mustang guy Posted November 24, 2014 Share Posted November 24, 2014 (edited) There are lots of folks who are stuck with dead amps in these things. I found some information, and over a period of time, I plan on repairing this thing, and if possible reducing the chances that it will happen again. I was not willing to throw out the baby with the bathwater, so I have done a bit of research. AVS forum has been very active on the topic, so I am going to summarize what I learn and post it here. A great little PDF was done here: Repair of an Epik Subwoofer Amplifier. This was my launching point, since the same exact symptoms happened to this sub. I've removed the amp. Here is the process to get to the FET's: Here is the little fuse holder on them. There are two fuses inside. If your sub is dead, it's likely one of these is blown. Never fear, the non-blown one is a spare! Remove these #2 phillips screws. The plate amp will lift out. I lifted it out by grabbing the terminal posts. Mine wasn't stuck, but some are. Be vigilant... Disconnect the power from the subs so you can lift the plate amp out: You will need to remove the heat sink from the plate amp face to be able to remove the FETs. Here are the screws: These screws on the back of the heat sink between the plate amp face and heat sink, hold the FETs tight against the heat sink. The white stuff is thermal compound. The thermal compound is between the faceplate and the heat sink to allow the heat to better dissipate to "outside" the enclosure. Remove these screws. Take the hold down metal strips out and set aside: I had to clip the vinyl tie to get the heatsink and power PCB away from the faceplate. This was common sense, but I just wanted to add it. You can always put a new tie when it all goes back together: Remove the PCB from the heatsink: Edited December 3, 2014 by mustang guy Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 24, 2014 Author Share Posted November 24, 2014 (edited) Now you can get to the FET's: I found out that all of these are not FETs. They are in order bottom to top: 1&2: MOSFET - Toshiba K4107 3: F12C20C Switchmode dual fast recovery power rectifier - Cathode 4: F12C20C Switchmode dual fast recovery power rectifier - Anode 5&6: N-Channel HEXFET power MOSFETs 7: TIP31C power transistor I tested the FET's in circuit and out. Here is what I got: 1: 651 Ohm 2: 4.1 Ohm 5: 15.49 Mohm 6: 18 Mohm I tested them out of circuit both with diode tester and with typical ohmeter. 1: OL 2: 3.8 Ohm 5: OL 6: OL My research has taught me that resistance between the outside leads on a typical MOSFET should be in the millions or Open Lead (OL). This proves that MOSFET #2 is bad bad bad! 3.8 Ohms is a shorted out FET. Gotta get a new one! Here is a picture of one of the big MOSFET's out of it's silicone shield: The shield is flimsy. I think it insulates electricity while conducting thermal heat. Check out this picture of the burnt glue under one the cap that goes to the bad FET: I tested both of the 330uF caps with my handy dandy ESR tester. THANKFULLY they tested OK... Here is my order from mouser.com, which includes a 8cm A/C fan. I'm not sure how or where it will be mounted. I suppose I'll figure that one out when I get it: Shipping was $7.99 bringing the total to $36.95 While I am waiting for parts, I thought about the whole steel vs aluminum holding those MOSFET's in. I made a hold down, and it took a little under an hour. Here is my process: Find a heat sink in an old computer. This was a bios heatsink in an old Acer: It's a lot bigger than the hold down thing, so it needs cut down. Edited November 28, 2014 by mustang guy Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 24, 2014 Author Share Posted November 24, 2014 (edited) Now that it is cut to size, it needs threads. My Epik Legend uses a 4mm x 0.70 machine screw.Mounting the fan so it blows offset from the PCB that faces it. This will also circulate cool air across the entire plate amp.NOTE: The fan hits the amp PCB's. I measured wrong!!! Nuts! Anyhow, I have room to mount the fan below the braces, so all is well. I will update pictures once I get it in.Using sil pads between the FET's etc and the main heat sink:reply space reserved...I have 5 left..... There is a 10 picture limit, and I want all this together... I should be able to get everything to fit in 30 pictures. Edited December 3, 2014 by mustang guy Quote Link to comment Share on other sites More sharing options...
swapface Posted November 24, 2014 Share Posted November 24, 2014 Excellent, thanks for the How-To on this. Being a owner of a Epic Legend I suspect it is only a matter of time................ Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 24, 2014 Author Share Posted November 24, 2014 (edited) Right! Perhaps some improvement on the heat sink and possibly a fan will keep this from happening to you. I do not like the hold-downs they use for the FETs (the plates with holes in them in the picture with screws). They are steel and a bit heavy, which means they will retain heat, and keep getting hotter. I would prefer aluminum. I will be looking into that and a fan setup like I use in my Klipsch Promedia Ultra 5.1 sub. Edited November 24, 2014 by mustang guy Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted November 24, 2014 Share Posted November 24, 2014 Nice job. I always thought an amp in the sub was a bad ideal. Quote Link to comment Share on other sites More sharing options...
