Fixed my Dead Nakamichi Amp
05-19-13, 08:55 AM
#1
Driver School Candidate
Thread Starter
Fixed my Dead Nakamichi Amp
AH, I purchased a SC400 1995 that had a blown amp inside. Each time I turned
it on it would blow the 20 amp fuse under the hood.
Grabbed one off EBAY and it was blown, got my money back but the guy said
just keep the amp since he didn't want to pay return shipping.
Looking around, it seems the average cost to repair these ran around 300
bucks, or buy a used one for 200-400 dollars off EBAY and take a chance on it
being bad or going bad before long. Well, I don't mind paying a decent price for a decent piece of equipment, but Lexus wanted 1700 bucks for one and some of these guys on EBAY think they are made of gold. You can see where some even list that the amp was NOT tested and carries NO warranty...for 400 bucks? I don't think so.
I found 1 service that wanted 300-400 for repair. I assume they test and check, and replace every component that is bad, but I could not say for sure. For all I know they may replace a few bad parts and ship it back.
So, I opened it up and took a good look. No blown or burned components I
could see at all. This is a 336ULOA, looked new inside. But high humidity and water can fail components, as can heat.
Checked the side connector, it has 4 wires. Power, ground, and 2 wires to
the subwoofer. The Blue/Yellow pin and the Ground pin were shorted together. No
wonder the fuse popped at power on.
Inside the amp you can trace the circuits, and not having a schematic I
decided to go at it blindly. This amp seems to be a 2 part amp built into 1.
Subwoofer section and main amp section. The subwoofer connector is the one on
the side and you can see where the Blue/Yellow power wire goes in, it also
connects power to the other main section connector straight from the fuse under
the hood. So you do not lose just the Sub output, with that shorted you lose function to the entire amp.
Would have been nice to have some separation, but it is what it is.
On the power output section you will see 8 Mosfet transistors, part number 2SD1445A
just to the right of 2 large flat diodes. There are 2 banks, each bank has 4 FETS. Tiny resisotrs in line with them
that you need to be careful with when putting the new ones in, don't fry them with the soldering iron
when pulling or replacing the FETS.
You will also see on the other side of the amp a plug in board with 12 transistors
that control the main section for speakers that have coils in line with them..
These all checked fine. However do not confuse these with the 8 FETS I am
speaking of on the other side of the amp. Those I would check if my amp only worked on one side
or had issues other than being shorted internally like mine was.
I took a meter and checked the transistors and the good ones showed 27.8 at
the left side of the FET and 2.0 on the right when tested to the middle pin,
collector. I had an entire bank of these at dead short across both sides to the
collector, and measures to ground it was the same. Blown FETS.
So I clipped some off and tested them as well to make sure nothing in the
board internally was shorting them out. Nope, it was the FETS. Again, there were
no burn marks or holes in them, nothing visually that would point to them as
bad. 5 out of 8 were blown, but I replaced them all as any prior damage probably degraded the others.
I also tested the large input diode and caps on the board, as well as the
resistors. I also checked the IC legs on the switching regulator IC in the
middle of the board, though that would not carry enough current to blow a fuse,
it could have been damaged ( which you can still buy if needed ) and all were
fine.
That original FET is obsolete. You can source some from China off EBAY for 2
bucks a pop, or find a solid replacement which is what I did.
So I cross referenced the FETS and ordered 8 - NTE377 FETS from Radio Shack
store on EBAY. 23 bucks total shipped.
The difference is...the original FETS were plastic backed, the new ones are
metal. However, they come with the insulating mica sheets and tiny bushing to
put in the screw hole so it won't short out.
After removing these, I checked the resistance again on the throughput holes
on the board and all was still fine.
I soldered them in, placed the mica sheets on, slapped some thermal grease in
between and resoldered them. Took me about 3 hours total to desolder, clean,
recheck the circuit board and solder them back in. I tested the 2 pins on the
connector and instead of showing a dead short between them, had a reading of
27.8 across them. Checked the amp on a friends car and his showed the same.
Plugged it in, turned it on and half heartedly expected a fuse to blow. Nope!
Radio came on and I had SOUND again! Connected the CD changer, it works like new
again for 23 bucks, some soldering supplies and a few
hours time.
I cannot say what originally caused the issue but when I got the car from an
old woman it had been sitting. There was some water in the trunk area by the
amp. Best probability is water hit it, shorted the FETS and killed it, also
causing a dead short to the input power and ground pins on the side connector.
Luckily no other components had been killed.
Now, I have a friend who has replaced his amp for the same reason. We took
his old amp, tore it apart, same thing...just a slight difference on the
resistance readings, but a dead short to the same 2 pins and 3 bad FETS. Replaced
those and Wa-La...it worked like new again. His also had been hit by water and
zapped. No other components in either of these needed replacing. .
