Today the wife and I decided to hit up some local yard sales since the weather was so nice. Glad that we did, as I scored a nice SX-636 for a good price. I have been working on restoring a SX-737 and figured the SX-636 would be a nice addition to have. When I got it home I opened it up and gave all of the controls a cleaning with deoxit. I was surprised how clean this unit was inside! I powered it up on the DBT and checked the DC offset. Everything checked out ok, less than 10mV offset in both channels, so I went ahead and put it on full line power. After letting it warm up for 15 minutes or so I dialed in the bias to 20mV per the shop manuals instruction, and then hooked up some cheap test speakers. To my delight the amp played perfectly. I listened to it for about an hour or so and then the left channel started to crackle and pop. I hurried up and turned off the channel A speaker button, and powered down the unit for a closer look. Looking closer at the power amp board I can see that someone before me has already replaced the 2SC1451 transistors with two NTE 287 transistors. Nothing else looks to have been touched inside. Are these acceptable replacement parts? I know that on my SX-737 I replaced them with KSC3503, as advised by this forum. I cannot read the printing on the differential pair transistors Q1-Q4, so I don't know for sure if they were also replaced or not. I suspect that they were not replaced, and they look to have the 2SA726 pin layout even though the workshop manual suggests that they should be 2SA763. I have some KSA-992 transistors on hand. I was thinking of matching some up and using them to replace Q1-Q4. What do you guys think? Is replacing the differential pair transistors a good next step, or should the NTE transistors be changed out? Unfortunately, I do not have any more KSC3503 transistors on hand otherwise I think I would just change all 6.