Fixed My Blown Nakamichi Amplifier!

[Kurbs]

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 fial 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.