INFINITY RS-12 SUBWOOFER troubleshooting – Fuse blown – relay not closing

INFINITY RS-12 Troubleshooting; For all problems (except blowing fuses)

After amplifier removal from cabinet, pull back plastic cover to examine unit. It is usually not necessary to completely detach the cover from the amp assembly.
CHECK TO SEE THAT RELAYS ARE CLOSING NORMALLY AND CONTACTS ARE
CLOSING: (Power LED goes ON, two audible "clicks" are heard 2 - 5 seconds after power is turned on):

 1. Attach power cord, connect AC signal (with Y-cable) into low level inputs; 100 mV @ 100Hz or music signal. Turn UUT on.
To confirm Relay 101 (power supply) is functional, check voltage across relay coil (see drawing for location) for 19 - 21 VDC. Then check voltages across R105/R106. RS-10: 12 - 13 VDC; RS-12: 22 - 23 VDC. If voltage is considerably higher there is a short causing the entire power supply voltage to appear across R105/R106. Or if voltage is absent see "NEITHER RELAY IS CLOSING" below.
To confirm Relay 102 (speaker output) is functional, check voltage across relay coil (see drawing for location) for 19 - 21 VDC. Attach 2 - 4 O power resistor to output cable of UUT. Check voltage across R147 for less than 50 mVAC. Little or no voltage across R147 assures R102 contacts are closing normally. (See drawing for location of R147.) If voltage is absent from relay coil see "FIRST RELAY (RLA 101) IS CLOSING, SECOND RELAY (RLA 102) IS NOT CLOSING"

Neither relay is closing

1. Check to see if LED is on, if not check to see if line fuse is blown. Replace if necessary — if fuse blows again see section IV: "UNIT BLOWS FUSES". If line fuse is not blown and LED is not on, check all AC connections at terminals and condition of line thermistor (should have DCR of 9 - 12 O). Otherwise replace toroidal transformer if required.

2. If LED is on and 21 VAC is present at the amp board (blue & black wires), and if 19 - 21 VDC is present at Relay 101 coil but no "click" is heard and rest of unit does not energize, replace Relay 101.

3. If no DC voltage is present at Relay 101 coil, check DC voltage across C103/C105 for +/- 15 - 17VDC. If voltages are very low unplug ribbon connector to filter board and re-check. If voltage rises to correct value than one of two conditions exist: A. C102, C104 are shorted or defective (most likely), or B. There is a short on the filter board, probably IC 201 (LF 347N). Replace and re-test.
Note: IC sockets are recommended for the new IC's.

If after IC replacement only one correct voltage is present check/replace appropriate Zener diode (D108 for C103, D111 for C105). 4. If DC voltages across C103/C105 are correct and Relay 101 is not closing: A. Unplug ribbon cable attached to input board. B. Insert a 15k resistor into leads 5 & 8 of the unplugged female connector end. (See drawing) C. Turn on UUT. If Relay 101 still doesn't close, check condition of D130 with "diode check" function on DMM; if shorted, replace. If O.K. replace Q116. 5. If DC voltage is present at Relay 101 coil and if it still doesn't closes, replace Relay 101.

First relay (rla 101) is closing; second relay (rla 102) is not closing

1. Check for 21 volts across Relay 102 coil . If present, replace Relay 102.

2. If no voltage is present: Check DC voltage across R105/R106. If voltage is substantially higher than (12-13 volts for RS10; 22-23 volts for RS12) there is a short on the 16v circuit.

To troubleshoot

A. Unplug ribbon cable attached to input board.

B. Insert a 15k resistor into leads 5 & 8 of the unplugged female connector end.  

C. Turn on UUT, second relay may now turn on and R105/R106 voltages may be correct. If so, replace IC 201 on input board.

D. If voltage is still high check zener diode (D116 for R106), (D115 for R105) for voltage abnormalities. Forward bias of zener can be checked with "diode check" function on DMM in the circuit or by checking for +/- 16 volts across zeners when energized. If either diode is defective, replace.

3. If no voltage is present at Relay 102 and voltages are correct across R105/R106:

Check for +15-16 volts on pin 8 of IC 101 (LF 347N) on main amp board.

Check for +1.7 to 1.9 volts on pin 9.

Check for +8 volts on pin 10.
If any of the above voltages are abnormal

Check DC voltage across R116 (33KO). (See drawing for location) Less than 50mV should be present. If higher, relay will not energize. DC is coming through the amplifier section. Check and replace semiconductors as necessary.

If there is negligible voltage across R116, (normal condition) check junctions of Q102, Q103 with "diode check" function on DMM in the circuit. Replace if defective. If O.K., replace IC 101 (LF 347N) on main amp board.
If all voltages are correct on IC 101:

Check junction of D118 "diode check" function on DMM in the circuit. Replace if defective. If O.K., replace Q101.

Unit blows fuses

Two conditions are possible: UUT blows fuses instantly when power switch is thrown, or UUT blows fuses when power supply relay is energized (due to an input signal).

Unit blows fuses instantly with no input signal

Desolder, remove power supply wires from main amp board (blue & black). Replace fuse, power up unit again. If fuse blows, replace shorted toroidal power transformer.

Unit blows fuses immediately after first relay is energized

A. Problem is usually shorted output or driver transistor or diode semi-bridge on main amp board.

These consist of: 2SC 3856 / 2SA 1492 TO-218 package; BD 911/912 TO-220 package;  FEN 16 / FEP 16 TO-220 package

Note: First check condition of D102,103.

All junctions can be tested in the circuit with a "diode check" function on a DMM. If a short is found; however, remember some devices are in parallel and will have to be removed to confirm the short.

B. Occasionally the output transistors seem O.K. or after a replacements high currents will continue to blow fuses. The problem is probably with one of the biasing transistors Q104,105,106,107,108,109.
Recommended final inspection

Check for a burnt or deformed thermistor (attached to power cord plug & switch). Replace if defective. Make sure the toroidal transformer is tight and will not move easily by hand. Make sure the Phillips screws holding the main amp board (on the front of the faceplate) are tight. (Loose screws mean poor transistor heat sinking, possible premature failure). Make sure the correct line fuse is in place; RS-10: 3A, RS-12: 4A — (all ratings at 120v). Make sure the red, red/black output wires are still maintaining an airtight seal as they thread through the plastic amp cover.