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.