So I now have a fully functional Nakamichi Amp in my car for 23 bucks in
parts and some time. I would suggest testing the other components and looking
for burn marks or obviously toasted caps and resistors, diodes, etc...but in
both these cases it was simply the FETS. The NTE377 was the closest match I
could find readily available, though you can find some similarly matched FETS as
well, most were around 3-4 bucks a pop or more and these ran me less than 3.
I hope this helps for those with the Nakamichi premium system folks who have
a blown amp and do not want to pay 300 and up for a used or repaired amp..
it on it would blow the 20 amp fuse under the hood.
Grabbed one off EBAY and it was blown, got my money back but the guy said
just keep the amp since he didn't want to pay return shipping.
Looking around, it seems the average cost to repair these ran around 300
bucks, or buy a used one for 200-400 dollars off EBAY and take a chance on it
being bad or going bad before long. Well, I don't mind paying a decent price for a decent piece of equipment, but Lexus wanted 1700 bucks for one and some of these guys on EBAY think they are made of gold. You can see where some even list that the amp was NOT tested and carries NO warranty...for 400 bucks? I don't think so.
I found 1 service that wanted 300-400 for repair. I assume they test and check, and replace every component that is bad, but I could not say for sure. For all I know they may replace a few bad parts and ship it back.
So, I opened it up and took a good look. No blown or burned components I
could see at all. This is a 336ULOA, looked new inside. But high humidity and water can fail components, as can heat.
Checked the side connector, it has 4 wires. Power, ground, and 2 wires to
the subwoofer. The Blue/Yellow pin and the Ground pin were shorted together. No
wonder the fuse popped at power on.
Inside the amp you can trace the circuits, and not having a schematic I
decided to go at it blindly. This amp seems to be a 2 part amp built into 1.
Subwoofer section and main amp section. The subwoofer connector is the one on
the side and you can see where the Blue/Yellow power wire goes in, it also
connects power to the other main section connector straight from the fuse under
the hood. So you do not lose just the Sub output, with that shorted you lose function to the entire amp.
Would have been nice to have some separation, but it is what it is.
On the power output section you will see 8 Mosfet transistors, part number 2SD1445A
just to the right of 2 large flat diodes. There are 2 banks, each bank has 4 FETS. Tiny resisotrs in line with them
that you need to be careful with when putting the new ones in, don't fry them with the soldering iron
when pulling or replacing the FETS.
You will also see on the other side of the amp a plug in board with 12 transistors
that control the main section for speakers that have coils in line with them..
These all checked fine. However do not confuse these with the 8 FETS I am
speaking of on the other side of the amp. Those I would check if my amp only worked on one side
or had issues other than being shorted internally like mine was.
I took a meter and checked the transistors and the good ones showed 27.8 at
the left side of the FET and 2.0 on the right when tested to the middle pin,
collector. I had an entire bank of these at dead short across both sides to the
collector, and measures to ground it was the same. Blown FETS.
So I clipped some off and tested them as well to make sure nothing in the
board internally was shorting them out. Nope, it was the FETS. Again, there were
no burn marks or holes in them, nothing visually that would point to them as
bad. 5 out of 8 were blown, but I replaced them all as any prior damage probably degraded the others.
I also tested the large input diode and caps on the board, as well as the
resistors. I also checked the IC legs on the switching regulator IC in the
middle of the board, though that would not carry enough current to blow a fuse,
it could have been damaged ( which you can still buy if needed ) and all were
fine.
That original FET is obsolete. You can source some from China off EBAY for 2
bucks a pop, or find a solid replacement which is what I did.
So I cross referenced the FETS and ordered 8 - NTE377 FETS from Radio Shack
store on EBAY. 23 bucks total shipped.
The difference is...the original FETS were plastic backed, the new ones are
metal. However, they come with the insulating mica sheets and tiny bushing to
put in the screw hole so it won't short out.
After removing these, I checked the resistance again on the throughput holes
on the board and all was still fine.
I soldered them in, placed the mica sheets on, slapped some thermal grease in
between and resoldered them. Took me about 3 hours total to desolder, clean,
recheck the circuit board and solder them back in. I tested the 2 pins on the
connector and instead of showing a dead short between them, had a reading of
27.8 across them. Checked the amp on a friends car and his showed the same.
Plugged it in, turned it on and half heartedly expected a fuse to blow. Nope!
Radio came on and I had SOUND again! Connected the CD changer, it works like new
again for 23 bucks, some soldering supplies and a few
hours time.
I cannot say what originally caused the issue but when I got the car from an
old woman it had been sitting. There was some water in the trunk area by the
amp. Best probability is water hit it, shorted the FETS and killed it, also
causing a dead short to the input power and ground pins on the side connector.
