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Sansui AU7700 Troubleshooting!

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patrickchen

New Member
Hello Audio Karma members!!!

This is my first thread, and was hoping to get some help on troubleshooting a Sansui AU7700. My right channel seems to be on the fritz. Sometimes when I turn on the unit I get some fizzing and the channel will be at about half power, if I power the unit off it sometimes will come back to full power.

My thoughts are that it might be a transistor or cap that needs to be replaced. I'm a bit scared to do any of this, although I've had some experience with soldering, I've never tried on a more complex unit like this one.

So I've narrowed it down to the power section at least, the preamp out does not have this problem.

Anyway thanks again members, I love this forum and think there's invaluable input here.

Happy listening!

Pat
 
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patrickchen said:
Hello Audio Karma members!!!

This is my first thread, and was hoping to get some help on troubleshooting a Sansui AU7700. My right channel seems to be on the fritz. Sometimes when I turn on the unit I get some fizzing and the channel will be at about half power, if I power the unit off it sometimes will come back to full power.

My thoughts are that it might be a transistor or cap that needs to be replaced. I'm a bit scared to do any of this, although I've had some experience with soldering, I've never tried on a more complex unit like this one.

So I've narrowed it down to the power section at least, the preamp out does not have this problem.

Anyway thanks again members, I love this forum and think there's invaluable input here.

Happy listening!

Pat
Click to expand...

Welcome to AK!
A bad speaker relay will cause this, and it's a common problem. You have to de-solder the old one and solder a new one in. On the relay the part number is printed. If it is MY -something or LY something, Digikey has them. Those are made by Omron. If it is a DEC relay or other, you will have to post a photo of the bottom so I can give you the equivalent Omron part.
 
dr*audio said:
Welcome to AK!
A bad speaker relay will cause this, and it's a common problem. You have to de-solder the old one and solder a new one in. On the relay the part number is printed. If it is MY -something or LY something, Digikey has them. Those are made by Omron. If it is a DEC relay or other, you will have to post a photo of the bottom so I can give you the equivalent Omron part.
Click to expand...

Thanks for the reply!!!

Would the bad speaker relay also affect the headphone input? I'm getting the same symptoms when I plug in headphones.

I'll have to find out where the relays are, I've got a service manual so I'll check there.
 
patrickchen said:
Thanks for the reply!!!

Would the bad speaker relay also affect the headphone input? I'm getting the same symptoms when I plug in headphones.
Click to expand...
Yes it would also affect the headphones. The headphones are simply padded from the output after the speaker relay.
 
You can test this theory by tapping on the relay with a non metallic probe, like a BIC pen, and see if the channel comes in or gets worse. If so, the contacts in the relay are worn and the relay must be replaced. The relay is a plastic rectangular box, about 1.5" x 1" I can't remember where it is on the AU-7700.
 
This is great information. Thanks! I have a friend that is going to look at it. I'll mention replacing the relay. Would the service manual have the right info for getting a replacement?
 
patrickchen said:
This is great information. Thanks! I have a friend that is going to look at it. I'll mention replacing the relay. Would the service manual have the right info for getting a replacement?
Click to expand...

No, the service manual will only have the Sansui part number. It will not tell you directly, you have to figure it out.
I looked at the service manual. From the schematic and the PCB layout it looks like you need a 48V relay. The Omron replacement is MY2-02-DC48. In stock at Mouser: http://www.mouser.com/ProductDetail/Omron/MY2-02-DC48/?qs=%2fha2pyFaduh4drsd3o8eXHNQyAND5DYQA0S95mHZYWOkrremdKfptQ%3d%3d
 
I would first try to clean the relay before ordering parts. Probably not as permanent a solution or a definitive way of isolating the relay as the problem, but easy to do and doesn't cost you anything. I got a Realistic STA-2000 working again by cleaning the relay but it didn't help the SX-838 I did.
 
The 7700 is very similar to the AU-7900. This unit needs to be restored. There are components on the power supply board that need to be replaced; diodes resistors and caps as well as a new relay.
 
Thanks for the correction.:thmbsp: I'm sure you are absolutely right. But then again, replacing the relay is definately not a permanent solution either. It might take 30 or 40 years, but it'll fail again.

