A few weeks ago, I fixed the issue with the protection circuit. I forgot to document the repair, so I’m going to go back later and write up what the issues were if anyone else needs to read that. Since then, it’s sounded pretty good, but recently after a while of listening (about 1-2 hours) the left channel will drop out due to some noise. It makes it unbearable to listen to, and the volume knob doesn’t do anything to lower the static, just the right channel.
So, there are two amplification blocks, as is pretty standard for receivers or amps. The first stage is a pre-amp, to bring all the inputs to the proper level, an then there is usually a second stage of amplification. Of course, this differs on the design and etc.. but this at least gives us a place to start.
So, to isolate where the problem is, there are a few methods. A method we can use on the SA-9100 is that the pre and post amps are actually separated and accessible on the back panel, so we can see the output from each one. Not all amps have this feature, but this is one of the quickest ways to isolate the issue. Another way is to just use the headphones out on the front of the stereo- headphones don’t take much power to run, so usually the headphone jack is connected to the preamp, from my understanding. In my case, it was something in the preamp stage that was causing issues.
Now that we have a starting spot, we can dive in. The schematics for this amp are incredibly in depth and give us a ton of info on the system. A great place to begin is the block diagram. This breaks down all the functions into the core parts and gives a quick walkthrough of how everything works. On the left are the stereo inputs. You can see all the inputs get joined in the switching board, and there is little active circuitry. From there, the signals get routed through an EQ assembly that applies the filters from the front panel according to the knobs you’ve turned, and then they get passed to the first amplification stage (or second, if you are using a turntable and the phono stage).
This is called the control amp. I don’t know why, it seems more obvious to call it a pre-amp, but I didn’t design it. The first problem with this subassembly schematic is that they didn’t list which channel is the left and which is the right. However, this won’t be too much of a problem for today! I was at an electronics shop earlier this week and I saw a neat product that I thought could help. It’s a cold spray that you put on thermally noisy components and when the problem goes away, you know you’ve found which one is thermally sensitive. It can also be used for finding cold solder joints that break when it gets colder and the materials shrink.
I started with a transistor Q6. I started here because I saw that the solder wasn’t OEM on the board and that a new transistor was put there, so there was a chance that this problem had occured before and was replaced in the service process. In fact, it was this transistor. Spraying with the circuit freeze completely removed the noise from the channel. It took longer to open the case than it did to find the trouble part (Q6).
So, the original part should have been 2SC1312. This is a component with 35V CE, 5V EB ratings. The one it was replaced with was a 2SC1344, which is from a similar family, but instead rated for 50V across the CE junction. I didn’t have either of these on hand, but I did have some close-ish replacements from earlier repairs on this system. In the power supply and protection circuits (note: there are two revisions, if yours doesn’t match the service manual for SA-9100, use the protection circuit from SX-737, they are the same), they commonly use 2SC1318, which is like the 1344 but with 7V base-emitter rating. Since the base of this will be modulated by the signal, and it won’t just be sitting at saturation all the time, I don’t think this should be too much of an issue. It might cause some clipping at high volume, but for casual use, it will be fine. Do take note of the pinout of the transistor, since the older packages tend to mix and match which lead is connected to what.
Voila! It works and sounds great. I’ve had it running all afternoon and it’s a dream again. This stereo really sounds remarkable compared to all the others I’ve owned, even against some other vintage powerhouses like the Sony STR6035. I also just love how it doesn’t have a display, just knobs and levers.
Anyway, I hope this helps someone revive their stereo, get a lead on their own problem, or just get a grip on some troubleshooting and repair methodology.
I did try to videotape this, but my camera kept shutting off mid-shoot. I don’t have a light on the front indicating recording, so I couldn’t tell when it stopped. I’d like to move into a more video-format direction, but also I like writing and being able to organize in a bit more organic fashion for me. I might try uploading and editing some bits just for practice, and maybe in the future you will see some video clips of me working on stuff.