My birthday is tomorrow, and I need help...

[Romeo Wolf]

2 days ago, I bought a 1974 Sherwood S-7900A Dynaquad receiver on Craigslist for $175. The seller claimed everything worked. Sure, it did. I liked it and bought it. Took it home with me until the next day, when I went to hook my speakers up to it.

Instead of music, was a constant destructive 120Hz buzz that sounded like it could blow my Yamaha's. At this time, I had my Yamaha NS-AP5700BLF's hooked up to it. They were hooked up properly and we even triple checked. Same result. Then we saw it: the receiver could only support 4, 8 and 16 Ohm speakers. The Yamaha's were 6 ohms, so that meant it could have been causing the problem, and later we brought up my huge Sanyos (seen on my avatar) that were 8 Ohms. Hooked them up, hopefully it should be fixed.

Nope. It still had that same hum again! We didn't even have any turnables or anything! Only the speakers, speaker wire and receiver. The CRT Television was sitting between the Sanyos in my room. We figured that using a power strip should fix it. Nope. Same occurrence. We then realized that the rubber feet should be touching the surface, and it was sitting on carpet. We didn't have a good table to fix the receiver onto. There were the kinds that fold, but those were flimsy. So instead we put it on a portable cooler (the kind where you like take to the beach) Damnit, It's still there! WHYYYYYY????????

And now, we insist on using an Isolater or hum eliminator. But I'm highly doubtful this will solve the problem.

The seller seems to be a smoker, as her apartment reeked of cigarettes. So did the receiver, but it went away eventually after we bought it. She said it worked perfectly 4 days before we bought it, for like some DJ party with friends and vintage speakers and ****. But now that I have it, it does this. How come and can I fix it?

Edit: it's got a bare metal chassis. Not a wooden one.

 

[rebellovw]

Find a decent tech and get it rebuilt. It is no surprise that it isn't performing like it did when new - 1974 was a very long time ago. My Pioneer is in the same boat and is now just decorating my office.

If you cannot afford a rebuild box it up and save it for later when you do have the funds.

Stick with new gear for now.

Thanks

 

[rebellovw]

One thing you could probably do - is unplug it - remove the cover - and take some photo's and post them here for some advice. Be very careful and do not touch anything - just take pictures for now.

On my Pioneer SX650 - I cleaned the dust - de-oxit controls and such - and that worked for a few months - until finally it went belly up and needs parts replaced.

Happy Birthday.

 

[Bucky Badger]

Bummer that's happening to ya. I assume that you didn't test it when at the sellers house.
It shouldn't have a hum sitting on carpet, or the cooler.
Your 6 ohm speakers shouldn't be a problem either. The receiver supports speakers that are rated from 4-16 ohm's so that's not the problem.
I am also assuming that you have it connected to the Right Main and Left main speaker terminals on the back of the receiver. And the Main button is pushed in on the front.
Where are you located?

 

[Romeo Wolf]

One thing you could probably do - is unplug it - remove the cover - and take some photo's and post them here for some advice.

On my Pioneer SX650 - I cleaned the dust - de-oxit controls and such - and that worked for a few months - until finally it went belly up and needs parts replaced.

Apparently it is already unplugged. I have not opened it yet.

Unfortunately I'm unable to do photos at this time since my SD card is fried. Plus I'm getting a new phone. So that'll probably happen tomorrow.

 

[Romeo Wolf]

Bummer that's happening to ya. I assume that you didn't test it when at the sellers house.
It shouldn't have a hum sitting on carpet, or the cooler.
Your 6 ohm speakers shouldn't be a problem either. The receiver supports speakers that are rated from 4-16 ohm's so that's not the problem.
Where are you located?

I'm in Vancouver, WA. The seller didn't have any speakers to test it with. I'm not so sure if that's the case, since the receiver obviously says 4, 8 or 16 Ohms. Not 4-16 Ohms.

 

[Powertech]

As this buzzing noise appears to be global - i.e. both channels affected, and it is definitely within the amp, I would suggest that it is probably a power supply fault. It could just be a supply rectifier diode which has failed. I have even come across rectifier snubber capacitors to have failed and shorted out a rectifier. In this case, all that was needed was to cut the capacitors out and all was well. The materials costs of these type of faults is quite low. It may be quite a simple fault, so it would make sense to get a technician to look at it. If they report that it will be an expensive repair, you could always keep it as is and repair it later. At least you would have some idea of the cause of the fault and the probable cost of the repairs.

