After smoking and changing the motherboard, no heating at the heat bed. Voltage for heating element is not switched. No continuity after measurement T1. I put on the heatbet with 12 volts and gets warm (manual heating). what else do I have to do about the temperature on the main board (main board Trigorilla 2540 I only replaced the steppers). can someone tell me what problems I have so that the Anycubic i3 Mega prints again.
schmitt-45

"No continuity after measurement T1."
This should be bed thermistor and should read a resistance. 100k at 25 degrees C, will decrease as temperature increases

M105 should display room temp for bed before it will heat up.