Audiobahn A16000V amp in Protection

[adelphia83]

Purchased a used amp off Ebay which has been working good for about a week. Like all Audiobahn amps, these get hot as a firecracker when playing for a long time at their lowest rated impedance. I expected it to go into thermal shutdown as my other Audiobahn amp does regularly.

This one didn't-- It went into protection and even with ample time for cooling remained in protection. Then while messing with it, I let some of the smoke out-- With the amp in protection, I removed the RCA cables which sent smoke out the sides of the amp. Please help me fix this thing.

Being quite handy with electronics, I ripped the amp apart to see what happened.

A) The power and output transistors look good. No scorching or burning of any kind and secured solidly to the aluminum frame for heat dissipation. Thermal interface material used throughout.

B) Two wire wound resistors are fried. By the looks of things, there is one resistor per output transistor. This amp uses six 8A PNP and NPN output transistors.

I'll try to explain the layout of the PCB. The power MOSFETs are on the left, 6 on top, 6 on bottom. These are labeled FQP 50N06. There's two transistors on each side of the center of the board (FR1003C), but I'm not sure their purpose.

The output transistors are on the right side of the PCB, 6 NPN type on top, and 6 PNP type on bottom. The numbers are K D718 and K B688 respectively. There is one wire wound resistor near each output transistor. Two of the wire wound resistors are fried for some reason. If X is an output transistor, and R is a working wirewound resistor, the blank spot is the fried one as located on the circuit board:

XXXXXX
RRRR R
RRRR R
XXXXXX

What would cause the fried resistors? The amp was working fine until it went into overheat protection. The rest of the circuit board and components look flawless. Last time I tried messing with the amp, it was blowing 80A worth of fuses on the outside of the amp when connected to power.

What would replacement cost be for any failed components?

Anyone with electronics knowledge chime in. If there's anything further I can test, let me know. Comments / ideas / suggestions would be greatly appreciated.

[adelphia83]

I used a MM to test resistance at each leg of the output transistors.

I don't remember which legs had which readings but 10 of the 12 transistors had significant resistance going between PIN 1 and 3, and between the middle leg and one of the outside. It had 0.8 ohms going from the other outside leg to middle leg.

The two transistors nearest the two burnt wire round resistors had 0.8 ohms measurable at all three legs no matter which ones I tried. I'm assuming these two transistors are toast as they appear shorted.

Another thing I must mention is that I had the amp powered up with the MOSFETs in free air, not attached to the heatsink for about 20 seconds doing testing. I didn't smell anything burning, but I figured I would mention this if it involved replacing any other components. I noticed that when the amp was first connected it will draw a LOT of current for about three seconds, then go back to normal.

Hope this helps narrow down the problem.

[Perry Babin]

If you still read 0.8 ohms between the legs of any individual transistor, you have at least one shorted transistor in each group of output transistors. Sometimes you can find the shorted one by very carefully measuring the resistance (where it read 0.8 ohms). The one with the lowest resistance (0.6 for one vs 0.8 for the others) is the shorted one.

If you can't find the shorted one like that, you can pull one leg of the emitter resistors. That will disconnect the outputs from the others in the group and you'll be able to find the shorted one (if it's shorted 2-3).

Pull the defective output transistors from the amp, resolder all of the emitter resistors that are not open and power up the amp. If it works properly, you'll need to order a complete set of output transistors and a complete set of emitter resistors. If the amp does not power up normally, we'll have to do a bit more troubleshooting.

To save other components, have the transistors tightly clamped to the heatsink before applying power. When you apply power, do so via a single 10 amp fuse.

[adelphia83]

After pulling a leg of each emitter resistor, I was unable to measure resistance on 10 of the 12 output transistors. The display read "1" at every leg, which I assume to mean out of range.

The two other output transistors (these two have the burned up emitter resistors) measured 0.6 ohms between all legs. I cut them out of the circuit as you suggested, and resoldered the emitter resistors to the other output transistors.

I replaced the board back onto the frame and tightened all transistors back to the heatsink. I used a 10A automotive battery charger as a tester with a 10A fuse installed.

I started by applying power without the remote lead in place. After several seconds I could smell smoke and cut the power. I sat and pondered whether it may have only been in my head and then reapplied power. This time I saw the smoke coming from the area of three power transistors, and then suddenly the battery charger went to full load (heard the loud hum and the output needle shoot to the top), then blew the 10A fuse a second after.

