Yamaha RX797 AMP Repair
A regular automotive customer brought in this Yamaha RX 797 amplifier with a no power situation. Firstly, I was able to download a service manual to make the diagnosis a little easier. Yamaha RX-797 service manual link
This included a self-diagnosis function to guide the repair. However, this was a no power symptom. First things first. Checked the power cord for continuity – OK. Checked the fuse – OK.
This system uses a small switch mode power supply triggered by the front panel power button to activate a relay, turning on a big, chunky transformer. The transformer primary side thermal fuse was tested and found to be OK. All the secondary windings on the transformer were checked for correct resistance.
Now I’d like to say that I was able to diagnose it down to the individual component through testing, experience and skill, but that wouldn’t be the truth.
I did some research…..
Others in the field had found that one of the capacitors in the power supply section tends to fail.
After removing the C254 polypropylene film cap, which should have a value of 22nF, this 630V capacitor let the amp down by only producing 3.38nF of capacitance.
I was able to find several of these units in stock at my local Jaycar store.
After installation, the amp gladly powered up and all the features, except one, worked fine. As soon as I pressed the ‘CD Direct’ button, the display would slowly fade after a few seconds. I was just about to do further testing, diving deep into the reason for the fault, but then………..
I did some research…..
I wondered if the owners’ manual might be able to help. I flipped through the pages and found exactly what I was looking for.
Therefore, there was no need to look further into what at first, I thought was a fault.
One great feature that is offered was a diagnostic mode. According to the service manual, pressing the ‘Master’, ‘AM/FM’ and ’8’ on the channel selection button all at the same time would enter this mode.
After entering the main menu, pressing the #6 button brought up a sub menu.
The code shown in the manual indicated that the amp had experienced overheating conditions on several occasions.
After consulting the Protection History, several codes came up – ‘There is a history of protection function excessive heatsink temperature.’
So, why was this overheating? I used my thermal camera to try and isolate the hot spot – or what I thought was the hot spot.
It showed up to 110˚C in protection mode, but during normal operation it dropped down to a lower temperature.
The component in question, Q130, is a 2SD2375 transistor. After checking the data sheet, I was surprised to see that the temperature range was -55˚ to +150˚C. So contrary to what I had thought, this transistor was actually within the working range.
Also, after checking the schematic, I could see the transistor was part of a 15-volt rail. The best way to check the transistor operation was to test the 15 volts. It was rock steady, so there was no need to be concerned about that component anymore.
But the question remained – Why had it displayed an overheating code?
While speaking to the customer, explaining what I had done during the repair, I did comment on how clean it was inside. Normally you would expect a reasonable amount of dust over the inside of the amp. He stated that he covered it with a silk cloth. Whether he had left the cloth on during operation or not, I couldn’t say, but it does seem logical that that may have been the cause of the overheating.
The customer is happy to report that after a month, the amplifier is working perfectly. I guess one lesson I learned from this repair is to research, research and research. If I hadn’t done research, I might not have found the faulty capacitor. If I hadn’t done further research, I might have started to look for a fault that the CD Direct seemed to create with the display.
If I hadn’t done further research, I might have tried to chase a faulty transistor that was actually working within the temperature range and finally, if I hadn’t done further research, (speaking to the customer), I might have not discovered the actual reason behind the overtemperature codes.
So, once again………. always make sure you do research. It might just make your day a little bit easier.
If you are interested in this repair, you can see a video on my channel following the link below.
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article on Samsung Swollen Battery Replacement