 

[Bucky Badger]

You can take it to Doug at Audio Specialties or Freds Sound of music if you want to get it looked at.
Doug will give you a free estimate.

 

[Romeo Wolf]

You can take it to Doug at Audio Specialties or Freds Sound of music if you want to get it looked at.
Doug will give you a free estimate.

There's one in Portland. Should do good. Will check it out!

 

[Bucky Badger]

Audio specialties is probably closer just fyi.
I am assuming that you also have checked the fuse.

 

[shelly_d]

It is most probably a power supply issue. The most expensive thing in a power supply is the transformer, but that symptom is more indicative of a rectifier or filter issue. The chances are excellent that the power transforer is fine. Parts tend to be relatively cheap. It may very well have worked 4 years ago, but the caps in a filter can go bad with age, even without use. Not trying to make excesses for a bad seller here, just bringing up some real, but IMHO low probability possibilities.

If it really is a 120 Hz hum, that points very hard at the power supply caps. The unfiltered signal out of the rectifier is 120 pulses per second pulsed DC. If partially filtered, then you have a large 120 Hz AC signal riding atop the DC voltage out of the power supply. That is what I suspect is happening here. If I were doing this I would check the operation of the rectifier diodes, see if there are any devices paralleling them and check those. Once the rectifier is working correctly then proceed to replace all the caps in the power supply filter. (These are old electrolytics and near end of life anyway, and usually not expensive.) I believe this would correct the problem.

(I'm beginning to fell like Click and Clack, the Tappet Brothers. I miss that show. RIP Tom)

Shelly_D

 

[GChief]

since the receiver obviously says 4, 8 or 16 Ohms. Not 4-16 Ohms

Does not matter, the 6 ohm Speakers are not the issue.

 

[goodolpg]

I agree.
It's not the 6 ohm speakers.
It's not the TV.
It's not the dog.
It's not the President.
Did you actually LISTEN to it when you went to pick it up?
If not, you should have. It sure sounds like it is broke and needs repair.

 

[faber12]

I'm not so sure if that's the case, since the receiver obviously says 4, 8 or 16 Ohms. Not 4-16 Ohms.

LOL

Young man, you have much to learn. Remember recently when you said you don't use AudioKarma to ask questions? Now that you are asking, you should listen respectfully to the answers.

 

[Bucky Badger]

I'm not so sure if that's the case, since the receiver obviously says 4, 8 or 16 Ohms. Not 4-16 Ohms.

Sorry for any confusion. A 6 ohm speaker falls within the covered range the receiver can handle. If the receiver accepts 4 ohm speaker's ON UP TO 16 ohm speakers, a 6 ohm speaker falls within that range and are safe to use.
As goodolpg, others, and I have stated, it's not the speakers causing the problem.
Let this be a learning thing for ya. Whenever you are buying a receiver off of CL, ALWAYS test the receiver by bringing some speakers with you. (small ones) OR, make sure you have some time to return the receiver if not working.
24-48-72 hrs. Unless of course you know the person you are buying from.
You can also bring a pair of headphones and an ipod or your phone and the appropriate cable to plug into that device (1/8th" phono plug to 2 RCA) and then plug the RCA's into the AUX or Tape In on the receiver. It's an easy way to see if it at least you get some sound out of the receiver.
Buying a receiver from someone you don't know that say's that it's working or was working when put away, is kind of asking for possible trouble. As you have now found out.
I hope that you at least plugged it in and looked to see if the lights worked and the tuning meter moved when rotating the tuning dial with the receiver set on FM.
It sounds like this will need to be looked at by someone that knows what to look for, and possibly need to be fixed.
Good luck and Happy Birthday.
BTW, it doesn't matter if all 4 rubber feet are touching anything. They are just rubber feet and shouldn't matter to the functionality of the receiver.

 

[shelly_d]

The HiFi engine has your service manual available to download. I took a quick look at the power supply in it. There are a grand total of 5 diodes to check and perhaps 8 or 10 caps that could be the culprits. I suspect the diodes are going to be fine. The most likely culprit would be the caps. They probably cost in the range of about $0.90 to $3.00 each depending upon value and voltage rating. Do you know how to use a soldering iron?

