Hey all - I'm an experienced electronic tech who is new to the site. I am attempting to repair a friend's SX-626. The right channel is working, the left channel is not. I have traced the problem to the main output board, and am inputting a 2kHz 400mV audio signal through the AUX input, and using the right (working) channel as a "model" to compare to the left channel (not working). I've done a little looking around, and this is what I have found thus far:
Static checks (no power) on the power output transformers Q9 - Q12 appear okay, as do all intermediate transistors that have been statically checked (no power). With the exception of Q3, are all biasing properly at about 0.6v (base to emitter and collector look good).
Input audio is seen and is the same at the base of Q2 (right channel) and Q1 (the left channel with no output to speaker).
The unit powers on with no fuses blowing, and I am currently using 0.75 amp fuses (instead of the 2 amp fuses as per spec) for additional protection - even the .75 fuses do not open on power up.
The problem appears to be centered around Q3, which had very high bias voltages of 1.3 volts and 1.0 volts on the static check from base to emitter and collector (no power on). With power on, it does not appear to be conducting audio. When powered, I see audio at the base of Q3, with 0.0v at the emitter, and 55 Vdc with no audio at the collector.
Comparing this to Q4, (Q3’s counterpart on the working right channel) I see the inputted 400mV audio (riding on 1Vdc) on the base, 0.7 Vdc on the emitter (with no audio), and the amplified volume-controlled audio (15V sine wave) on 15Vdc on the collector.
Also, just for information - static resistance check at VR3 on the left channel, (100-ohm pot) is 41 ohms, somewhat higher than its working counterpart on the right channel VR4 (50 ohm pot) at 14 ohms, although VR3 is 100 ohms while VR4 is 50 ohms (?) so this may be normal.
None of the pots (VR1 through VR4) have been moved from their original position.
All other components checked show no problem (resistors, capacitors, diodes, etc.) My plan is to replace Q3 first, and see where that takes me. Comments, ideas? Thanks.
Static checks (no power) on the power output transformers Q9 - Q12 appear okay, as do all intermediate transistors that have been statically checked (no power). With the exception of Q3, are all biasing properly at about 0.6v (base to emitter and collector look good).
Input audio is seen and is the same at the base of Q2 (right channel) and Q1 (the left channel with no output to speaker).
The unit powers on with no fuses blowing, and I am currently using 0.75 amp fuses (instead of the 2 amp fuses as per spec) for additional protection - even the .75 fuses do not open on power up.
The problem appears to be centered around Q3, which had very high bias voltages of 1.3 volts and 1.0 volts on the static check from base to emitter and collector (no power on). With power on, it does not appear to be conducting audio. When powered, I see audio at the base of Q3, with 0.0v at the emitter, and 55 Vdc with no audio at the collector.
Comparing this to Q4, (Q3’s counterpart on the working right channel) I see the inputted 400mV audio (riding on 1Vdc) on the base, 0.7 Vdc on the emitter (with no audio), and the amplified volume-controlled audio (15V sine wave) on 15Vdc on the collector.
Also, just for information - static resistance check at VR3 on the left channel, (100-ohm pot) is 41 ohms, somewhat higher than its working counterpart on the right channel VR4 (50 ohm pot) at 14 ohms, although VR3 is 100 ohms while VR4 is 50 ohms (?) so this may be normal.
None of the pots (VR1 through VR4) have been moved from their original position.
All other components checked show no problem (resistors, capacitors, diodes, etc.) My plan is to replace Q3 first, and see where that takes me. Comments, ideas? Thanks.
