Need assistance, speaker system repair (Logitech Z5500)

[LeWidget]

Hi Guys,

I have a pair of Logitech Z-5500 5.1 speakers, but, it appears the 'Centre' speaker out doesn't work. Luckily, I happen to have a second pair of Z-5500 speakers that do work and have used these to help rule out possible faults.

Now, for those who aren't familiar with these speakers, the setup/design is basically,

AC power > Sub and everything connects into the sub (speakers and control pod.. ill post some pics)

When I play music through the 'faulty' speakers, all other speakers and sub work fine, so I haven't bother connecting them to my primary speaker-system, but with the centre speaker (of the faulty unit) and the control pod, I've connected them to my working speaker-system and they work fine. I've also tested the speaker cable, seems to work fine too..

So... Im guessing it's possible a fault with section within the audio output section of the circuit?

I'm a bit of a noobi, but I did solder a capacitor in the correct way once.. and it didnt blow up... so that has to count for something.,. right  .. but also, Im very unfamiliar with Audio related circuits (well.. any circuits)..

Just wondering, if I were to start fault finding, where should I start? what area of the circuit ? I've looked online for a schematic.. have found a couple, though not sure if that's a modded version or an earlier/later version of the wiring.. I read there's different revisions..





Suggestions ?

[Zbig]

How I would approach it:

• Connect the center speaker from broken set to the good set. Does it work? Then it's good, move on
• Connect the central unit from the broken set to the good set. Still works? Move on.
• If you arrived here, the center channel amp in your sub unit is probably busted. Confirm by using known good central unit and known good speaker with non-working sub/amp box.

Have you visually checked the pins of the multi-way connector on the cable coming from the central unit?

[LeWidget]

Hey Zbig, thanks for the reply

How I would approach it:

• Connect the center speaker from broken set to the good set. Does it work? Then it's good, move on
• Connect the central unit from the broken set to the good set. Still works? Move on.

Done. Tested both speakers and control pod in known working system and they work ok.

• If you arrived here, the center channel amp in your sub unit is probably busted. Confirm by using known good central unit and known good speaker with non-working sub/amp box.

Sorry, what's the "central unit"? You mean control pod ?

[Zbig]

Sorry, what's the "central unit"? You mean control pod ?

Yes, that's what I meant as it's much more than a controller. All the decoding, processing, D/A conversion, etc. happens there. But never mind, I digress. So I guess now would be the time to open up the subwoofer box and look for anything obviously broken in the power amplifier area (like a burnt transistor). Some photos could help

[macboy]

Did you happen to receive this unit in this condition, or did it just stop working one day?
I'm guessing the former.
I can see from the second photo that the center channel is connected to the third of five speaker terminals. I see in the first photo that the center then probably connects to blue wire on the right hand side of the photo. (Do you follow so far?). Follow that blue wire to the amplifier PCB. It goes to the top left corner of the PCB. Look there, I see maybe something missing, and a big mess around the heatsink where it used to be...

[SeanB]

See the blue wire, that runs from the centre speaker connector, and which finishes on the main board at the top left. then look next to it, at the metal bar, with what looks like a very exploded power amplifier IC under the bar. Probably the same IC as used in all the others, so tou just need to replace it and your speakers will work again.

Cause probably was a shorted cable between the speaker and the control unit, and it being run at high level into this for a while.

Going to be a long job to undo all those screws to get the heatsink out, then undo the bar, then remove the heatsinks to unsolder the IC and replace it.

[LeWidget]

oh wow, thanks guys, yeh you're right, looks like an IC should be there.. but where is it now  .. cant see it in the box. Good eyes, guys






Looks to be an ST-TDA7294, all the others are. $10.38 (free shipping) can't find it cheaper. (Australia)

http://au.rs-online.com/web/p/audio-amplifier-ics/1891172/

Will order one and see how that goes... hopefully nothing else went poof with it. 

[macboy]

don't be afraid to put a TDA7295 in there. The '95 is rated "80 Watt" and the '94 "100 Watt" for whatever that's worth, but in this application there will not be any difference. They share the same pinout and functionality.

Avoid buying from China etc. as fakes are not uncommon. The real thing (ST brand TDA7295) at Digikey is $4.34. Same at Mouser. You'll pay them a few bucks for shipping, but you can get your package in days, not weeks, and you know you have the real deal.

edit: I completely neglected your location when responding. Nonetheless, the advice above stands, and RS has the TDA7295 for AU$6.80.  Also, try to do a better job soldering than the factory did, it's shamefully bad.