In the mean time, cleaning the relay allowed me (someone with extremely little knowledge) to isolate the problem and get my receiver up and running again.:music: It hasn't missed a beat in the last couple of months since I cleaned it.:banana: That also saved me the time and expense of tracking down a new relay and installing it without being sure that is the problem. Besides that, is it not the number one mantra around here when an amp is cutting out to clean ALL the switches and pots?

As an update, my tech found a cracked solder joint on the SX-828 so the relay wasn't the problem. The sound on the left channel would work intermitently when the relay was tapped, that's why I suspected it. I did get some carbon off the contacts so I'll call it preventative maintanence.:D
 
Sriskie said:
Thanks for the correction.:thmbsp: I'm sure you are absolutely right. But then again, replacing the relay is definately not a permanent solution either. It might take 30 or 40 years, but it'll fail again.

In the mean time, cleaning the relay allowed me (someone with extremely little knowledge) to isolate the problem and get my receiver up and running again.:music: It hasn't missed a beat in the last couple of months since I cleaned it.:banana: That also saved me the time and expense of tracking down a new relay and installing it without being sure that is the problem. Besides that, is it not the number one mantra around here when an amp is cutting out to clean ALL the switches and pots?

As an update, my tech found a cracked solder joint on the SX-828 so the relay wasn't the problem. The sound on the left channel would work intermitently when the relay was tapped, that's why I suspected it. I did get some carbon off the contacts so I'll call it preventative maintanence.:D
Click to expand...

In order to clean the relay you have to desolder it, clean it and solder it back in. And then it will go intermittent in about a month because the plating on the contacts is worn off and it will start oxidizing again. Then you have to replace the relay;desolder it and solder in the new one. Waste of time and effort.
To check the relay all you have to do is tap on it with a non metallic probe like a BIC pen. If tapping on it causes the channel to come back in or go out, the relay is bad. In your case there was another issue that tapping the relay provoked, but that is usually not the case.
 
Forgive me DR. Audio, I have a nasty habit of questioning those who know WAY more than me until I understand.:ntwrthy:

I've cleaned these relays without having to desolder them as per instructions from forums such as this...a fairly unintimidating job for someone as inexperienced as myself. You're saying that there is absolutely no validity in doing so and it is always a very temporary fix at best?
 
There are a very few occasions when cleaning the contacts with a piece of thin card soaked in DeOxit will sort out an intermittant relay contact problem. There are too many so called 'success stories' floating around where someone's 'cleaning method' (e.g. with 800 grit abrasive paper) has clearly removed all trace of the contact plating, vital for a durable low resistance path - and then it's usually only a matter of time (days/ a week or two) before the relay contacts get oxidised and fail.

Change the relay.
 
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Sriskie said:
Forgive me DR. Audio, I have a nasty habit of questioning those who know WAY more than me until I understand.:ntwrthy:

I've cleaned these relays without having to desolder them as per instructions from forums such as this...a fairly unintimidating job for someone as inexperienced as myself. You're saying that there is absolutely no validity in doing so and it is always a very temporary fix at best?
Click to expand...

Yes. I got burned on this myself and then had units come back and had to do the relay. On relays in low energy circuits, like in a preamp or CD player you can get away with cleaning because the contacts aren't usually burned or worn, dirt or cigarette crud got in there. But in an amplifier, the contacts in the relay have high voltage across them and the gold plating gets burned off in time. Then if you clean the contacts, since the plating is gone, they get tarnished again, rapidly. So in a month you have the same problem again. If you are doing it on a customer's unit, you don't want to have a recall. If it's your own, you could try it, but why would you want to pull your system apart again in a month to replace the relay? Get it done right the first time, I say.:thmbsp:
BTW, I know sometimes you can easily get the relay cover off without de-soldering it, but many times there just isn't room to get in there to do that.
 
monoethylene said:
Hm..I think you have to take a 24VDC relay..
Click to expand...

From the schematic based on the rating on the filter cap, (50V I think) and I think they have a 100 Ohm resistor in series with the relay. I went through and calculated the voltage drop across the resistor based on the coil current, I think it was 15mA, so if my memory is correct you have close to 48V on the relay. You should measure it to be sure.
 
monoethylene said:
Hm..I ve just measured the voltage drop over D11 and it is about 24VDC. It should be a 24VDC MY02 relay..
Click to expand...

Ok, I'd go with that. The filter cap on that supply is rated at 50V so I figured a 48V supply. So the correct part number is MY2-02-DC24.
 
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