KPA500 T/R Switch Investigation October 2023

Ian G4GIR

Problem with KPA500 No Power Output was realised by the Asterisk (*) going out when drive applied. The reason was due to R23 20 Ohm resistor being open circuit in the Attenuator input circuit to the PA module. This was found by feeding a low level drive into the amp and probing the path to the Gates of the PA Devices, drive disappeared at the junction of R23 and R22.

During investigations before the open circuit R23 was found, in collaboration with Andy K3WYC, voltages were found to be incorrect within the T/R Switch circuit. Finally D10, an 1N5408, was found to have high reverse leakage current. This was replaced and expected voltages were restored.

TP2 TP3 TP4
RXTX RXTX RXTX
Expected results :- >=200V ~0.8V ~4.6V ~1.7V ~3.9V >=200V
Faulty D10 :- 30.2V 0.8V 4.67V 1.74V 3.93V 221V
Good D10 :- 225V 0.81V 4.67V 1.74V 3.92V 225V
Schematics showing the voltage measurements before and after D10 replacement have been created.

An indication of this D10 failure is that P2 has a TX voltage of 193V instead of 221V, resulting in incorrect TX voltage levels at D7/L2, D10/L1, TP2, R7/Q4, P2, and Q3/R24.

Elecraft Test Procedure

Tests were carried out to confirm Elecraft's figures for TP2 and TP4. This test is believed to be carried out with P2 (270V) disconnected and a short across P7. Expected and Actual figures are as follows. (Note the 11.3V result is probably due to the Amps lower 12 Volt supply versus Elecraft test 13.xx Volt supply.)

TP2 TP4
RXTX RXTX
Expected results :- 13.1 0.8 3.9 13
Faulty D10 :- 11.30.82 3.9111.31
Good D10 :- 11.320.817 3.9211.32
A faulty D10 cannot be determined using Elecraft's test procedure.