Repairing a Speakercraft MZC-66 - Page 1
Forum section: electronics / repair

author: MacAttak
author_rank: Unknown
timestamp: April 08, 2013, 01:25:04 am
content: 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 callwhich 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'tlookdamaged. I am thinking that for $14 I might as well just replace them as a precaution, but again not sure if that is warranted.

author: MacAttak
author_rank: Unknown
timestamp: April 08, 2013, 01:35:41 am
content: 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 offbecause 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).

author: amyk
author_rank: Super Contributor
timestamp: April 08, 2013, 11:13:37 am
content: 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...

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 08, 2013, 12:36:19 pm
content: It's almost as if something has been poured in there, did you check inside the top cover?

author: Sander
author_rank: Contributor
timestamp: April 08, 2013, 04:48:29 pm
content: 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.

author: MacAttak
author_rank: Unknown
timestamp: April 08, 2013, 04:52:19 pm
content: 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.

author: G7PSK
author_rank: Super Contributor
timestamp: April 08, 2013, 05:47:11 pm
content: 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.

author: MacAttak
author_rank: Unknown
timestamp: May 02, 2013, 04:44:40 am
content: 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 boardand 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 purchasesI 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!!

author: notaxdu
author_rank: Newbie
timestamp: July 06, 2013, 01:12:42 am
content: 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.

author: MacAttak
author_rank: Unknown
timestamp: July 06, 2013, 05:58:08 pm
content: Hi notaxdu, I sent you a PM through this forum to see if I can help you out!

author: Experimentonomen
author_rank: Regular Contributor
timestamp: July 07, 2013, 08:23:07 pm
content: 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.

author: MacAttak
author_rank: Unknown
timestamp: July 07, 2013, 09:13:52 pm
content: 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.

author: Experimentonomen
author_rank: Regular Contributor
timestamp: July 08, 2013, 09:38:36 pm
content: 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.

author: MacAttak
author_rank: Unknown
timestamp: July 09, 2013, 12:36:36 am
content: 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 periodBut 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.

author: true
author_rank: Frequent Contributor
timestamp: July 09, 2013, 02:18:38 am
content: Quote from: MacAttak on July 09, 2013, 12:36:36 amThey 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

author: MacAttak
author_rank: Unknown
timestamp: July 09, 2013, 02:54:09 am
content: Yeah, I've heard that the newer ones don't need the case mod for airflow. Or at least they have a longer MTBF.Zone 1 probably fails the most often because it's the one everybody usesMany people don't have all of them hooked up, and it's natural to start with 1 for the most used area.I ran across one fellow in Canada who was advertising in an appliance repair forum to repair bad channels in the MZC units. He wanted an obscene amount of money for it - repairing all 6 zones of a MZC-66 would end up costing well over $1,000. That's what motivated me to try and fix my own. I was surprised with just how straightforward those output modules are, and how cheap/easy they can be to fix - only about $6 per zone as long as it's just the caps.

author: biggirl
author_rank: Newbie
timestamp: November 24, 2013, 02:04:52 am
content: I am experiencing the popping noise on one of the channels. Is there a video or blog entry you could suggest to show me how to change the caps?

author: MacAttak
author_rank: Unknown
timestamp: November 25, 2013, 04:19:48 am
content: This video is a good one. It gives answers to many of the questions you would probably have.If you look at the photos I posted in this thread, the first one shows the amplifier with the outer cover removed. You will want to remove the small rectangular daughterboard in the back that corresponds with the audio zone you are having problems with. Be careful to unplug the connector wires that are attached to the daughterboard - there is a big one at the front end, and a smaller one at the rear. There are two screws that hold the board down to the chassis, plus a screw in the back panel where the output jacks are.In the photo you can see that I have removed two of those daughterboards. The second and third photos are of the removed daughterboards (with the heatsink blocks removed from them).Note: be careful poking your hands around the rest of the amplifier - especially the board next to that large coil in the middle of the amp. Those large capacitors can hold a serious charge for quite a while so you don't want to be touching them.Once you get the daughterboard out, you will need to remove the large aluminum heatsink block from it. There are only two screws holding it on.Replace all nine of those capacitors - it's a hassle to figure out exactly which ones might be bad and they are very cheap to replace, so just do them all at the same time. Be sure to replace them with NEW capacitors from a quality source. Don't use parts found on ebay (these are often counterfeit) or your junk drawer from 1980 (capacitors degrade with age). Nichicon is a good brand. I bought mine from Digi-Key (http://www.digikey.com/), a reputable source for electronic parts in the US.Be sure to pay attention to the ratings of each capacitor you are replacing, and use the same capacitance rating and voltage rating (you can use a higher voltage part if you need to, but not a higher capacitance part). Also, pay attention to the physical sizes of the capacitors you are replacing so that when you order the new ones you won't have problems with them fitting on the board (there isn't much room for oversized parts).It's not very hard to repair as long as you are careful and pay close attention to those things.