Now it will do the same thing (full load input power) and blow a new fuse every time I reapply power.

Thanks Perry for all your support on this forum, it's greatly appreciated!

[adelphia83]

A picture (or two) are worth a thousand words.





Notice how the resistors nearest the three power transistors on the left look a little toasty. A resistance reading reveals that all three are now shorted.


On the other side you can see which output transistors I've clipped out (none of the legs are touching even though it looks like they may be in one of the images). I've since resoldered all the emitter resistors for testing. You can also see the resistors that have burned.

[Perry Babin]

It appears that the power supply FETs have failed. They were OK when the amp was going into protect but now they will have to be replaced.

When they fail, it's common for the gate to short to the drain (which is connected to B+, via the transformer). When the gate driver transistor tries to pull the gate back to ground, the resistors overheat and sometimes fail. 3 have in this amp. You need to replace all 6 FETs on that side of the amp.

If both power supplies produce positive and negative voltage, you can pull the FETs on the side with the burned resistors. If nothing is wrong with the other supply, it should produce rail voltage and allow further testing.

If there is a single pair of driver transistors for both power supplies, one of the driver transistors may have been damaged by the blown FETs. If the amp draws excessive current after removing the defective FETs, follow the traces back from the burned gate resistors until you find the driver transistor (a 2sa1266?), pull it and check it.

[adelphia83]

I went and ordered replacements for all power FETs, emitter resistors, and output transistors.

I could not easily identify which were the driver transistors, in fact I don't see an identical pair anywhere on the board other than the TIP42C's on the audio side of the board. I haven't pulled the board up to check the traces from the resistor to driver transistor, but I'll let you know what I find.

Two things I ran across when ordering: Could not find an exact match for K D718 K B688. I wound up ordering their 2SD718 and 2SDB688 equivalents. Also the emitter resistors are not clearly labeled as to their value, other than being 4 watt. The good ones and bad ones measure 0.5ohm. I ordered a wirewound 4 watt 0.5ohm resistor from Newark, though I can't be sure it's the exact same part. Will any of this present a problem?

As I checked the resistors on the power supply side, the bad ones tested 25 ohm, and all the good ones BUT one, tested 100 ohm. There was one on the other side of the burned ones that tested 25 ohm though it didn't appear to be burnt. Should I replace that as well?

This forum is a great resource. What normally causes these bad output transistors? By the looks of things, is this a poorly designed amp? Also why does this board employ two power supplies, or is this a common thing? Once again thanks for the help.

[Perry Babin]

The emitter resistors are almost certainly 0.1 ohm. It appears that they have R1J printed on them. If that's correct, they are 0.1 ohm 5% tolerance.

When measuring low resistance values with a standard multimeter, you need to touch the meter leads together to determine what '0' is for that meter. Then subtract that from the resistance you read across the resistors.

The resistors may be reading low because the FETs are defective. You would have to remove the FETs or one leg of the resistors to get an accurate reading. Even if the blackened ones are within tolerance, I'd replace them.

Outputs generally fail when the amp is driven too hard or driven into an impedance lower than that recommended by the manufacturer. The protection circuits should protect the amp but most aren't sensitive enough to do it 100% of the time.

The build quality of this amp is about average for a budget amplifier. The biggest problems (IMO) with an amp like this are the poor quality, single sided circuit board and the thin heatsinks (not sure on this amp). The single sided boards develop broken solder connections. The aluminum under the outputs is too thin to dissipate heat quickly.

[adelphia83]

Great info. One more thing before I get to work on this pulling things apart and replacing components.

What is the function of a wirewound resistor?

To order a replacement emitter resistor, what properties am I looking for? I know that I need a 4 watt 0.1 ohm-- Other than that are there any other fundamental attributes I need to be looking for? Does "wirewound" make a difference?

The original part has proven terribly difficult to find. Many thanks again.

[Perry Babin]

The following would probably be OK.
71-CW2B-0.1 Mouser

They are rated to 3.75w. You can also use 5w resistors. Check the dimensions of the resistors you order to confirm that they'll fit the board.

In amplifiers, the emitter resistors help ensure equal current sharing for the parallel components.

Wirewound can typically handle surges (above their power rating) better than metal film or metal oxide film resistors. In most cases, the type of resistor isn't important as long as the power rating is equal/greater and the value is the same as the resistor they're replacing.