Amp hum

[schuhjm]

Hi, I have an old Sanyo Plus series A35 amp. Recently it's developed a hum on the speakers. I suspect a bad ground somewhere, but I don't have a schematic, and not sure where to poke around without one. I did check at least the ground to the output heatsink and it appears OK.

The hum is independent of any other component being hooked into the amp, or pluged into as well. It's also essentially independent of the volume control.

Does anyone have any advice, or access to a schematic of this amp?

Thanks

[cunningham]

Sounds to me like the filter caps are about to go Bye Bye. If you measure the rail voltage, which is DC, and measure with an AC meter, you can get an idea if it is ripple on the rail voltages causing the hum. (Scope would be better) This could be caused by bad filter caps and replacing them soon would probably help. Electrolitic capacitors will break down over time and absorb moisture and not work anymore. This is a common problem with old amps. Not that the amp is broken, just some worn out parts, like fixing a used car.

[schuhjm]

sounds like a good place to start, thanks. I recall the darlingtons were replaced sometime in the last 4 years, but I don't recall whether the caps were done.

The amp's closed up again, but I assume the filter caps are the large ones? Too bad I don't have a scope, but I problably would have already rooted around and diagoned them already if I had access to one.

thanks,
JS

[cunningham]

I know how you feel, I have no scope either. I don't have an extra $1200 to spare. School is very expensive. I could probably buy a cheap one, but I prefer to save and get a better one. Besides, I would like to get into using radio, like 100-150Mhz. Scope this fast is expensive.

Anyway, yes the filter caps are the large ones, and are connected to the rectifier diodes from the main transformer. Sometimes bad electrolitic caps tend to swell and the top of the 'can' will bulge up. I used to maintain and fix arcade games and monitor chasis circuits. Basically TV's. It is always a good idea to check for any bulging electrolitics when the circuit starts to go hay-wire. Doesn't mean that all bad caps will bulge up though.

good luck

[schuhjm]

The first thing I looked for were obvious defects. I would have noticed bulging caps, especially the big ones. Didn't see any other problem except a lot of dust.

I feel kinda stupid being a EE, and not diving into it, but I guess I know better than to root around without knowing the schematics, and not having the tools to do it properly. The Vdc / Vrms comparison should help, if the noise signal is large enough.

I expect it will be a while before I dig into it again, Too busy with home remodeling. Moving the stereo was a good opportunity to open it up. Next time will probably be when I get really annoyed with the background hum, or maybe I'll just keep the volume up for awhile...

Thanks again.

J

[Mr. Bungle]

I also have hum in speakers. When I turn speakers on, membrane of right speaker move inside... What is the problem?!

P.S. Does any know any technical info (power,year,price) of it?!?

[anatech]

Mr. Bungle,
You have DC offset on that channel. Do not run the amp with any speakers connected until you fix this. Do you have any test gear and experience?

Hi schuhjm,
Another thing that can cause oscillation is HF oscillation. Time to get the amp to an oscilloscope and have a peak. I am sure you know someone with this equipment. You do not need a schematic yet.

-Chris

[Mr. Bungle]

Hmmm, very little... But I have one friend that could help me...

[anatech]

Hi Mr. Bungle,
Well, it's great to have a friend that can help. Look over his shoulder and learn. If the amp is not drawing excessive current, this would normally mean the voltage amp has a problem. Don't jump in & change the semiconductors as this is not always what is needed. Keep an open mind.
-Chris

[Mr. Bungle]

Thanks on help....