author: angelstreet
author_rank: Newbie
timestamp: December 22, 2013, 02:22:08 pm
content: I have a MZC 66 with two channels gone. Same MO as what is posted. Any chance of repairing for a fee?

author: informative
author_rank: Newbie
timestamp: February 04, 2014, 03:03:17 am
content: Replace all nine of those capacitors - it's a hassle to figure out exactly which ones might be bad and they are very cheap to replace, so just do them all at the same time. Be sure to replace them with NEW capacitors from a quality source. Don't use parts found on ebay (these are often counterfeit) or your junk drawer from 1980 (capacitors degrade with age). Nichicon is a good brand. I bought mine from Digi-Key (http://www.digikey.com/), a reputable source for electronic parts in the US.Be sure to pay attention to the ratings of each capacitor you are replacing, and use the same capacitance rating and voltage rating (you can use a higher voltage part if you need to, but not a higher capacitance part). Also, pay attention to the physical sizes of the capacitors you are replacing so that when you order the new ones you won't have problems with them fitting on the board (there isn't much room for oversized parts).=============MacAttak,THANK YOU for this very helpful and informative post. We did an extensive remodel on our house in 2008. With the studs bare, I thought it was the best time to wire the house for "whole home audio," so after looking at a number of solutions, I purchased a "b-stock" Speakercraft MZC-66 in late 2008 through an authorized reseller (who also installed it... and is now out of business) for a painfully large amount of money (for me, anyway). After five years of gentle use (once or twice a month -- only one or two zones at a time) I noticed my speakers making a loud "tick tick tick tick" sound when the power to the main unit was on, even if that zone with those speakers wasn't being used.There seems to be no shortage of people complaining about the high cost and crappy quality of Speakercraft products -- but few resources for advice on how to get them repaired, other than to send them back and pay as much to have speakercraft repair them as a new unit costs. No thanks.When I first discovered the problem, I was searching for a solution and stumbled upon this post of yours, as well as a listing by an independent retailer of the amplifier cards on a popular internet auction site. Without any electronics tinkering experience, the $100 brand new card sounded like the better idea. So I bought one, and when it arrived I started to troubleshoot (pulling out the speaker wires one zone at a time) to figure out which zone / card needed to be replaced. BUT -- painfully -- I discovered in the processes that I didn't have one, but FOUR amplifier cards that were fried. Although I was readily willing to part with $100 to repair this thing, the idea of spending $400 on it wasn't so appealing.Having read your post several times now, and giving myself a crash course on soldering (and desoldering) -- I think I'm up to doing the repair myself -- and even adding a new cooling fan with a heat activated switch. The one thing that I'm having trouble figuring out is which capacitors to order. It's a foreign language to me.So... if it isn't a big hassle, and in the spirit of helping out another guy that was suckered by Speakercraft (and perhaps others like me who might stumble on this in the future), could I trouble you to provide a little more info:(1) You provided a link to a site where I can get capacitors. Could you please provide detailed info on which ones to select? It looks like there are two sizes to replace on those boards. No? I would love to know how to order the same ones you used for the replacement.(2) Did you add a fan? I'm planning to do that while I have it apart. I think I have that part of the project all figured out using an external power supply that powers a fan on a thermal switch. That said -- if you did that as well (or something like it) I'd love to hear the details... and also any tips for this relative noob on tapping into the internal power supply.(3) How's your's working after you did the repair? Any issues?Thanks again for the great post,Jim

