- I found that my voltage regulator (7805A) has about 2,5k resistance between the output and the ground. Is it means, that this regulator is bad and I should replace it (probably it was burnt when soldering )?
How did you measure the 2.5k resistance (in circuit, chip alone or what?
The only way to test if a voltage regulator is OK, is to add a couple of capacitors (100..470nF from input terminal to ground terminal and around 22µF from output terminal to ground terminal) and test the output voltage at say 0.5A (eg. 10 Ohm, 3W resistor) and vary the input voltage between 8V and 10V while having a voltmeter over the output and ground terminals (in parallel with the load resistor).
You can't just put an Ohm-meter over its terminals like if it was a transistor.