Linear 1 amp Labtec PSU PCB Burnt

[Krankshaft]

I have a Labtec 2.0 with woofer speaker system (only bought it for $20 bucks a few years back) and it came with this 1 amp 15 volt linear mammoth. Its not the greatest thing out there but it serves its purpose.

As you can see its much larger than its switcher counterpart and heavy enough to break some toes if you drop it .

Seriously I would think this billet of iron alone would cost 20 bucks .

It has always run quite hot so I decided to open it and check it out.

This is what I found inside the circuit is pretty standard a bridge rectifier, capacitor, a fuse, and a resistor.

As you can see the PCB has turned black around the resistor this alarms me.

The heat resistant tubing around the output wires kind of tells me that the manufacturer was expecting this kind of thing.

I'm used to some minor discoloration but black tells me that this resistor is getting real hot.

Also I take it that the plastic surrounding the fuse was once clear and the heat discolored it.

Also what is this resistor for? It can't be a fuseable resistor because there is already a fuse.

I was thinking of upgrading the resistor to a higher wattage for better heat dissipation but then if it is some type of fuseable resistor I would be defeating its purpose.

Is this no big deal or something to watch out for?

[Krankshaft]

Re: Linear 1 amp Labtec PSU PCB Burnt

I'm thinking of just using one of my 1 amp switcher (pictured) in its place but since this device is negative tip .

I'd rather see if I can continue using the linear.

Come on who uses negative tip anymore other then just to be annoying?

Or is it because that prevents your customer from finding the adapter locally so they go back to the manufacturer?

I'm going to need to open the switcher and reverse the wires if I end up using it.

No problem but I'm lazy I'll take plug and play any day .

[starfury1]

Re: Linear 1 amp Labtec PSU PCB Burnt

Don't know the device and only guessing with the psu

I can only see 3 Diodes, is one hidden on the other side of the cap?
Bridge requires 4.
Since I can't see the bottom of the pcb its a bit hard to workout whats connected to what
The resistor looks to be a low value of some kind and yeah agree nice place to stuff a resistor.
I would replace it and mount it higher off the pcb
PCB is a bit heat damaged, it looks more dark brown and not charcoaled, so although not good think it will be ok.

I dont know what the resistor is for , depends on I suppose how its hooked up
In series I suppose it would some what drop voltage and may be a crude form of short protection on the connector (from the possible occasional short).

I dont think its a fusible one......not 100% sure but I thought they used green body for that...(could be wrong)

Not sure on the switcher...think they are a little crude as wall warts and since its audio may feed trash into the system, but I don't know really one way or the other

I suppose the risk is there..... but you could try it

the above just guess work on my part

HTH anyway

Cheers

[gg1978]

Re: Linear 1 amp Labtec PSU PCB Burnt

From what you describe, you have an unregulated linear supply there. The output voltage may well be over 20VDC under no-load conditions, and droop to 15V under full load, or close to it.. The resistor may be a load resistor, to prevent the PSU from having a no load condition, or it may be acting as a current limiter..

[starfury1]

Re: Linear 1 amp Labtec PSU PCB Burnt

thats possible too....cant work out the colours they look like brown and blacks but considering the heat they could be discoloured....

[gonzo0815]

Re: Linear 1 amp Labtec PSU PCB Burnt

It is normal for cheap hard paper PCB`s to get darker when heated. Probably the resistors purpose is to provide a load and prevent high voltage.

[kc8adu]

Re: Linear 1 amp Labtec PSU PCB Burnt

It is normal for cheap hard paper PCB`s to get darker when heated. Probably the resistors purpose is to provide a load and prevent high voltage.

correct on both counts.

[999999999]

Re: Linear 1 amp Labtec PSU PCB Burnt

This is a pretty common problem for moderate or higher powered speaker sets with a sealed wart, that they run hot enough to brown the PCB, sometimes popping the cap and other times tripping the thermal fuse in the transformer windings one time too many. Personally I take the top half of the cover (after popped off) and put a ton of tiny holes in it on a drill press, being carefull not to put any around the high voltage plug pins or that transformer winding if there's any chance of kids poking things into the holes, just a few holes around the PCB and low voltage area is better than none. Then again I can't tell but there might already be vent slits in it?

You could check the amp chip and it's rail caps to see their max ratings for a determination of peak voltage it can use, but basically that resistor is there mostly to improve linearity (but still an unregulated, not "linear" supply), if it weren't there the initial peak voltage would drop a larger amount under load peaks, somewhat mitigated if there was massive capacitance in the amp which I doubt.

Considering how it looks to have ran hot, I would replace the capacitor while you have it open. Also check the resistor solder pads for delamination, separation from the board. If necessary, put a liberal blob of heat resistant epoxy on the bottom over that area to reinforce it (after cleaning, take an eraser and rub hard on the area then wipe with rubbing alcohol), trying not to get it on any other solder joints so if any other part was heat stressed and needed replaced later, it won't be such a bad job - but the resistor itself is pretty tolerant of heat and looks ok as best I can see it in the picture.

I would not substitute a switching PSU, the PSRR of the chipamp is going to be worse at high frequency noise the switcher outputs. You could convert it to a linear supply with some loss of total power, but I kinda doubt the powered amp is good enough to make that very worthwhile. You might not hear the difference though, using the switcher, and some people actually prefer the change (get offended if you tell them they're liking the added distortion) to a switcher with some amps.

[starfury1]

Re: Linear 1 amp Labtec PSU PCB Burnt

So its a "constant load" resistor, guess the 3 diodes were what I was contemplating
(should have wake up thought, makes sense heat wise too)

humm interesting 9"s thanks,
I knew switcher technology had somewhat change as in running higher frequencies so didnt have a clue on the impliactions these days
Sure a wall wart version would be somewhat el cheapo thought and not good for audio.
but considering what he was running he might get away with it.

good point ....since you got it open you may as well change the cap,

thanks for the feedback guys

Cheers

[999999999]

Re: Linear 1 amp Labtec PSU PCB Burnt

I think there is another diode we can't see, behind the 'lytic cap. That would also make sense with the positive trace running around the edge of the board with the position of the three we do see.

IMO a wall wart can be ok if the amp doesn't need much output power but it's pretty easy to feel it's underpowered at less than 6W per channel. At least getting the transformer away from the low level signals and keeping the extra heat out of the amp case can be good.

[starfury1]

Re: Linear 1 amp Labtec PSU PCB Burnt

I think there is another diode we can't see, behind the 'lytic cap.

I thought thats were it might be since he said bridge.

just that you can make a FW CT setup and that would leave a diode so I was ponding that when I opened my big trap about the resistor

Cheers