Author Topic: Repairing a Speakercraft MZC-66  (Read 3892 times)

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Offline MacAttak

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Repairing a Speakercraft MZC-66
« on: April 08, 2013, 11:25:04 AM »
This is my first real repair challenge, so please forgive me for any questions that might seem too basic.


I have a Speakercraft MZC-66 multi-zone audio amplifier system. I bought it 7 years ago when my house was built, and had digital keypads and ceiling speakers installed in 5 areas (kitchen, 3 bedrooms, and outdoor patio) along with all of the wiring needed for it. It worked great. Wife loved it. It's really a remarkable piece of home audio gear.

But after several years (the warranty is only a 2-year manufacturer's warranty) individual zones started going out. Instead of the expected audio in a zone, you would only get a loud crackling noise. I could switch around the output terminals in the rear of the amplifier to verify that it wasn't blown speakers or fried control panels - is is the individual zone amplifier modules in the unit that have been going out one by one. So far two out of six zones are dead, with another on the verge of dying.

I could send it off to the manufacturer, and it would likely cost from $500 to $700 for that repair. They likely would make me also pay for a "technician" to do a service call ::) which would be another couple hundred $. I could also buy a new model of the same device - for around $2,000. Yowsers.

So I figured might as well try to fix it myself first. So I've done some research and it seems that this is a fairly common problem with these amplifiers built around 2005-2007. The internal amplifier "cards" fail after a few years, one at a time. It is generally attributed to poor air ventilation and faulty parts. So with that in mind, I took the cover off and started investigating.

Here you can see the insides after taking the cover off. This is a view from the top rear - you can see the dozens of input/output jacks and terminals at the bottom of the photo. The large board in the front of the unit has a number of digital parts - microcontrollers and such - used to learn and control up to six IR input devices and drive IR output to any of the six output zones (think of it like a beast of a programmable remote control). To the right of center is the main power supply. Just below center you can see six sets of heavy guage wires coming from a wide (horizontal) power distribution bus board - each set goes to one of the six individual amplifier cards. In this photo, I have already removed cards #2 and #4 (the blown ones). The remaining four cards are underneath the large aluminium heatsinks. The thin circuit board strip across the back is there to break out all of the I/O connectors (hence the ribbon cable that connects it up with the main digital board).



Knowing about how heat affects electrolytic caps, I was expecting to find some blown out caps on these amplifier boards - and thats exactly what I think I have found. I used my el-cheapo $20 ebay "ESR" meter to check some of the larger caps, and some tested in-spec while others were VERY high (like 7 or 8 ohms). I couldn't find any obvious physical failures though - no bulges or splits - however the two failed cards do have a nasty residue on them. I *think* this residue is the dried-out remains of electrolyte... is that a correct interpretation? Here you can see photos of both failed cards with the residue on the edge. The other four (functional) cards do not have this - they are clean:





Notice also that the amplifier chip on the second photo appears to have been reworked. There is a ton of flux and you can also see where solder flowed onto the nearby unused capacitor pads. I'm not sure why this would be done since this unit was supposedly brand-new in a sealed factory box before it was installed. Perhaps the manufacturer installed repaired card modules in new systems to save money??

So my plan is to just replace all of the electrolytic caps on both cards (even the small caps). They all sit right next to the massive heatsink (removed for the two above pictures), so I figure I might as well just replace them all.

I am wondering though if I should just replace the amplifier chip too while I am in there? They are TDA8920BTH (http://www.nxp.com/documents/data_sheet/TDA8920B.pdf) which appear to be in stock at digikey for only $7 each. Or should I not worry about these? The data sheet on them claims "full short-circuit proof across load and to supply lines" along with thermal protection... and they don't look damaged. I am thinking that for $14 I might as well just replace them as a precaution, but again not sure if that is warranted.

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Offline MacAttak

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Re: Repairing a Speakercraft MZC-66
« Reply #1 on: April 08, 2013, 11:35:41 AM »
I also wanted to add something else that I found (which I was not expecting). In the very first photo if you look closely at the far back right corner you will see an IC that looks to be in quite bad shape. Here is a close-up shot of it:



My presumption is that chip is fried pretty well (just a cheap quad op-amp MAX4395 ESD). But I'm not sure why - the traces mostly seem related to the RCA jacks in the bottom of the photo - and I don't have anything hooked up to those. Nor have I ever had anything hooked up to them. I'm not really sure what it could be affecting as it is nowhere near the two blown-out amplifier cards and I am not aware of any other nonworking features of the device.