[SeanB]

Old one looks like it was cut out, so there has been somebody there before. just buy the cheaper one and put it in, no way that will be doing 80W anyway per channel.

[LeWidget]

Thanks guys . If that's the case, Ill save a few bucks and grab the 95' . From what i can tell, it controls the front speaker only(?).. that wouldn't require a large amount of wattage ?

[Twoflower]

I agree to SeanB that it looks like it was cut it out. But there's one additional thing that would worry me: There's some black snodder around the pins. I could imagine that the former owner let some magic smoke escape there. If that's the case it could be more complicated than just add the new amp.

I would check the following points before spend some money:
1. Check the impedance of the speaker. So unsolder the cables to the speaker itself and measure the impedance (<10 Ohm)
2. See if you can drive the sub-speaker with a signal to hear if it is damaged.

If that's OK, I would be careful while powering on the amplifier. It could be that the cause of the damage is still there and can cause a rapid unscheduled disintegration event.

EDIT: Can you check if the other amps have a glimmer-sheet between the heatsink and the amplifier housing? If yes you should order that too. And thermal grease (cheap stuff is OK) if you don't have some.

[LeWidget]

Thanks Guys

I agree to SeanB that it looks like it was cut it out. But there's one additional thing that would worry me: There's some black snodder around the pins. I could imagine that the former owner let some magic smoke escape there. If that's the case it could be more complicated than just add the new amp.

Hmm.. true.. wonder if they did try to repair it before :/. The black stuff I was gonna try cleaning off with some IPA.. hopefully it's not damage to the board itself or the pads. 

I would check the following points before spend some money:
1. Check the impedance of the speaker. So unsolder the cables to the speaker itself and measure the impedance (<10 Ohm)
2. See if you can drive the sub-speaker with a signal to hear if it is damaged.

If that's OK, I would be careful while powering on the amplifier. It could be that the cause of the damage is still there and can cause a rapid unscheduled disintegration event.

EDIT: Can you check if the other amps have a glimmer-sheet between the heatsink and the amplifier housing? If yes you should order that too. And thermal grease (cheap stuff is OK) if you don't have some.

mmm not too sure what you mean. Am I to check the impedance of the small centre speaker or the sub speaker ?

Also, with the sub, can I play some music through it to see if it goes doof doof or should there be a specific signal ? (Analog or Digital.. or doesn't matter)? When I was testing at the start, I ran the 'Test' function on the system which throws out white noise into the speakers/sub .. nothing sounded strange though Ill give it another shot just in case i missed something.

If all that checks out ok, and I get the parts in and soldered, is there anything I can do/test before turning it on ?

Also, with the glimmer-sheet, I'll check today if my other (newer) unit has it. Are glimmer-sheets electronic specific ?

Thanks again, much appreciated all the feedback

[Twoflower]

Right, you already wrote that you tested the center speaker and it works fine. So unless you have some test equipment (oscilloscope) there's little you can do. Check carefully if there's something odd looking. That could be everything, even a slightly discolored part/PCB around a part could be a hint to have a closer look.

Sorry for using the German word Glimmer, in English that's Mica. These Mica sheets are used to electrically insulate the heatsink from the part. They are common electrical parts. Because they're very brittle they are replaced by electrically insulating but thermal conductive polymers. Probably you can just buy a insulating Thermal Pad and you're done. But clean the white stuff well before, that's the old thermal grease.

[LeWidget]

Right, you already wrote that you tested the center speaker and it works fine. So unless you have some test equipment (oscilloscope) there's little you can do. Check carefully if there's something odd looking. That could be everything, even a slightly discolored part/PCB around a part could be a hint to have a closer look.

Sorry for using the German word Glimmer, in English that's Mica. These Mica sheets are used to electrically insulate the heatsink from the part. They are common electrical parts. Because they're very brittle they are replaced by electrically insulating but thermal conductive polymers. Probably you can just buy a insulating Thermal Pad and you're done. But clean the white stuff well before, that's the old thermal grease.

Thanks . No Oscillocope yet  .. one day Ill get tot that stage. Should I still check impedance of the centre speaker ? Apparently it should be rated at 8ohms

 Ill check around too for any suspicious colours... just trace the circuit or anywhere on the PCB ?

Noprobs about the German :p . I started learning it via Duolingo.. ich bin ein banana! . With the Mica insulator, what thickness should I use? I can get  http://au.element14.com/keystone/4673/transistor-insulator/dp/3723963  which is pretty thin.. 0.076mm ?.. they seem pretty cheap too.. 14cents each for those in the link .. replace all or just the 1x? Come to think of it.. I think its one long strip they all share...