Luckily no other components had been killed.
Now, I have a friend who has replaced his amp for the same reason. We took
his old amp, tore it apart, same thing...just a slight difference on the
resistance readings, but a dead short to the same 2 pins and 3 bad FETS. Replaced
those and Wa-La...it worked like new again. His also had been hit by water and
zapped. No other components in either of these needed replacing. .
So I now have a fully functional Nakamichi Amp in my car for 23 bucks in
parts and some time. I would suggest testing the other components and looking
for burn marks or obviously toasted caps and resistors, diodes, etc...but in
both these cases it was simply the FETS. The NTE377 was the closest match I
could find readily available, though you can find some similarly matched FETS as
well, most were around 3-4 bucks a pop or more and these ran me less than 3.
I hope this helps for those with the Nakamichi premium system folks who have
a blown amp and do not want to pay 300 and up for a used or repaired amp..
Last edited by kurbs62; 05-19-13 at 09:09 AM.
The following 3 users liked this post by kurbs62:
05-24-13, 02:59 PM
#2
Wished you lived in FLA, i would bring mine by for you to check it out. It is not completely blown, but at times, the base will go out, and all I have is low mids, other times it chips in and works fine for a while. I know it is not the sub, as I connect known working one and had the same issue. It is not the radio, as I changed it out, only thing I have not changed is the AMP.
05-25-13, 03:24 AM
#3
Driver School Candidate
Thread Starter
Well, if you open the amp up you will see it set up as 2 amps inside. On the side connector with 4 wires, that is the circuit that controls the sub. If you were to look at the transistor bank you could check for loose solder or a burned cap or resistor. It could also be as simple as a bad wire from the amp to the subwoofer...there are a reddish and black wires in that side plug. The blue/yellow and brown are power and ground. You may want to take a meter to them from the amp to the sub and see if they are bad. If not wiggle those and see if the wiring is goofy, or wiggle them by the subwoofer and see. If so its a short distance from the amp to the subwoofer if they need to be replaced. But I would try that first. Then if not there, open the amp and check for bad solder joints or obvious burned components.
05-25-13, 12:27 PM
#5
So curbs, get to the point return, pre-paid shipping aside. how much are you gonna charge to do this service. In the process of buying an SC and the seller already told me the radio doesn't work. got a head unit on the cheap JIC but this isn't my first SC. The the amps of the two before couldn't swim either. Lemme know something fair. Would love to get this doene right. Will PM the request as well. Thanks for the info.
05-27-13, 04:59 AM
#6
Driver School Candidate
Thread Starter
Well, I have a very heavy schedule between work and home and grand kids. If you do not feel up to it someone adept at electronics can replace these fairly easy. I was lucky, because no other components were blown. There are also 3 IC chips ( and I have some if anyone needs them ) and the resistors and diodes and smaller transistors can be purchased if need be. But if more than the fets are blown and you see burned traces, etc...that is when I might buy a good used one depending on the damage.
Careful on EBAY though, mine and other peoples have bought "tested and good" amps that were DOA. In both of the amps I repaired, it was only the fets that shorted due to water. But for 23 bucks and some time, its a cheap weekend project. I will try to get around and post some pics tho to make it easier.
Careful on EBAY though, mine and other peoples have bought "tested and good" amps that were DOA. In both of the amps I repaired, it was only the fets that shorted due to water. But for 23 bucks and some time, its a cheap weekend project. I will try to get around and post some pics tho to make it easier.
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06-03-13, 07:09 PM
#8
Took my amp apart everything looked brand new, tried testing the Fets, after watching a few youtube videos and reading a few websites, but I still don't think I am doing it right, as I am not getting the readings you got Kurbs. So my procedure was, use a shielded wire and touch to the outer left and right contacts, then use my multimeter (touch the negative to the middle contact, positive to the right; with the negative still touching the middle contact, move the positive probe to the left side and back to tot he right. My numbers seems to be like high 400. this is with meter set to continuity. Am I doing some thing wrong?
06-15-13, 06:49 AM
#9
Driver School Candidate
Thread Starter
Your readings may very well be different than mine. Do you see any difference in any of the fets, such as all readings the same but maybe 2 or 3 out of whack? Are you setting the meter on Ohms or diode setting? Bad Mosfets are often short-circuited drain to gate. May want to clip one close to the board if it looks suspect and test it out of circuit.
I wish I had a schematic but no luck finding one. Otherwise I could have detailed troubleshooting much more in depth using an o'scope.
I wish I had a schematic but no luck finding one. Otherwise I could have detailed troubleshooting much more in depth using an o'scope.
