Amplifier repair

[pman92]

I'm not sure if anyone on here knows much about car audio but you may be able to help anyway.
The sub in my VS has recently stopped working, and started making popping noises approx every second or so. The protection light on the amp is on, flashes off breifly as it pops, then comes straight back on.
It's not an input problem as it does it with the RCA's disconnected, it's not a woofer short as it measures perfect at 4 ohms and the protection light still behaves the same with the woofer disconnected. Power and earth is good.
I've pulled the amp out and taken the cover off. There's a small black 2 wire component bolted to the heat sink that appears melted. Everything else visually looks fine. I'm not surprised as I've got this amp hot numerous times (it's an old class AB and isn't mounted in an overly vented place, and it's fully driven with the sub on it).

The amp is a jaycar response precision aa0424. The PCB is printed "RS1802 Main 0411 Rev.1A". The melted component is labeled TH, it's a black 2 connection component half encased in a metal case and bolted to the heat sync. There is a capacitor labebeled C4P in parrell with it.
I'm not sure if the metal case is also an electrical connection but the heat sync is grounded to the PCB in another location.
I'll attach a picture in the next post

[pman92]

Does anyone know what it is so I can hopefully buy another one and fix it?

[vlad01]

That looks like a temp sensor (thermistor) and I have seen them look like this as some are made like that. The sensor is just set in resin on a mounting tab.

Your fault will be else where.

[pman92]

I think your right. A Google search has made me aware of 'reference designators'/refdes's. Apparently TH is common for thermistor. It measures several kilo ohms so it's certainly not shorted.

Further searching has led me to learn the most common fault is transistor failure. Apparently the rows of smaller transistors bolted to the heat sync near the power cable/fuse end are power supply transistors. The larger ones near the RCA input end are output transistors.
There's 12 output transistors in total. 6x B718 and 6xB688. I'm assuming 3 of each for each of the 2 channels.
The 3 B688 on one side all measure 0 ohms between each connection. I'm assuming they are shorted but I'll unsolder them tomorrow and recheck. I'm guessing it's pretty unlikely they are meant to be like that.
What's the likelihood of 3 transistors all failing together? They are positioned next to each other and are at the end of the heat sink.

I'm learning a fair bit doing this. Hopefully can save a few hundred $$$ aswell.

[vlad01]

well depends on the transistor arrangement, if they are series, one goes it will take the rest out in the chain. Pretty sure they would run parallel. Back in the days when I fixed amps as a job the older bipolar transistor amps had a bias voltage on the base of the transistors, normally 0.6V and that basically kept the transistors in a just off state but barely poofteenth switched on. If that bias was set too high the output transistors would conduct current to the supply rails and heat the the transistors, which in turn make them conduct more and a runaway situation makes complete failure inevitable. This causes all transistors to suddenly fail and short.

The fact you found them to read as a short is certainly the problem.

[The1]

Itl be one of the drivers on the channel, they are under the heatsink next to that rail temp sensor, fixed quite a few of these jaycar amps with same problem, usual issue is sub or speakers that can't handle the output, people running them past what they handle and with the speaker constantly clipping it overheats them, the temp sensor cutout is to hot in these.

[The1]

Their parallel in the ones I've done, only a few fail. Also I scrape the crappy thermal pad off, use arctic silver and screw them with shorter scews directly to the case, they are full plastic case the new ones I use, also apply arctic silver and screw the temp sensor down, im far from an electronics expert though lol