Thermal paste.. im all over it.. got some here already

Thanks again, much appreciated

[Twoflower]

Just check if any devices look like they could have been damaged. For example burnt resistors, electrolytic capacitors that leaked, ... Do a visual inspection and look for something odd. It's a bit like finding Waldo but not knowing if Waldo is there

About the mica sheet. I would use this one: http://au.element14.com/multicomp/mk3305/insulating-kit-mica-to-3p-to-218/dp/520238 as the size would match better. The metal plate of the TDA has a size of 19.6mm x 17.5mm. I wouldn't bother to exchange the others if they're looking OK. Once in place they can stay there for ever. But as this material is brittle and the sheets are thin, be careful not to bend them to much. And if the thermal grease you use is conductive (some 'high performance' stuff for CPU heat-sinks are): make sure you don't bridge the mica sheet with it.

Nice, I'm using Duolingo too. In my case I try to sledgehammer some Spanish words into my brain -> "Tu oso bebe cerveza." As you see the first Spanish sentences are very useful too 

[LeWidget]

Thanks Twoflower  

I had a visual at the boards, but can't see anything strange, looked clean. There was 1x capacitor, C101 (seen in pic above de-soldered section), has glue on it, but half of it has lifted from the board, and the capacitor wiggles at the leg>cap junction .. not sure what cap it is but looking at it, I assume some movement of it is normal ?

[EDIT] the pin in the square pad was a bit stubborn.. had to up the soldering iron temp.. any reason why it might have been harder to remove?





I noticed some of the legs were'nt soldered on one side, is that a problem? Im assuming not if they're soldered on the other side, and they've been working for a few years with issue?



Regarding the Mica sheet, I noticed 3 different sizes used for the IC's .. can't remember which ones go where, some fell off when I removed the heatsink. i also think a couple of then seem slightly damaged.. so was thinking maybe just replace all of them??
The link in you post is for a minimum 10x Mica sheets @ $1.39ea ($13.90).. but I came across these 2x (which seem identical to each other), that seems to have very similar specs/thickness as MK3305, but around 1/6 the price.

http://au.element14.com/multicomp/mk3306/insulating-kit-mica-to-220/dp/520214

http://au.element14.com/multicomp/mk3306/insulating-kit-mica-to-220/dp/52021402

I'm not familiar with them, but is there much difference ? Would they still do the job well ?

Also, with the old mica sheets, they have holes which line up with the screws that secure the heatsink to the support bar. If I replace them, there won't be any holes.. will that matter? Will the new Mica sheets still be ok sitting between the heatsink and IC? I dont think the holes do much except help line them up for easier installtion??



With the heatsink, when I removed it, it left behind some grey substance, im assuming its thermal paste? The grey strips on the heatsink themselves (seen in pic) are different, they seem to be some sort of tape.. maybe some sort of thermal tape that goes thermal paste > tape > heatsink ? If so, safe to scrape away the old paste from the sub and just apply some new stuff ?





Thanks again, much appreciated

[Twoflower]

The squared pin was the ground pin. If you look closely it is connected to the huge copper area which is the so called ground plane. It sucks all the heat out of your soldering iron. So your observation is quiet normal.

The C101 is more the L100. If the solder-joints are OK, just fix the inductor with a drop of glue to the PCB to prevent ripping it of the PCB.

The missing solder is OK. The parts are soldered from the other side. No worries, everything is fine here.

OK now to the things that make me wonder what the previous owner had done. Why the different sizes he had placed them in there. I don't think Logitech uses different sizes there. But I could be wrong here.

The Mica sheets you selected are too small (19x15mm) for the Amplifier ICs (19,6 x 17,5mm). And the one I selected earlier had also holes in areas you don't want to have them. As I mentioned earlier I would go for the polymer/silicone stuff. This sheets can be cut to match your size, just buy one large one. You can easily add holes were needed without the danger of cracking them. For example WLFT 404 53 X 53, probably 53mm x 53mm. But that would be a little tight for cutting 2x3 sheets out of it. I would add at least a 1mm margin around the IC. And you should check if you can get the ones with glue on one side and attach the pads to the heatsink. That make later repair easier as the WLFT 404 type has adhesive on both sides. But the E14 search options don't help much here. This one from DigiKey doesn't look too bad: LI2000A THERM TAPE 100X100X0.2MM. And: You must not use thermal grease with this pads, even if you use the ones with adhesive on one side. Eventually someone else has a better option as others may have better experience with thermal materials.

