Recently bought on b6ac on ebay. did some lipo charging and discharge.
it did eventually had the auto discharge 1S then over voltage problem loop and all four buttons not responding. after i did the STOP/Enter button while powering. ( bad idea. dont do it! ).
found this page http://blog.lincomatic.com/?p=1725 and i did tried the solution there.
and failed to repair it with 6s battery way.
then i decided to open it and i change the burnt resistor. and removed the irfz44n.
- i powered it and the discharge did finish without the over voltage error. NOW MENU Is back. all four button functioning. so i put back the irfz44n. upto today. b6ac is working. thou am not sure its calibrated properly. anyways, as long as it works for now. All is ok 🙂
Hope this info help you guys that have the same problem.
mcu4life 
Althou the loop menu is fixed. Voltage is not calibrated properly. i had to do the 6s solution and all is ok for now.
can you mention or give me a picture where is the resistor was burnt?. because mu imax b6ac same condition as yours.
the big resistors on the lower right bottom. the one on the right.