author: MacAttak
author_rank: Unknown
timestamp: August 09, 2014, 11:44:52 pm
content: I get a PM about this every few months or so when someone stumbles onto the thread after a Google search. Always the same questions, so I'll just post here to make it easier for those folks who simply just want to get their amplifier working again...1. Can you tell me which exact part numbers you used for your repair?My memory isn't perfect, so I pulled up my web orders from this period of time. I think each amplifier card has nine capacitors. The large two in the middle of the card are 470uF/35v. The medium-sized one next to those two is a 47uF/63v. Two other medium-sized caps towards the end of the board are 47uF/50v. And next to those are the four small caps, which are 1uF/50v. I used a Rubycon for the 63v cap, and Nichicon for the others. Here are the exact part numbers from DigiKey, along with unit price at the time (based on how many I bought) and a link:Physical size, capacitance rating and voltage rating are ALL important for operation of the device. Larger parts simply will not fit. Lower voltage rating caps will be undersized and likely explode when turned on. And correct capacitance ratings are necessary for proper functioning of the unit. Be careful when/if substituting for alternative parts.493-4669-1-ND ($1.254 x2)http://www.digikey.com/product-detail/en/UKA1V471MPD1TD/493-4669-1-ND1189-1511-ND ($0.37 x1)http://www.digikey.com/product-detail/en/63ZLJ47M6.3X11/1189-1511-ND493-4660-1-ND ($0.677 x2)http://www.digikey.com/product-detail/en/UKA1H470MED1TD/493-4660-1-NDUKL1H010KDDANA-ND ($0.239 x4)http://www.digikey.com/product-detail/en/UKL1H010KDDANA/UKL1H010KDDANA-NDTotals:2x $1.2541x $0.372x $0.6774x $0.239= $5.19 per card repaired (each amp channel has its own card)The op-amp chip that was replaced was a MAX4395:MAX4395ESD+TCT-ND (2.95)http://www.digikey.com/product-detail/en/MAX4395ESD%2BT/MAX4395ESD%2BTCT-ND2. Did you retrofit a fan into your unit?No, I didn't feel like it was worth it for how infrequently we use the system.3. Is it still working?Yes. We turn it on only a few times per year though, so VERY light use. Whole-house audio is one of those things we've discovered that seems like it would be awesome and used all the time, but in reality it almost never gets used.4. How long do you think it will continue working? Will it break again?I expect to get another 5 years or so out of it before channels start failing again. No reason to think that the replacement parts will fare a great deal better than the original parts. Although I did use higher-quality components in the repair than were originally there, the system layout itself is pretty poor with the amplifier cards cramped into a tight area (with no air circulation) while over 1/2 of the case interior is more or less unused open space. Heat will surely build up when it's turned on, and those caps will heat up and have a shortened lifespan for sure.Installing a forced-air cooling fan would likely make the repair last a few years longer. But really, it's only $5 per card to repair it and the work/parts for the extra cooling would be quite a bit more than that (and would introduce some noise). I'll just repair it again in a few years if I need to.5. Can I pay you to repair mine for me?No, sorry, I don't feel comfortable doing that. Electronics is 100% just a hobby for me at this time, and I cannot commit to providing a paid service. It should be very easy to find a local electronics repair shop that would be willing to do the work - there is plenty of information in this thread to give them what is needed to perform the work, including exact part numbers. This repair is exceptionally simple to do - the hardest part is getting those damned little amp cards out without breaking a board connector.It's also REALLY expensive to ship one of these huge units around. And likely to get damaged in the process.

author: jediok
author_rank: Newbie
timestamp: September 20, 2015, 06:57:57 pm
content: Macattack,I am operating my MZC-66 in pre-out mode only on all six zones using an external 12 channel power amplifier, due to several faulty internal amp boards. I sometimes hear a faint clicking sound in the noise floor. I was wondering if the unit would operate more safely, more quietly, and still be fully functional in pre-out mode if I simply disconnected each of the six internal amp boards at the 5-wire clock and power connection block point, each labeled "MOD", "OSC", "GND", "+27V", "-27V"? I could also disconnect the corresponding preamp "R","G","L" input wire connection on each, if advisable.

author: MacAttak
author_rank: Unknown
timestamp: September 20, 2015, 11:10:42 pm
content: You could disconnect those, and I don't think it would hurt anything. I doubt it would help either though. The clicking is most probably interference being picked up from something else, especially if your signal wiring to the external amp is long. Shouldn't hurt to try it and see if it helps /shrug

author: jediok
author_rank: Newbie
timestamp: September 24, 2015, 04:39:27 pm
content: Disconnected power supply plug from all six amp boards and secured each with tape in open position, but left preamp connection alone due to level of difficulty to disconnect tiny, fragile plug/socket. Unit now operates normally in preamp mode. Noise floor seems quieter, no clicking at present, but more importantly, I worry less now by having known faulty components out of the circuit paths.