Heritage_Head Posted November 24, 2014 Share Posted November 24, 2014 Interested in what you find. Always wondered what was so wrong with the amps. One day I would hear this and the next it would be that. Quote Link to comment Share on other sites More sharing options...
wvu80 Posted November 24, 2014 Share Posted November 24, 2014 At a couple of bucks a piece, I presume you'll be replacing all 5 FET's, but it begs the question; are the FET's defective, or is lack of cooling the problem? Your picture is the second one I've seen where that thermal paste is dried up and looks really ineffective. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 24, 2014 Share Posted November 24, 2014 Great tutorial Craig. How do you test the FET's? 1 Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 24, 2014 Author Share Posted November 24, 2014 At a couple of bucks a piece, I presume you'll be replacing all 5 FET's, but it begs the question; are the FET's defective, or is lack of cooling the problem? Your picture is the second one I've seen where that thermal paste is dried up and looks really ineffective. The thermal grease isn't dried up, but there seems to be too much of it. Also, the steel plates that clamp the FETs down from the back store heat rather than dissipate it, which is bad engineering, IMO. I will try to remedy that. Great tutorial Craig. How do you test the FET's? Here is my plan, but it may change. In circuit, I will use a simple ohmeter and see if they offer 1M or more resistance with the gate CAP discharged. In circuit, I will diode test them in open and closed position. I have already tested them in-circuit, but didn't write the data down. Most were above 1MOhm as I remember. I think the two big ones read something like 20Ohms and 675 Ohms. If they don't offer enough resistance, then they are shorted out I understand. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 26, 2014 Share Posted November 26, 2014 Bump for info. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 26, 2014 Author Share Posted November 26, 2014 (edited) I made my order. The total for this repair including an A/C fan to cool is $36.95. I have no idea if it will work! Keep your fingers crossed. I am not going to use thermal compound between the FET's and other electronics and the heat sink. Instead I am using a product called Bergquist Sil Pad K-10. It has a very high thermal conductivity as well as electrical insulation. The boot was just keeping the heat in, IMO. I have update post #2 above with new images and information. The silicon grease is to apply between the heat sink and the plate amp face. Edited November 26, 2014 by mustang guy Quote Link to comment Share on other sites More sharing options...
babadono Posted November 26, 2014 Share Posted November 26, 2014 I'd say double plus 1 on everything so far Craig. Excellent job. +1 on the Sil Pad idea. You need to get the heat away from the devices not keep it in the little sleeping bag arrangement they had going on. I think the designers were not too worried about the steel plate on the front of the devices as it was only holding the metal tabs on the back of the FETS in contact with the main heatsink. But it can't hurt to make it out of aluminum. Maybe there should be some fins on the main heatsink? My concern is after your efforts, it is just a insufficient design to drive the load. Do guys on the other forums say they fix'em and they stay fixed? Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 27, 2014 Share Posted November 27, 2014 You always hear the long time members complain that a lot of the good discussions and knowledgeable posters have gone. Guys like Craig are more than filling the void. I enjoy your threads sir. Carl Quote Link to comment Share on other sites More sharing options...
Quiet_Hollow Posted November 27, 2014 Share Posted November 27, 2014 Great tutorial Craig. Excellent job. +2 Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 28, 2014 Author Share Posted November 28, 2014 Thanks guys. I appreciate your kind words and emoticons. I added some stuff. That pc of steel that was holding the MOSFET's in is a gonner. I re-purposed some old computer junk. Check out the pictures in posts 2 and 3 above... Quote Link to comment Share on other sites More sharing options...
wvu80 Posted November 28, 2014 Share Posted November 28, 2014 Can you mount the fan above your new heat sink, like the CPU fans? Of will this fan cool the entire board? Are you doing your project at the shop? I'd like to drop by and check it out if I'm out by the shop in the next few days. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted November 28, 2014 Share Posted November 28, 2014 Great tutorial Craig. Excellent job. +2 +3 You've inspired myself to re-cap my HK 330B receiver, now that I have a few bucks. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 30, 2014 Author Share Posted November 30, 2014 (edited) Can you mount the fan above your new heat sink, like the CPU fans? Of will this fan cool the entire board? Are you doing your project at the shop? I'd like to drop by and check it out if I'm out by the shop in the next few days. I got the parts today. The fan is perfect for it. Not too powerful, whisper quiet, ball bearing, unidirectional, and best of all 120VAC. The fan will be mounted to the enclosure separate from the plate amp, but blowing towards it. I already know how I'm gonna do it. It'll be slick. Great tutorial Craig. Excellent job. +2 +3 You've inspired myself to re-cap my HK 330B receiver, now that I have a few bucks. It's really quite a bit of fun learning about that stuff. Be careful, there are lots of ways it can bite you. I never shocked myself and stopped my heartbeat, but I have jumped a few times! When you put the caps in BE SURE TO GET THE POLARITY CORRECT! They do tend to explode... edit: I assumed you were a beginner. Something tells me you are not... My memory aint all that good... Edited November 30, 2014 by mustang guy Quote Link to comment Share on other sites More sharing options...
mike stehr Posted November 30, 2014 Share Posted November 30, 2014 It's really quite a bit of fun learning about that stuff. Be careful, there are lots of ways it can bite you. I never shocked myself and stopped my heartbeat, but I have jumped a few times! When you put the caps in BE SURE TO GET THE POLARITY CORRECT! They do tend to explode... edit: I assumed you were a beginner. Something tells me you are not... My memory aint all that good... It doesn't bother me to be told to take heed to polarity orientation, you can never be TOO careful. Fortunately, the boards that are silkscreened have the schematic part #, value/voltage and polarity marked on them. I already ordered parts from Mouser, mostly Nichicon audio grade capacitors...I had some 1uf 50volt caps stashed away to get things started. It seems to be a fairly simple project so far. The only custom fabrication I'll have to do is mount a snap-in cap in place of the main reservoir cap, a old big Elna 2200uF 63volt unit. I can mount the snap-in on a piece of perf-board, and then mount that to the chassis. Along with a re-configuration of the ground leads with respect to the negative terminal of the Elna 2200uF cap. They have like way too many wires soldered to that poor little ground terminal of the Elna. I would imagine this is the way it was wired at the factory. It's a Taiwan made model, series B1. Quote Link to comment Share on other sites More sharing options...
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