Advice Needed Humming subwoofer

[battenfan]

A search for Teac Powermax hum yields a few results, maybe it's designed this way. Probably power supply caps. Has it always been this way or gotten worse? I don't believe in ground loop if there's nothing connected. Does it go away if you connect cables or try to short the input?

 

[Captain Jack]

OK guys, I've ordered a 3300uF cap along with a ferrite rod to clip around the power cable.

Also, the power cable has NO grounding at all as it's a 2-core cable with a Yooropean plug (bought from PC World!) and an adapter. Inside there's a 25v transformer... we'll see if the new cap with a ferrite ring resolves it.

What I could really do with is an oscilloscope to see where the noise is coming from (assuming the mains since it does it with nothing connected as well).

 

[bim]

Engineering definitions:

Hummer, a device that plugs into the AC wall outlet or uses battery's to power it.
Puppy, anything else that does not us any form of AC, DC or dark matter to power it. (can also be used to describe a unknown hummer, such as "Whats that puppy do?)







Sorry........My fault, found the half full bottle of scotch left over from last night........

I thought it was a 4 wheel drive american car!!

 

[Waveguide]

My guess is the same as others that the filter capacitor(s) in the power supply
have dried up and no longer make a DC voltage from the output of the rectifier.
http://electronics.stackexchange.com/questions/73863/cap-value-for-full-wave-rectifier-circuit
(check "waveform without capacitor" in the link)
A multimeter that can measure AC voltage is enough to do a quick check.
The AC voltage over the capacitor should be a few volts or less.
At low power (no audio signal) the AC voltage will be close to zero if the filter
capacitor is OK.

RFI will never give a low frequency hum unless the source is amplitude modulated
with the exact frequency of the hum. (I once worked in a really fancy EMI-lab)

 

[Captain Jack]

Replaced the rectifier capacitor and it's as new with no humming at all

Thanks all for advice!

 

[2cvbloke]

When in doubt, always blame the capacitors...

 

[Captain Jack]

That's usually the cause of many, many issues.. especially when something is a good few years old (this Teac sub is nearly 10 years old). And a cheap fix too!

 

[2cvbloke]

Yep, failing capacitors can produce all kinds of odd behaviour in electronics, from sound faults to complete failure to power up the device (E.G. the old sky receiver "stuck in standby" fault), I need to re-cap my two Amiga A600s (just need to get myself a decent soldering iron first since my last one died), they started exhibiting odd behaviour, one would pay FDD sounds back through the audio channels, the other would randomly fail to start up, both of which had the same failed caps, so, they're getting upgraded to Tantalum caps where possible...

 

[Golfather]

Hi guys

Sorry for bumping an old post, I think i have the same problem with humming with my TEAC Powermax 1300
Can you provide details of what rectifier capacitor spec was used, and which one on the board was replaced please?

Many thanks!

 

[Captain Jack]

Can't remember. It was the same spec as the ones that were in there. I have another issue with it now where the volume just goes up and down all the time. Haven't looked at it yet - not sure if I should just cut my losses with it.

 

[Terryl]

A bad electrolytic cap can sometimes be located by the shape of it's top, when they go bad they sometimes dome or bulge up at the tops.

Look for the largest can type caps, these are close to the power supply transformer, but be very careful as these caps can carry a charge after the unit is unplugged, wait 10 to 15 minuets before trying to remove one.

Try and replace with the same rating in mFd and voltage, you can always go with a higher rating and or voltage but not less.

And if your unit has a switching power supply (no large power transformer) don't try to fix it, these types of power supply are very dangerous to work on unless you know what your doing.

Example of bad vs good caps.

 

[Golfather]

Thanks guys !
After opening it up seen the following board.

I presume its the single large 3300uF35V capacitor, although its not bulging as expected?

Just to clarify, Im getting a slight humming in background, not loud, but its noticable. However when volume cranked up with music its not audible. Only on low volumes you can hear it through the music.

Will replacing this cap rectify this issue do you think?


Cheers!

 

[Terryl]

Well I don't see any caps that may be a problem, when they go bad they bulge up on top. However I do see a burnt up resistor, the big one marked R36(???) this is in a voltage regulator circuit, it looks to be a 100 ohm (or 1000 ohm) 1 watt, this may be your problem, it looks like old age has hit, I also see a few others that look to have taken a bit of heat.

If you know the make and model number you could do a google check for a replacement PCB, there are a few markings for the board.

 

[Golfather]

Thanks for the reply

R36 (the big burnt looking one) is measuring at 377ohm so I would think thats OK?
Rear tracks look ok too

So I guess we are back at the big cap? Or is there anything else I should be checking?

 

[Channel Hopper]

Have you eliminated earth pickup, or 'loop' . A new piece of equipment connected , even a different cable ?

Try the amplifier / speakers on their own, in a different room , connected to just an MP3 player or mobile phone.

 

[Golfather]

Yes will try moving to another socket, also have a ferrite core around the mains cable as an additional measure but no difference.

Is it worth changing the transformer to an external power supply? I swear i could hear a slight hum with the transformer powered but with no connection to board or speaker!

Can readily get a 25v 1.5A for a LG soundbar on ebay.

 

[Channel Hopper]

Small steps first, just move the kit into another room and try.

 

[Terryl]

After looking closely at your ""Board 3.jpg"" photo (I copied and zoomed in on that area) I may have seen the problem, I retract my statement on the caps looking OK, the cap marked ""C32"" look to be bad, it is a bit domed up and looks to have maybe squirted electrolyte out some at the bottom, also the PCB looks to have taken a bit of heat in that area.



Try replacing that cap first.

 

[Channel Hopper]

Well spotted. It does look 'bulged ' in the next image (5)

https://satellites.co.uk/forums/attachments/board5-jpg.129806/