author: brinzlee
author_rank: Newbie
timestamp: August 21, 2020, 03:06:39 pm
content: I know this is a very old thread....I have a blown up amp....well and truly...., the caps dried out and took out the chip, the caps, a resistor and the two inductors....I have managed to repair most of it, the only one I am trying to identify are the two inductor chokes....one is a common mode (green torroid) and the other isn't (red torroid). does anybody know its current value and how many uH they are.....any help would be appreciated.....Thanks

author: johnhoef
author_rank: Newbie
timestamp: December 15, 2023, 03:53:50 pm
content: Macattak, I hope you are still with us here. It's been a while! I'm dealing with the exact same issue you dealt with. In my case, two of the amp boards have two caps burned, literally. They are C6 and C7. These are small SMD style low value ceramic caps. I have no idea of their value. Also, the circuit board traces are burned. A schematic would be VERY useful in determining how to repair this. Would you happen to have a schematic you can share with me? Or at a minimum, do you know the value of these two caps? Here's a photo:

author: johnhoef
author_rank: Newbie
timestamp: December 15, 2023, 04:34:23 pm
content: I did some poking round with my meter, as well as looking around with a strong magnifier. It looks like C6 and C7 are two capacitors connected in series, across the plus and minus 27 volt rail. Measuring the capacitance across these two pins, I find roughly the same readings on one of the good boards, compared to the damaged board, with the two capacitors removed. I'll guess these two caps are a very small value, and being across the two rails, probably not that important if I make a guess as to their value. Probably in the double digit picofarad range...say, 20pf?

author: johnhoef
author_rank: Newbie
timestamp: December 15, 2023, 04:51:25 pm
content: I looked up the spec sheet for the amp chip. They have an example circuit with 100nf caps across the power supply, so looks like these tiny caps could be .1mfd each?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 15, 2023, 04:53:58 pm
content: Quote from: johnhoef on December 15, 2023, 04:51:25 pmCould very well be the case, and it will not hurt to try. Still wondering how they were destroyed though, is your supply rail still in tolerance?

author: johnhoef
author_rank: Newbie
timestamp: December 17, 2023, 03:11:48 am
content: What happened was one or more of the capacitors actually shorted causing a high current flow on the power supply traces. Those tiny caps were simply in the way, and burned up along with the burning traces! Recapping solved all problems with all six power amps. So, I strongly advise a total recap of these power amps! The .1mfd caps work fine as replacement of the burned caps, and as advised in the spec sheet, .1mfd is the correct value for these caps. I also replaced a 450mfd bulging cap on the power supply board. What a hassle.....I had to dismount the power transformer to gain access to the bottom of that board! I have the unit reassembled, but have no way to test the entire unit, so hoping all will work well, once the customer installs it.

author: johnhoef
author_rank: Newbie
timestamp: December 17, 2023, 03:25:12 am
content: Just thought I would add a few more suggestions for any of you that are planning on performing this repair. The power amps will not work unless there is 5volts on pin 6 of the amp chip. This is the "mode". As I recall, it's the red wire of the 5 wire connector on each board. I rigged a two resistor network, and connected it across the +28 volt supply. Then just used a clip lead to connect the resulting 5v to the mode pin on each board. Do unplug the 6 conductor cable that supplies the mode logic so you're not feeding this 5v into the logic circuitry!!According to the amp chip specs, the "osc", pin 7 needs to be dealt with. They recommend using a 30k resistor connected from pin 7 to the negative rail. Speakercraft left this pin floating! So, I also left it floating, and the power amps worked fine that way. But, in my opinion, this is poor design, not to follow the manufacturer recommendations!! The thought occurred to me to add these resistors, but decided to leave it alone.

author: MacAttak
author_rank: Unknown
timestamp: January 03, 2024, 05:04:22 pm
content: Glad you solved your problem, and sorry I couldn't assist.I dismantled my whole workshop a few years ago (gave all my gear away to a local makerspace). I just didn't have room for it anymore after moving to a new place, and my eyes aren't as good as they used to be. So I decided to hang up my hat on electronics.I also left that Speakercraft amp in the home that I sold, so don't have it (or the specs) any longer to refer to.