07-03-13, 07:18 AM
#10
Driver School Candidate
Thread Starter
Took my amp apart everything looked brand new, tried testing the Fets, after watching a few youtube videos and reading a few websites, but I still don't think I am doing it right, as I am not getting the readings you got Kurbs. So my procedure was, use a shielded wire and touch to the outer left and right contacts, then use my multimeter (touch the negative to the middle contact, positive to the right; with the negative still touching the middle contact, move the positive probe to the left side and back to tot he right. My numbers seems to be like high 400. this is with meter set to continuity. Am I doing some thing wrong?
07-30-13, 02:25 AM
#11
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Kurbs you are a life saver fixed my amp up great...
Removing old ones...
New ones in place ready to solder...
BUT I have another problem I was out of 20a fuses for obvious reasons so I put in a 30a
seemed fine till I turned it up and it fried 4 resistors (and 2 fets on the left side) and since 2 are printed and not marked with rings & they were pretty much fried to hell I'm not sure of the ohms they appear to be a 1/2 watt (maybe 1/4? got 42ohm reading) and a 1/8 watt (got 990 & 997 ohm reading) resister in line with each other they are located behind the same heatsink the fets attach to on each side of the big cap not next to it but passed the 4 resistors in a row the larger ones were really fried I tried 1/8 watt 1kohm and 1/2 watt 47ohm resistors (and replaced the left bank of fets again) the 1/8 watts seemed fine but the 1/2 watts started smoking pretty good the amp worked for a minute on a 15a fuse does anyone know what the proper resistors are?
Removing old ones...
New ones in place ready to solder...
BUT I have another problem I was out of 20a fuses for obvious reasons so I put in a 30a
seemed fine till I turned it up and it fried 4 resistors (and 2 fets on the left side) and since 2 are printed and not marked with rings & they were pretty much fried to hell I'm not sure of the ohms they appear to be a 1/2 watt (maybe 1/4? got 42ohm reading) and a 1/8 watt (got 990 & 997 ohm reading) resister in line with each other they are located behind the same heatsink the fets attach to on each side of the big cap not next to it but passed the 4 resistors in a row the larger ones were really fried I tried 1/8 watt 1kohm and 1/2 watt 47ohm resistors (and replaced the left bank of fets again) the 1/8 watts seemed fine but the 1/2 watts started smoking pretty good the amp worked for a minute on a 15a fuse does anyone know what the proper resistors are?
07-30-13, 02:40 PM
#12
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Here is a little visual aid I may be completely wrong with the wattage's 
EDIT: Ok I ordered one from ebay I'll figure it out
EDIT: Ok I ordered one from ebay I'll figure it out
Last edited by bobbygamer; 08-03-13 at 07:50 PM.
08-05-13, 03:16 AM
#13
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Something caused those resistors to fry...did you recheck the solder joints on the new FETS? Or the resistors may have been bad from the 1st time it blew....I would be interested in the outcome.
08-05-13, 09:23 AM
#14
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Well I had it up pretty damn loud it was working for a day all the solder joints were great, the old original fets were all toast but one I think if I had a 20a fuse in there it would have just popped that I have a stockpile of 20a's now. New amp off ebay should be here sometime this week there were a few on there for $100 
had to jump on it
Edit:
Got new amp it does not work right either the strange thing about this one is it phases in and out continuously very fast but does make sound (Also the fets checked out fine) before I send it back I checked the resistors and they ARE MARKED 22ohm on the large ones and 100ohm on the small ones this amp is also slightly different then mine in several places these resistor markings confused me even further because after another check of my fried ones both small ones clearly are 1k not 100ohms the larger ones were the most fried and I got a reading of 42 ohms on one of them ANY input would be appreciated hopefully the next amp I get will work fine here's a pic of the resistors in the new amp:
had to jump on itEdit:
Got new amp it does not work right either the strange thing about this one is it phases in and out continuously very fast but does make sound (Also the fets checked out fine) before I send it back I checked the resistors and they ARE MARKED 22ohm on the large ones and 100ohm on the small ones this amp is also slightly different then mine in several places these resistor markings confused me even further because after another check of my fried ones both small ones clearly are 1k not 100ohms the larger ones were the most fried and I got a reading of 42 ohms on one of them ANY input would be appreciated hopefully the next amp I get will work fine here's a pic of the resistors in the new amp:
Last edited by bobbygamer; 08-12-13 at 05:53 AM. Reason: Update
09-04-13, 06:02 PM
#15
Fet testing
Look at this video and tell me if the test proceedure is correct, and by the test and response, are those fets bad. The proceedure I was using is CBE - BE, then BC, then CE. The letters mentioned are the prongs on the transistor.
http://www.youtube.com/watch?v=FuJhH...ature=youtu.be
http://www.youtube.com/watch?v=FuJhH...ature=youtu.be