DrTim
Active Member
I needed a project to do that can be done inside so I can avoid the Florida summer heat and keep me off the couch. Believe me, the heat requires respect. Remembered that I had a SX-939 (Jan, 1974) and its European cousin, the SX-9930 (Oct, 1975) under a blanket in the carport. These have been out there for about 4 years. Don’t remember exactly how I acquired them but I do remember it was at different times. The 9930 was in better shape both cosmetically and internally than the 939. The 939 had the rice krispy sound and never got put into the rack. I had the 9930 set up for a couple months but it was lacking. Sounded labored and muddy. That’s why it ended up under the blanket in the carport.
Four years ago, I looked at the interior of the 939 thinking about doing a recap. I was overwhelmed. After years of reading the threads here and the recent recap threads, I feel confident that I have the desire, tools and ability to at least follow instructions enough to get both these units back to working condition. I plan on doing this slow as I have all summer to work on it. No hurry here.
The plan is to work both units one step at a time on separate tables. I will start with the power supply board on the 939 and when that is fully done and tested, then the power supply for the 9930 will be done and tested before moving on to the next section.
The other threads will give me guidance and info, but I will have questions along the way which I hope someone can help me along. Hopefully, I will not become a thorn in the side with some of my questions.
Starting with the power supply board AWR-052-0. The board for the 939 got very hot and burnt. I did a voltage check on the pins for both units and the 9930 checked out fine. However, the voltage for the 939 board was all pretty close except for Pin 4 which would run away with its voltage starting at +12 dcv and keep climbing past +60dcv. I’m wondering if that would be an indication that a couple of the resistors (like maybe R15 or R16) have failed? Should I find the problem of the voltage at Pin 4 before recapping and replacing the 3 transistors? I will test several of the resistors nearest the heat sinks to see where they stand during the recap and replace any that are not within 5 to 10% of specs. Am I on the right track? Resistors that you would replace because of the past heat? I know about raising them off the board. Suggestions appreciated and welcomed.
Four years ago, I looked at the interior of the 939 thinking about doing a recap. I was overwhelmed. After years of reading the threads here and the recent recap threads, I feel confident that I have the desire, tools and ability to at least follow instructions enough to get both these units back to working condition. I plan on doing this slow as I have all summer to work on it. No hurry here.
The plan is to work both units one step at a time on separate tables. I will start with the power supply board on the 939 and when that is fully done and tested, then the power supply for the 9930 will be done and tested before moving on to the next section.
The other threads will give me guidance and info, but I will have questions along the way which I hope someone can help me along. Hopefully, I will not become a thorn in the side with some of my questions.
Starting with the power supply board AWR-052-0. The board for the 939 got very hot and burnt. I did a voltage check on the pins for both units and the 9930 checked out fine. However, the voltage for the 939 board was all pretty close except for Pin 4 which would run away with its voltage starting at +12 dcv and keep climbing past +60dcv. I’m wondering if that would be an indication that a couple of the resistors (like maybe R15 or R16) have failed? Should I find the problem of the voltage at Pin 4 before recapping and replacing the 3 transistors? I will test several of the resistors nearest the heat sinks to see where they stand during the recap and replace any that are not within 5 to 10% of specs. Am I on the right track? Resistors that you would replace because of the past heat? I know about raising them off the board. Suggestions appreciated and welcomed.