trying to fix or replace a bad power adapter - please help

[newtoCaps101]

Re: trying to fix or replace a bad power adapter - please help

Look man, this decoder would normally run even on 3.3v or 5v, the decoding stuff doesn't use more than 5v.
But the decoder also has an audio amplifier for the headphones output and for the 5.1-7.1, whatever it has there as connectivity.

The electronics that decodes the dolby signal and makes 5.1 or whatever the decoder does powers itself from an internal converter somewhere, which takes power the box receives and generates 3.3v or 5v or whatever low voltage is used. So that doesn't care about what you use at input.

The audio amplifier used for headphones output and so on does however care how big the voltage is. Depending on what type of amplifier it is, the higher the voltage at input, the more watts the amplfier can produce and the louder the volume.

So you can probably safely use even a 12v DC, but don't be surprised if you turn the volume up a bit and it sounds bad.

Great explanation thanks. now the thing is actually a decoder, it only decodes and of course controls the volume level, the actual amplification is done on my subwoofer which has a built in amp, this think is optional and goes "between" the computer and the subwoofer and speaker set to "Decode" things to dolby digital, pro logic and provide more options and effects. you know? But good point with the headphone amp there.

So do you think if I used the 12V the sound would become bad at say higher levels? thats something I dont want but I mean this does control the sound levels but by no means its a amp, its purely a decoder. Do you think I should try to mod the ugly big laptop 15V power supply with the connector or just use the 12V adapter he included? Would it be better if I were to stay closer to the desired volts? I dont know really.

[budm]

Re: trying to fix or replace a bad power adapter - please help

You can jump the thermal fuse (there should be printing on the body of the fuse for the temperature which the fuse will go open circuit, usually around 115c or so) which then you will no longer have any protection against transformer over heating. I would at least install a fuse for the protection. The thermal is not connected acorss Hot and Neutral, it is in series withe transformer winding.
Since the circuits does not use bi-polar power supply (+/-), it just uses a bridge rectifier to convert 16VAC into about 22vdc, so you can feed DC into the unit without problem, you will lose about 1.2V drop at the output of the rectifier due to Diode voltage drops. The input does not care about the polarity of the DC feeding it either since the bridge will automatically route the corect polarity of the DC to feed the circuits.

[newtoCaps101]

Re: trying to fix or replace a bad power adapter - please help

You can jump the thermal fuse (there should be printing on the body of the fuse for the temperature which the fuse will go open circuit, usually around 115c or so) which then you will no longer have any protection against transformer over heating. I would at least install a fuse for the protection. The thermal is not connected acorss Hot and Neutral, it is in series withe transformer winding.
Since the circuits does not use bi-polar power supply (+/-), it just uses a bridge rectifier to convert 16VAC into about 22vdc, so you can feed DC into the unit without problem, you will lose about 1.2V drop at the output of the rectifier due to Diode voltage drops. The input does not care about the polarity of the DC feeding it either since the bridge will automatically route the corect polarity of the DC to feed the circuits.

thanks but look at the last page, We are past the point of using the old transformer as I accidentally broke a enamel coated wire, soleft with the 2 options. spend $50 buy a 16 VAC adapter like josh said, or use either my 15v adapter or the 12V adapter my friend included, said he used it for a few years.

suggestions?

[budm]

Re: trying to fix or replace a bad power adapter - please help

As I stated and explained, you can feed DC into that power jack.

[newtoCaps101]

Re: trying to fix or replace a bad power adapter - please help

You are right, I must have missed that part sorry. so it doesn't matter if I feed it VDC or VAC since the bridge rectifier will convert it into 22VDC? So Does it matter or is it better I use the 15VDC adapter or the 12VDC adapter? And what can I do about the voltage drop? will be it a issue that may cause any audio problems or damage the device in the long run?

[joshnz]

Re: trying to fix or replace a bad power adapter - please help

so it doesn't matter if I feed it VDC or VAC since the bridge rectifier will convert it into 22VDC?

No all the bridge rectifier does is convert ac to dc.
if you feed it 16 VAC the dc side get's 22.5 VDC with no load,

if you feed it 24 VDC the dc side get's 22.8 VDC

The lower voltage is from losses in the bridge rectifier.

[budm]

Re: trying to fix or replace a bad power adapter - please help

Can I see better pictures of the bottom and top side of the board, straight shot? I want to see what that voltage regulator on that heatsink is.

[mariushm]

Re: trying to fix or replace a bad power adapter - please help

The bridge rectifier doesn't convert anything to 22v DC.

A bridge rectifier converts AC to DC, or passes DC through. In both cases, there are losses in the bridge rectifier.

The losses in the bridge rectifier are about 0.7-1.2v on each diode, depending on how much current goes through the rectifier and in a bridge rectifier there are always two of four diodes conducting, so you get 1.4-2.4v loss after conversion or after DC passes through.

On, AC conversion to DC, the generated voltage is about 1.41 x AC voltage.

So 16v AC goes in -> it's rectified to 1.41 x 16 = 22.5v -> remove losses from diodes - > you get 22.5 - 1.4-2.4v -> the decoder has about 20v



Those four diodes are the bridge rectifier, the thing labeled "load" is the decoder, the 10uF capacitor is the big capacitor near the rectifier. So with AC voltage you can see two out of four diodes running, then the other two diodes run and so on, and you get DC voltage, albeit a bit wavy...that's where that large capacitor near the rectifier comes into play... in the picture above i set it on 10uF which is too small, just for you to see.

If you use a 15v DC adapter, there's no AC conversion, so you still have only 15v DC before taking out the losses from diodes which are still in the circuit, so the decoder has 15v - 1.40-2.4v ... about 11.4-13.6v





If you decide on using the decoder ONLY with DC voltage, you can go ahead and desolder the bridge rectifier from the board, and connect the wires directly to the + and - holes in the board. This way, there are no longer any losses in the diodes of the bridge rectifier because you removed the rectifier.

So any DC voltage you send, that's what the decoder will get.

You can even reuse the jack in the back of the decoder.

Desolder the bridge rectifier ... you're left with four holes in the board :

[ + ] [ ~ ] [ ~ ] [ - ]

Solder four wires to these four holes.

If you have a multimeter, you can put it on DC voltage measurement.
Plug a DC adapter in the back of the decoder, then put red probe on the wire from the first ~ wire and the black probe on the second ~ wire.

If the multimeter shows a positive voltage, connect the wire from the first ~ to + , and the wire from second ~ to the -

If the multimeter shows a negative voltage, the wire from first ~ goes to -, and the wire from second ~ goes to +.

[budm]

Re: trying to fix or replace a bad power adapter - please help

It looks like the output of the bridge feeds the Voltage regulator, depends on the Vreg, there should be about 5v or so drop across the Vreg for the regulation head room. The reason I suggest to feed the power jack directly from external DC source is that he does not have to worry about polarity of the external power supply, the only bad thing will be the voltage drops of the bridge. I would like to know the P/N that regulator IC or if it uses discreet pass transistor for the voltage regulation.