What causes 'thermal' faults?

[StillKeen]

I don't have any amps with this issue, but I see some on eBay from time to time. Now, talking very generally about this, what are some of the causes? I can see that the problem must result from 1) the amp's method of dissipating the heat is compromised (so faulty fan, dried out thermal paste ... etc) or 2) the amp is producing more heat than it can dissipate ... so more heat that it's designed to produce ... which I would put into electrical component fault.

I know little about electronics, being a mechanical engineer, but I'd have thought a blown output transistor (which seems to sometimes be something that people are fixing on many brands of amps) would not produce extra heat ... infact you'd lose a channel or part of a channel and have less heat? (or does it load up the others to where they're less efficient and produce more heat?).

I've seen some websites and references to books that discuss amp repair, and I'm planning on looking into them further, but just curious as to some general rules about where to look with a broken amp. Perhaps the question of 'would you buy a broken amp to fix that was either 1) thermal shutdown, 2) missing one channel or 3) doesn't power up at all?' (and not counting amps where there is visible burnt board/components to be seen.

Thanks,

[Francious70]

The most common cause of dead amplifiers is a clipped signal. Second I would think is not giving it enough wattage to produce the power you're asking for.

[Eric D]

I am going to guess that 90% of output transistors fail in what results as a dead short. When this happens, they take all the current the power supply can give them and turn it directly into heat. In an amplifier which has current monitoring of its power supply, it can detect this and shut down. In plenty of amplifiers the power supply just keeps pumping current to the short and the amp heats up. The proper size fuse on the amp should protect from this, but many people put bigger fuses in there, and the amp just cooks more.

I have seen blown transistors become so hot they have actually melted into the aluminum heatsink of the amplifier. It takes some serious heat to do this.

When I buy a broken amp, I most often buy it based on the cosmetic appearance of the amplifier. Most people are not amplifier techs, so if they claim it "thermals", or "has distortion", that can be such a broad range of things. I just look at it as the amp either works, or does not.

Generally the cosmetic appearance says more about the abuse of the amplifier than its actual electrical use does.

[StillKeen]

Eric, Francious, thanks for the info. I'd not realized that the amp may still 'work' with blown transistors. From a mechanical engineers perspective, electrical things either work or they don't ...

When I started the thread, it was out of curiosity before possibly buying a broken amp to play around with. This was due to my recent ''success'' with fixing a 475 that was missing rail capacitors. Thanks to members of the forum, I got the right capacitors, soldered them in, did the power caps too, and successfully powered up the amp to run some speakers off a car battery. It sounded fine, the sub (rear channels were not doing any work though, as I didn't have a decent battery to test them off).

With the amp appearing to be all fixed, we installed it this week, and unfortunately found that the sub sounded very distorted, even at low volumes. We went through the following process:

1) checked sub for mechanical damage (as it was a cheap secondhand sub, essentially free in a box). Nothing obvious, and still measures 4ohms.
2) switched sub to the front channels, and found it ran fine.
3) put front speakers on the rear/sub output (with appropriate gains) and found there was a ton of distortion

[youtube]http://youtu.be/rWkslOabGbQe[/youtube]
http://www.youtube.com/watch?v=rWkslOabGbQ

This is all done at very low volumes, below normal listening volumes. I know in the video it sounds like there is a ton of bass, but this is honestly about 1/3 of the volume you'd have when driving along ... The sub is out on the ground behind the car, you're hearing the front 6'' making bass, and just the microphone is close.

Now I should point out that the gain for the rear was adjusted up a little from minimum, but still pretty low. My friends headed off home tonight, so we didn't get a chance to try with the gain turned down lower ... could this distortion just be too high gain? With just the good front channels working, the amp doesn't get hot, it barely get's above room temperature, it's not even warm to the touch.

Any immediate thoughts on the cause of the fault? My friend has a electronic engineer friend who has fixed many amps before, so the next step may be contacting him, but it's really something we'd like to fix on our own, and he's often busy.

Help please.

[Eric D]

Do both the right and left rear outputs have distortion, or just one side?

[StillKeen]

Hi

Both have it. We thought possibly the right was worse , but they may have been equally as bad.

My friend is going to try switching the RCA (the head unit only has one set) and putting it directly to the rear inputs, rather than it passing through the front inputs and going internally to the rear channels.

Thanks.

[quasig]

An update on this thread.... this is my amp, and last night we confirmed that only the right channel is affected. Currently the sub is running of the left channel only, and it seems to be OK (albeit lacking in power).

[Eric D]

I would guess one transistor in the output of that channel is damaged. Transistors either die by shorting out, or burning open. When shorted your amp won't work, when open the amp will work, but with heavy distortion on the bad channel (from only have of the signal being reproduced by the output stage).

There are other things which could be wrong, but I am pretty confident it is a bad transistor.

[StillKeen]

Eric, thanks, that's brilliant. I think it'll be opened up by the electronic engineer next ...

Thanks.

[rolandk]

Do all the red bias LED's light up? 2 per channel, 8 total. If any are out, the LED's themselves have been known to go bad. Measure across with a diode checker, they should light up.
Are the output transistors active? If so, check the bias and compare to the known good channels. If not, ohm out all the fused resistors. Replace any bad ones as well as the transistors they are connected to.
If all this is good, check the 100Ω resistors near the RCA inputs for damage. ZX Ti amps are also notorious for bad rotary switches and crossover frequency pots.