Luxman SQ 700X Integrated Amplifier
I've had this quite a long time, it came with the right hand channel not working. The output and driver transistors were blown and there was visible damage to associated components, resistors, and a diode.
I went to the trouble of sourcing the correct out put and drivers, they were/are no longer made but I found new old stock from an internet supplier; I installed them but initial tests suggested there was a few more problems that needed looking at probably what caused the failure in the first place. It got put aside for quite a while...I finally pulled it out again, checked my previous work and found a few more issues, some capacitors were leaking and well out of spec. I checked some of the small signal transistors as they may well have been leaky, they were however OK for now; this is quite an old unit 1970's, and they may still prove to be more noisy than a modern replacement. once the unit is up and running.
So time to test: I sometimes/often start powering up from a Variac especially when the output devices are really hard to come by (other option is to install temporary common place devices for testing the rest of the circuit) I found that with only a low mains voltage applied there was a rising DC voltage on the speaker terminals, too much to run in to a pair of speakers. More checks... it took a little while and I could find no real reason, so I carefully increased the mains voltage, at a certain voltage the DC on the speaker terminals dropped to only Milli-volts. Much better, at full mains voltage I was able to adjust the DC offset with the variable resistor on the main circuit board I also set the IQ (Bias) to a safe level just to start.
I connected speakers and a source and beautiful music appeared!
Well that's the donkey work done, now to clean controls and find and fix the details, it really needs some capacitors in the power supply replacing, I need to order them, I don't have then in my parts bin.
MORE TO COME...
I went to the trouble of sourcing the correct out put and drivers, they were/are no longer made but I found new old stock from an internet supplier; I installed them but initial tests suggested there was a few more problems that needed looking at probably what caused the failure in the first place. It got put aside for quite a while...I finally pulled it out again, checked my previous work and found a few more issues, some capacitors were leaking and well out of spec. I checked some of the small signal transistors as they may well have been leaky, they were however OK for now; this is quite an old unit 1970's, and they may still prove to be more noisy than a modern replacement. once the unit is up and running.
So time to test: I sometimes/often start powering up from a Variac especially when the output devices are really hard to come by (other option is to install temporary common place devices for testing the rest of the circuit) I found that with only a low mains voltage applied there was a rising DC voltage on the speaker terminals, too much to run in to a pair of speakers. More checks... it took a little while and I could find no real reason, so I carefully increased the mains voltage, at a certain voltage the DC on the speaker terminals dropped to only Milli-volts. Much better, at full mains voltage I was able to adjust the DC offset with the variable resistor on the main circuit board I also set the IQ (Bias) to a safe level just to start.
I connected speakers and a source and beautiful music appeared!
Well that's the donkey work done, now to clean controls and find and fix the details, it really needs some capacitors in the power supply replacing, I need to order them, I don't have then in my parts bin.
MORE TO COME...