Dual 13V power supply... How critical is 13V ?

[Jolly 7]

For no apparent reason, the other night my cherished Sony Bluetooth speakers cum CD player went bang. The mains fuse in the plug was intact, but on opening it up, I found a couple of burnt out SMD resistors and PCB tracks on the built-in PSU board. It is unrepairable, but the print suggests it is a dual 13V supply. Could I get away by using a DIY dual 12V supply instead, if I can fit it in place of the old one ?

[Ambientnoise]

Quite possibly but before doing too much, ensure that the blow up wasn’t caused by the main unit failing somewhere. Maybe you could try powering with an temporary external psu or even a couple of 9v batteries to see if you get any signs of life? Monitor the supply current if you can.

[knobtwiddler]

Ambient noise has good advice. I see no reason why exactly 13v is crucial- in fact, it might be an arbitrary figure dictated by Sony's parts bin. The most likely outcome is losing about half a dB maximum output level.

What worries me more (as AN suggests) is that the fault is being caused by something upstream. This is most likely a switch-mode PSU. One idea is to look for a short between drain and source on the power MOSFET. This is a classic SMPS failure mode, which might bode well for circuitry upstream.

As AN suggests, a current-limiting bench PSU is desirable here. The only problem will be isolating the logic side from the amp, I.e. knowing which is taking the current when you test it. For example, you might think a lot of current is being used and the amp has a fault, but (assuming the logic has a separate regulator of sorts) the logic is surprisingly hungry. If you can isolate the amp and power it on it's own, that is a good idea.

[Jolly 7]

Thanks for the replies. Here are some pictures. The burn-out SMD resistors are not immediately below the mosfet, but under some electrolytic caps. The picture of the connector was taken after desoldering it from the PSU. From left to right, the connector corresponds to13v, 13v, ground, ground on the PCB.

[turretslug]

It looks as if those components could be in the drive circuitry to the power MOSFET- if this has gone short between pins, that could explain their abrupt demise. I agree with previous comments- I don't see why the 13V should be critical, but having worked on a number of Sony tuners that had a 13V rail, maybe it was a number that chimes nicely in the designer's mind. You could think of it as 12V with a bit in hand for decoupling...

[Jolly 7]

MOSFET power supplies in units such as these don't seem to be a very good idea then as they seem to be more prone to failure compared to standard PSUs ? Would there have been any disadvantage in using just 12V regulators instead ?

[turretslug]

Yeah, the supplied Acer PSU (LiteOn) for my netbook blew the plug fuse the other day, the replacement fuse also blew immediately, so I took the rubber mallet to it (the PSU that is). Sure enough, the MOSFET had committed spectacular seppuku with plenty of surrounding collateral mayhem. I suspect that "lowest bid" applies and everything is a bit marginal. Probably most survive the warranty period though....

[Diabolical Artificer]

Some of these SMPSU's look pretty straightforward , until you look underneath : ) Are they proper Sony caps in the PSU?

Back in the day when I lived on a boat we used to power battery tranny radio's meant for 3v tops op with car battery's, discharged ones that is. All went fine for years till we used a fully charged one, the radio obviously went bang. The moral of the story is most electronic things will be quite happy powered with less than optimum PSU's, to a point. Most have some sort of regulation after the power in anyway.

Andy.

[Electronpusher0]

Check the output circuit to make sure it is a dual supply, it could be a single 13V supply with the pins doubled to carry the current. Unfortunately the photo of the back of the board does not show the end with the connector.

13V could be a legacy from designing circuits for vehicles where 13V is a good compromise between a partially discharged 12V battery and the voltage expected when the vehicle is running and the battery charging.

Peter

[jjl]

Originally Posted by Jolly 7

MOSFET power supplies in units such as these don't seem to be a very good idea then as they seem to be more prone to failure compared to standard PSUs ? Would there have been any disadvantage in using just 12V regulators instead ?

Pretty much all mains powered electronic items use switch mode power supplies like this one these days. A well designed SMPSU offers cost, weight, size, reduced waste heat and power consumption advantages over a linear power supply.
I believe that various energy consumption regulations make linear mains power supplies pretty much impractical for consumer goods offered for sale at present.
The disadvantages of SMPSUs are complexity and that they often blow up spectacularly when they go wrong.

John

[Ambientnoise]

Good point that it may be a single 13v supply on duplicate pins. Check carefully before applying +/- 12v !

[turretslug]

I agree- looking at the output rectifiers and PCB underside, they look to be parallelled and fed from a single transformer winding, producing just a single output rail.

[Refugee]

I have got a computer screen that is marked 11 volts.
It has an internal SMPSU that is reliable and was powered by a power brick that had to be replaced every year or so.
I bought an unregulated power brick with a good quality iron transformer in it that looks like it came from a shortwave radio transmitter. It has a very low iron loss and remains stone cold if left powered 24/7.
The screen needs about 2 amps and runs on 9 to 14 volts with a bit of variation on the brightness. The backlight is the only bit on the unregulated supply.
It has been working for a few years now.

[Jolly 7]

Originally Posted by Diabolical Artificer

Some of these SMPSU's look pretty straightforward , until you look underneath : ) Are they proper Sony caps in the PSU?

Back in the day when I lived on a boat we used to power battery tranny radio's meant for 3v tops op with car battery's, discharged ones that is. All went fine for years till we used a fully charged one, the radio obviously went bang. The moral of the story is most electronic things will be quite happy powered with less than optimum PSU's, to a point. Most have some sort of regulation after the power in anyway.

Andy.

The caps are not Sony as far as I can tell. Possibly from a Chinese supplier.

[Skywave]

Originally Posted by Jolly 7

. . . the print suggests it is a dual 13V supply.

Just a thought. I wonder if that "13v." is on no-load? If it is, I could drop to 12v.(-ish) when delivering a substantial current.

Al.

[Jolly 7]

Thanks everyone for the very useful replies. My terminology was wrong, the PSU was not dual voltage but a single 13V source split into four wires, which I have now joined to make two i.e. only one positive and one negative. The good news is that everything upstream is working just fine. 9V wasn't enough to power it up and I used a spare external 12V 1.5A PSU to test it. The Sony CMT-X3CD is a right pain to open up, so I will be converting the unit to be powered from an external 12V DC source only. This particular unit was given to me by a gentleman who was about to throw it away ! A very nice sounding unit I must say and I am delighted to have given it a second life !!

[Maarten]

As a side note, I don't think there's such a thing as Sony capacitors.