REPAIR AND RESTORATION OF THE SR-400
18
WDØGOF 6/22/20014
SUBSYSTEM TROUBLESHOOTING AND TESTING
4-8. V4A RX FAULT ISOLATION
Assumption: You have injected 35uv @ 6 – 6.5 MHz into pin 1 of V6 and there was ½ watt audio output. And,
when you inject 100uv into pin 2 of V4 you do not get ½ watt output. Check for 260 Vdc pin 1 of V4. If there is
no 260 Vdc check pin 1 and 2 of T6. The B+ is supplied to V4 through R126 and T6. Pin 2 of V4A should be
zero volts dc in receive mode. If it is a high negative voltage, clean the contacts (pins 9, 1 and 5) of relay K2.
Pin 3 of V4 should have 1.5 vpp RF injection from the VFO and the DC bias should be approximately 11vdc. If
the DC bias is incorrect then either R30 or the tube is bad. If there is no injection RF on pin 3:
CYCLONE II suspect C53
CYCLONE III suspect CR30, R176 or C223.
4-9. V3 RX FAULT ISOLATION
WARNING DAMAGE TO TEST EQUIPMENT POSSIBLE. See note in 4-7
. Pull V3 and check the
voltage on the socket pin 6, it should be 260 vdc. If there is no 260 vdc then suspect T2, C42 or R23. With V3
pulled inject 6.25 MHz at 150uv into V3 socket pin 6. Tune the main tuning to approximately 250 on the black
scale, and peek the audio output. If you do not get ½ watt audio out then suspect C42, C44 or T2. If the
proceeding checks are good there is a bias problem or V3 is bad. If V3 is known to be good refer to the voltage
and resistance charts in the manual to isolate the fault.
4-10 V2 RX MIXER FAULT ISOLATION
WARNING DAMAGE TO TEST EQUIPMENT POSSIBLE. See note in 4-7
. Pull V2 and check the
voltage on the socket pin 6, it should be 260 vdc. If there is no 260 vdc then suspect T1, C30 or R16. With V2
pulled inject 6.25 MHz at 25uv into V2 socket pin 6. Tune the main tuning to approximately 250 on the black
scale, and peek the audio output. If you do not get ½ watt audio out then suspect C30, C31 or T1. If the
proceeding checks are good there is a bias problem, injection problem or V2 is bad. If V2 is known to be good
refer to the voltage and resistance charts in the manual to isolate the fault. If the tube voltages and resistances
are good check the mixer injection voltage. Pin 7 of V2 should have 1.5 vpp at approximately 4.600 MHz.
4-11 V1 RX FAULT ISOLATION
WARNING DAMAGE TO TEST EQUIPMENT POSSIBLE. See note in 4-7
. Pull V1 and check the
voltage on the socket pin 5, it should be 260 vdc. If there is no 260 vdc then suspect L6. With V2 pulled inject
7.25 MHz at 45uv into V1 socket pin 5. Tune the main tuning to approximately 250 on the black scale, and peek
the audio output. If there is no audio output then C15 is open. If there is audio output but you cannot get ½ watt
ground the tie point of R3, R5 and R7. If the audio increases more than 2db then the RF GAIN pot is at fault.
Check the voltages and resistances per the charts in the manual.