I'm planning to replace that IC just to be thorough, but figured I would point it out in case it makes sense to anyone that this would also be fried.

Between that questionable chip and what I saw with the two amp cards, I have to wonder if the manufacturer didn't sell me a "refurbished" unit that wasn't even properly serviced. And that kind of pisses me off >:( because I certainly didn't pay a reduced price for it - it was about $2,500 new (plus about another $5,000 for the rest of the system).

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Offline amyk

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Re: Repairing a Speakercraft MZC-66
« Reply #2 on: April 08, 2013, 09:13:37 PM »
That residue doesn't look like capacitor electrolyte. (If it was, which cap did it come from? That should be obvious to find.) It reminds me more of old glue that's become conductive...

Offline PA0PBZ

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Re: Repairing a Speakercraft MZC-66
« Reply #3 on: April 08, 2013, 10:36:19 PM »
It's almost as if something has been poured in there, did you check inside the top cover?

Offline Sander

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Re: Repairing a Speakercraft MZC-66
« Reply #4 on: April 09, 2013, 02:48:29 AM »
Maybe ,if they didn’t clean the board, it is flux that eroded the board/parts. I would clean the boards, mark the dirty parts (first parts to look at if cleaning doesn’t work) and test the device.

I’ve seen similar corrosion on boards that were used in a garage-soap dispenser. The soap had leaked onto the boards, solved the silkscreen and corroded metal parts.

Offline MacAttak

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Re: Repairing a Speakercraft MZC-66
« Reply #5 on: April 09, 2013, 02:52:19 AM »
The inside of the system is immaculately clean (not even any dust). The caustic spotting I photographed is the only exception to that. There isn't even any of that stray rubbery "goop" that you find on some boards.

It has been kept in a very controlled environment. There is no chance anything could have spilled into it.

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Offline G7PSK

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Re: Repairing a Speakercraft MZC-66
« Reply #6 on: April 09, 2013, 03:47:11 AM »
Is that condensed vapour from the big caps. in the power supply, the photo is not clear on that part of the equipment but the big caps look a bit discolored.

Offline MacAttak

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Re: Repairing a Speakercraft MZC-66
« Reply #7 on: May 02, 2013, 02:44:40 PM »
Following up on this project...

Four of the six amplifier modules were failing (not just the two I originally thought). All of them had the strange residue on them. I'm not certain what it was, but it cleaned up easily enough with IPA.

I tested all of the caps on one of the failed modules in-circuit and it was showing really high ESR on my $20 ESR/LCR cheapo component identifier gizmo. So I pulled all of them and tested them individually. The two large output caps were in really bad shape, and the other smaller caps weren't great but weren't totally gone yet. I spent a few hours probing around the board :-DMM and documenting the circuit to make sure I understood it. Gained a strong dislike of traces and microvias that are placed under large IC's  >:(. Those took a lot of probing and deductive reasoning to figure out. It also doesn't help that this dumb amplifier chip is numbered backwards - pin 1 is at the 1 o'clock position and it goes clockwise from there.

Anyhow, after figuring out the circuit I was pretty comfortable that the rest of the components were all likely unharmed, so I replaced all of the caps with higher quality Nichicon caps. I also replaced that quad op-amp in the input panel board that looked suspicious.

And now all six channels are working again, as good as new! :-+ It's a really nice feeling to have enough knowledge and confidence to tackle a useful project like this (instead of just another microcontroller & blinky lights project).

It also saved me at least $2,000 (the replacement parts were under $30 in total). So it pretty much just paid for the last few months of test equipment and other similar purchases :-+


I definitely would not have been able to repair this on my own without the huge amount of knowledge I've picked up from these forums over the past 6 months, so I was lucky to discover EEVBlog and the forums. So if you read this - Thanks Dave!!

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Offline notaxdu

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Re: Repairing a Speakercraft MZC-66
« Reply #8 on: July 06, 2013, 11:12:42 AM »
Hi MacAttak -

I have an MZC-88 with a similar problem.  I'm looking for assistance in repairing it - would you be interested in a paid gig?  I too have made an investment in the Speakercraft MZC system (controllers, keypads etc) - self-install and config etc.  I'm not able to locate a repair option...hoping you can assist. 

Thanks.

Offline MacAttak

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Re: Repairing a Speakercraft MZC-66
« Reply #9 on: July 07, 2013, 03:58:08 AM »
Hi notaxdu, I sent you a PM through this forum to see if I can help you out!

