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
https://www.manualslib.com/manual/846721/Yamaha-Rx-797.html
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
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24 Comments
Leave a Reply
George Nutzul
June 3, 2023 at 7:59 am
Thanks for a very well written article, Mark. And, you have a good sense of humor! You make a very good point that we should use the internet to our advantage in checking for known problems to speed up our diagnosis. Also, its very important to talk to the customer about how he was using the unit when the failure occurred. Often that could shed some light on why the failure occurred. Thanks again for the well written article, Mark.
Mark
June 3, 2023 at 4:52 pm
Thanks George,
In the Automotive field we call it interrogating the customer. It sounds harsh, but if you look up the definition of interrogate, it means to 'ask questions of (someone) closely'. That's the only way to get to the truth, the whole truth and nothing but the truth!
Eric Rice
June 3, 2023 at 7:59 am
Good job mate!!
Mark
June 3, 2023 at 4:52 pm
Cheers Eric
Parasuraman S
June 3, 2023 at 8:21 am
Vow! What a great article giving full insight into an Amp repair professionally! Excellent explanation! I fully agree with you that doing research is a part and parcel of our trade! Many, many thanks for this wonderful and very informative article!
Mark
June 3, 2023 at 4:53 pm
Thanks Parasuraman,
I always say that I don't know everything......but I do know how to research.
Andrew F. Ali
June 3, 2023 at 9:44 am
Very well done.
Mark
June 3, 2023 at 4:54 pm
Thanks Andrew
KOSTAS DONTSIOS
June 3, 2023 at 3:23 pm
Very nice research!!
Albert van Bemmelen
June 3, 2023 at 3:48 pm
Great repair Mark! And it reads like an adventurous story as well!
It also explains why I try to avoid such difficult audio amplifier sets that have so many hidden technical issues.
Mark
June 4, 2023 at 4:29 pm
Thanks Albert.
You can spend a lot of wasted time searching for difficult faults, but it's a good feeling if you are able to repair it.
Mark
June 3, 2023 at 4:55 pm
Thanks Kostas,
I used to hate study, but now I really enjoy the research side of things before I start my repairs.
Yogesh Panchal
June 3, 2023 at 5:43 pm
Hi Mark, Good Fix!
Have you confirmed with customer still he use silk cloth to cover up; is that causing error ?????
Mark
June 4, 2023 at 4:27 pm
Yes I have discussed with him the need to remove the cloth before using the amp. He has had no further issues with the amp.
Yogesh Panchal
June 3, 2023 at 6:16 pm
Now a days every electronics equipment uses some hidden operating codes through control panel combination keys for setting & diagnose.
Mark
June 4, 2023 at 4:28 pm
Yes, you are right Yogesh - even washing machines and microwave ovens!
Gustavo Contreras
June 6, 2023 at 10:01 am
Great article! Thanks for sharing
Gus
Mark
June 11, 2023 at 4:36 pm
Thanks Gus
Joven
June 7, 2023 at 12:37 pm
Troubleshooting calls for wisdom and common sense. Kudos to you, Mark! You did well. Satisfaction comes with every accomplishment.
Mark
June 11, 2023 at 4:37 pm
You are right Joven. I am just a hobbyist, but I get excited every time i make a successful repair
Waleed Rishmawi
June 7, 2023 at 4:01 pm
Research is is the best tool for our repair field because without it we go astray. Thanks for sharing and have a blessed day
Mark
June 11, 2023 at 4:38 pm
You are right Waleed. I think research is probably one of the most important parts of our repair. I always start with research first.
Maddy
September 18, 2023 at 7:02 pm
Thanks for that!
As a complete amateur I managed to repair my amp by following your instructions. Was a very straightforward fix.
Brilliant!
Craig
August 3, 2024 at 11:10 pm
Hi Mark,
I recently bought one of these old YAMAHA RX 797 receivers from Ebay. It has the same sort of issue the one you fixed had. When turning on the unit it powers on but after a few seconds powers off. I'm not an electronics repair person but I would like to have it fixed asap. You mentioned about the UK model having a different rating or type of capacitor compared to the US model. I have a friend that does some electronics repairs and he said if I order the correct capacitor he could install it for me. Where do you buy/source capacitors online? I will run the diagnostics test and see what it shows but I'm feeling it's the capacitor.