to plug power supply when doing heavy work on laptop its normal in alll laptops. They are also very sensitive to faults in the voltage supply in one or more rails, so by running the board with rails out of spec, OP is already risking new ICs giving up the ghost. I personally prefer to use laptop while connected to power source only.
While other options do indeed exist, the vast majority of issues affecting TR4 or TR5 or faulty 7805s are due to issues in the low RAM bank. You repeat the process until you spot the faulty chip(s).
If you get voltages back to normal levels, then the fault is in the leftmost half bank, otherwise it is located in the rightmost half bank. What you would do next is restore the tracks, and cut again disabling half the low RAM bank, four RAM ICs to the left. If the voltages levels are back to normal, then you know the low RAM bank is at fault. On the speccies, it is very easy to disable the low RAM bank by simply snipping the three rails to the right of the rightmost 4116. The 4116s are definitely known to run hot when shorted, so it is definitely a good idea to check. Yes, the two other voltages are definitely generated from the 5V rail, through TR4, TR5, coil, a few diodes.
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