In case anyone has similar issue:
My Prusa (V1) has been working perfectly for years.
Last time I used it, the hot end temperature was no greater than 7 degrees below the set point. I was able to finish the print by raising the set point 10 degrees.
Today, the temperature dropped when I started extruding; after about 90 seconds, the temp was too low to melt the filament.
I found several troubleshooting tips, such as verify the hot end resistance is less than 8 ohms, check the mosfet, etc.
In my situation, the AT power supply (pre-ATX) could no longer deliver enough power on the 12 volt rail. At the 12 volt input screw terminals on the sanguino board, the voltage was only 9 volts.
After swapping out the questionable power supply, the new power supply reads 11.68 volts at the screw terminals. This is another not quite so old AT power supply, but will tide me over until I can order a proper 12 volt supply. I guess after six years I can finally justify that this 3D printing thing is not just a flash in the pan and is worth spending money on.
Summary: If printer was working fine, but not the extruder temp does not reach set point or cannot maintain temperature, check the power supply voltage at the electronics board input. If the voltage is significantly lower (9 volts instead of 11.7 volts) the power supply might be going bad.
Cheers!