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Offline Experimentonomen

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Re: Repairing a Speakercraft MZC-66
« Reply #10 on: July 08, 2013, 06:23:07 AM »
As it appears this is some sorta class d amplifier, keep in mind that you need to replace the capacitors with proper LOW ERS types. Also it appears like its a separate modulator(PWM stage) board and  power module board.

However i also lean toward a failed output stage ic/pwm stage as a result of failing caps.

Maybe you could retrofit in some new fresh class d modules instead of repairing the old ones with likely has a fried output stage/pwm stage anyways. Hypex makes some very nice class d modules, if 70 euros + vat and shipping per channel is acceptable.

Offline MacAttak

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Re: Repairing a Speakercraft MZC-66
« Reply #11 on: July 08, 2013, 07:13:52 AM »
Yes, they are class D amp chips (TDA8920BTH) but they are almost never the source of failure in these units as far as I know. The chip used on these modules is pretty robust and isn't being overdriven.

99% of the time it is the large and medium size electrolytic caps that go high ESR since the manufacturer used somewhat cheap caps and the designer crammed them all right up next to those huge heatsinks. If you look at the first photo you will see that all of the output stage modules are tightly grouped together, with a long vertically mounted power supply pcb that effectively walls them off from any air circulation in the chassis. The power supply cables (with big chokes on them) loop over the top edge of that board. There is no fan in that section and almost no ventilation holes in the chassis. All of the output stage modules just bake back there in the heat.

One of the more popular "mods" to these is to simply add some airflow cutouts on either side of the case at the back, and install simple case fans to create a wind tunnel through all of those heatsink fins.

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Offline Experimentonomen

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Re: Repairing a Speakercraft MZC-66
« Reply #12 on: July 09, 2013, 07:38:36 AM »
I have a module with one of these class d ic's on it(TDA8920BTH) and i have never gotten it to even reach noticable heating of the small heatsink it came with(considerably smaller than the heatsinks in your amp), despite beeing a uber cheap china module off ebay with questionable capacitors.

And judging from your pics there seem to be footprints close to the ic for little SMD X7R or similar ceramic capacitors for local decoupling, i guess they dident populate these as they thought they werent needed and thus relied solely on the ESR of the main lytics which is bad as they are a mile away from the ic in RF terms, these ic's operate at 250-400kHz so that distance between the ic and main lytics has considerable stray inductance, likely contributing to the audible noise of the failing/failed modules.

Putting all of the HF decoupling duty on the main lytics further decrease their lifetime.

Offline MacAttak

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Re: Repairing a Speakercraft MZC-66
« Reply #13 on: July 09, 2013, 10:36:36 AM »
Oddly, these do get warm - but possibly due in part to how they cram all of the modules next to each other in a confined space with no meaningful ventilation. Not really "hot" - just warm.

Yes, I don't believe those topside caps were populated on any of the six modules that I have, but there were some populated pads on the underside of the board. I had traced them all out at one point to make sure I understood the circuit, but have since thrown that schematic away. C1, C2, and C3 were way out of spec - well over an order of magnitude or two above expected ESR (C3 also needed to be rated for higher voltage than the rest since it sits between the +27v and -27v rails). C4 and C5 were borderline, but still high. The other four smaller caps weren't so bad, but I replaced them anyways since they are inexpensive parts.

The electrolytics are placed about as close as they can be to the ic, given the heatsink they chose. They are pretty much pressing right up against it.

All of them were average to low quality electrolytics. I forget the brand, but from research it seemed to be commonly used in audio systems a few years ago - usually with a lifespan of only a few years with good performance at first but failing within 2 to 4 years. Pretty much just good enough to make it through the warranty period :--

But that would go along with the unpopulated caps if the manufacturer was just trying to squeeze a few more pennies of savings out of the build.

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Offline true

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Re: Repairing a Speakercraft MZC-66
« Reply #14 on: July 09, 2013, 12:18:38 PM »
Oddly, these do get warm - but possibly due in part to how they cram all of the modules next to each other in a confined space with no meaningful ventilation. Not really "hot" - just warm.

They started putting fans in these a few years ago; I think they are 120mm fans and they are located on the top of the units. I haven't see any of the fan-equipped units fail. I have had a number of older units fail (usually Ch1, and sometimes other channels).

Interesting to know it's likely just a cap issue as we have one customer who still uses a ton of these. Something I can fix in the shop is something easy I can get paid for ;)


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