If you want to stay with the Mica ones, clean them carefully from the old grease and see if there are potential cracks were the amplifier ICs have metal parts. If not you can still use them. The Worn out areas around the mounting holes seem to be fine.

The tape around the heat-sink is to make the chassis air-tight. You should keep it that way. Otherwise you'll lose some umpf. The only area were air from the inside of the box is allowed to escape is the bass reflex port.

[LeWidget]

Thanks Twoflower , might have a look at the MICA pads I took out and see what can be re-used, then replace what I need.

What are the main things to look for in general with these thermal pads/sheets?

Thicker the better (assuming it fits when it needs to) ?? http://au.rs-online.com/web/p/thermal-gap-pads/0446493/ 2mm.. maybe too thick?

or even http://au.element14.com/bergquist/hf225fac-0-004-ac-0404/hi-flow-225f-ac-004-4-x4-sheet/dp/1893465 .. too thin ?

[Twoflower]

I'm not the best person to ask about the thermal solutions. But I can share what I know. These pads has 4 main values:

1. Thermal conductivity: The better it transport the temperature the better (and usually more $$$). You probably don't need the best ones here.
2. Electrical insulation: As you only want to insulate the different amps from each other 1kV is more than enough.
3. Elasticity: The capability to fill uneven surfaces or even build tolerances (e.g. mounting the heat-sink on top of a surface mounted board with different devices) you need softer materials. As your devices are 'bent' to be flush to the heat sink and both surfaces are relatively smooth you don't need very flexible stuff.
4. Thickness: See above. Also because all amps are pressed flat on the heat sink you don't need much material between. Too thin material is hard to handle (e.g. try to apply a thin, flexible sticky film without air-bubbles and no folds). Even if I selected 2mm earlier I would see if you can get something in the range of 0.2 to 1mm. I just failed to find something reasonable at Digikey (I was looking for single sided sticky pads).

There is also the number of sticky sides (zero to two). In your case I would go for one sided. Non sticky ones are OK, if you can place the pads and make sure they don't move around while mounting. Double sided sticky: You can't change things without destroy the thermal pad (some are even very sticky). These are used to glue a heat sink on without the need to fix the heat sink.

The ones you selected are interesting stuff. It is a phase change material. So while the first (also later heating) the top and bottom side will melt and adapt to the surface. But the same problem like double sided sticky thermal pads: If you need to remove the heat sink: They glue the parts together and you must replace the pads. But of course it will work (I would go for the thinner ones).

It looks like it is hard to get single sided thermal pads. So probably you should go for the non sticky ones. How about this: Thermal Insulator, Gap Filler, Sheet, Carbon, 1.6 W/m.K, 0.5 mm.

[LeWidget]

Thanks Twoflower , the last link you posted looks good, though doesn't say how big the sheet is. I might send them a message tomorrow and see what sizes they come in

[Twoflower]

From full order number I assume its 150x150mm. But you should ask to be sure.

One thing that came into my mind. Since the Mica sheets and the thermal pad will have a different thickness. It seems that all amps are fixed with one metal bar. That could cause that the amps are not evenly pressed against the heat sink. Probably it's best to use the same stuff on all amps that are fixed with one metal bar. Just to avoid problems.

[Rasz]

TDA7295 ....
Avoid buying from China etc. as fakes are not uncommon.

wouldnt worry too much about that, even fakes make sound and OP cant be too concerned about quality. This speaker system is rated at 10% THD from the factory.

[LeWidget]

Thanks Twoflower , Ill double check to see how much room I have to play with between the IC and metal bar.

@Rasz, what's THD ?

[Rasz]

@Rasz, what's THD ?

exactly my point! you will be fine with any chip from ebay as a replacement

[Twoflower]

Rasz, I don't think that the responce from LeWidget was irony. He might really want to understand the therm THD.

@LeWidget: THD is short for Total Harmonic Distortion. Simply spoken that means the noise (deformation) of the output signal introduced by the amp. The lower this value is the better.

I'm quiet mixed with fakes. As there are two levels of fake: 1. Not meeting the specs. This could lead to bad sound or also to an early death. 2. You don't get even a amplifier. Both could led into buying the device again (best case) or fry other parts as well. And for a device that costs 2,50€ at a trustful store I wouldn't think twice save one or two bucks and risking other unforeseeable risks. But this decision has to be done by LeWidget