Supply voltage drops too low when transmitter is keyed

[rw9rn]

Hello. I bought the ready K-2 S / N 5815.
RX mode, the display shows 12.7 V 0.34 A. I connected a 50-ohm dummy load, set the control power to 5.0 watts, press and hold the TUNE. At the TX mode, the display shows 9.8 V 1.8 A. Power 5 watts. Set the POWER control 10.0 watts. At the TX mode, the display shows 7.9 V 2.74 A.
Why voltage drops so much?
On J3 jack (12 V DC) at the TX mode 12.7 V. Controlled DMM.
Where to start looking for faults?

Andy V. Borisenko RW9RN