Shelly_D

 

[Romeo Wolf]

Sorry for any confusion. A 6 ohm speaker falls within the covered range the receiver can handle. If the receiver accepts 4 ohm speaker's ON UP TO 16 ohm speakers, a 6 ohm speaker falls within that range and are safe to use.
As goodolpg, others, and I have stated, it's not the speakers causing the problem.
Let this be a learning thing for ya. Whenever you are buying a receiver off of CL, ALWAYS test the receiver by bringing some speakers with you. (small ones) OR, make sure you have some time to return the receiver if not working.
24-48-72 hrs. Unless of course you know the person you are buying from.
You can also bring a pair of headphones and an ipod or your phone and the appropriate cable to plug into that device (1/8th" phono plug to 2 RCA) and then plug the RCA's into the AUX or Tape In on the receiver. It's an easy way to see if it at least you get some sound out of the receiver.
Buying a receiver from someone you don't know that say's that it's working or was working when put away, is kind of asking for possible trouble. As you have now found out.
I hope that you at least plugged it in and looked to see if the lights worked and the tuning meter moved when rotating the tuning dial with the receiver set on FM.
It sounds like this will need to be looked at by someone that knows what to look for, and possibly need to be fixed.
Good luck and Happy Birthday.
BTW, it doesn't matter if all 4 rubber feet are touching anything. They are just rubber feet and shouldn't matter to the functionality of the receiver.

I just noticed. My father was looking at another receiver of the exact same make and model. That one was only $100, in perfect working condition (seller will send a video of it) on Facebook marketplace. That one actually has a serial number! Mine didn't. It was blank. Mine was a ridiculous $175 for a receiver that, no matter what we try to do, only buzzes when we press the button for the front or remote speakers. The lights do work, most of them. Tuning does work (stereo comes up on a couple stations) but we get no sound, unless we want some 90db speaker-blowing 120Hz buzz.

This $100 unit I'm looking forward to should actually put out stations when I bring it home. Otherwise, it may be interference Issues I'm dealing with, like speaker wire passing behind a 20" CRT TV that's plugged in, and between speakers twice it's height.

I might return the non-serial number receiver back to the lady and get my money back once she returns next weekend, or I might sell it as-is for perhaps the same price I paid for it: $175.

 

[Romeo Wolf]

I agree.
It's not the 6 ohm speakers.
It's not the TV.
It's not the dog.
It's not the President.
Did you actually LISTEN to it when you went to pick it up?
If not, you should have. It sure sounds like it is broke and needs repair.

Nope. The seller didn't have any speakers to test it with.

I recently found another perfect working 7900 that I discovered, had a serial number. Mine didn't. It was only $100. Mine was $175.

It would have been better to buy another receiver of the same model, which I'm looking forward to. The seller of the $100 one is busy rn, but he will send a video of it, hopefully actually working eventually. Sometimes, it's better to find and buy another of the same receiver for cheaper than it is to get an old one repaired. Then sell the old one as-is, for the price you paid for it. Maybe less.

Lesson learned.

 

[Romeo Wolf]

LOL

Young man, you have much to learn. Remember recently when you said you don't use AudioKarma to ask questions? Now that you are asking, you should listen respectfully to the answers.

I was lazy at that point and didn't feel like it. I actually DO ask questions, but perhaps in a dramatic way. Think r/askreddit or r/nosleep style combined together.

 

[Romeo Wolf]

The HiFi engine has your service manual available to download. I took a quick look at the power supply in it. There are a grand total of 5 diodes to check and perhaps 8 or 10 caps that could be the culprits. I suspect the diodes are going to be fine. The most likely culprit would be the caps. They probably cost in the range of about $0.90 to $3.00 each depending upon value and voltage rating. Do you know how to use a soldering iron?

Shelly_D

Basically, we have nothing for what is needed to repair a receiver. We only have a screwdriver, bruh. No soldering iron.

I know I sound like a dumbass but we don't have the experience or the richness to do all this, or even get it repaired.

D*ckmove: We found another receiver of the same model on Facebook marketplace that supposedly actually works, for $100, with a legit serial number. Mine didn't have one and was $175. We're messaging the seller w/the $100 receiver and Because he's busy today, he'll eventually send us a video of hopefully a demo. Looking forward to this next one! (Hopefully this $100 receiver won't need caps replaced for a while, like my $175 one does, maybe now, or I'll give it back to the smoking Lady and get my money back once she returns next week, or sell it for the price I paid for it, as-is, cause we're broke)