Assuming you have that particular version of the tester, my advice is DO NOT test anything on the zener test pins (K - A) until you have made the mods. You might get away with it if it is a higher voltage zener and you are fast but there is a good chance you will burn D2 and possibly damage the DC-DC boost IC.
There is
one item which
can be safely tested across the zener test pins (K-A) on every version of the Transistor Tester, modified or not:
This item is a 1k ohm resistorNo matter what condition the Transistor Tester is in, the absolute maximum current which will flow through this 1k ohm resistor will be safe. It will be no higher than 30mA.
In fact the “zener voltage” displayed by the Transistor Tester (for the 1k resistor) represents the mA flowing from K to A.
This is just Ohms Law: The voltage drop across a 1k ohm resistor is exactly 1V per mA.Transistor Testers with the original OSHW zener test circuit will display 2-3V for “zener voltage” with 1k resistor connected to K-A. The original OSHW zener test circuit is a 30V DC source feeding the “K” pin through an internal 10k ohm series resistor. The “A” pin is grounded. This internal 10k series resistor therefore limits the absolute maximum zener test current to 30V/10k = 3mA.
In contrast, unmodified (but working) LCR- units with APT32F172K8T6 MCU
and the poorly designed U7 circuit will display 5-8V “zener voltage” with the 1k resistor connected to K-A. This is because DC-DC converter U7 actually
does function as a constant current source (until it gets damaged).
An LCR-TC1 where the current-limiting portion of U7
has been damaged, but where the rest of the circuit still works, will apply about 30V across the 1k resistor. In this case the tester may display
“unknown or faulty component”. However, a multimeter connected in parallel with the 1k resistor will read 25-30V.
Finally, if D2 and/or U7 failed catastrophically, then the tester may report a zener voltage of either 0V or 4V.