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Why Do Outputs Blow?

Sriskie

Sriskie

Well-Known Member
Generally speaking, what is it that causes output transistors to blow? Do they tend to die a natural death? Bad drivers? Emitters? Problems with the rectifier circuit? Assuming there wasn't shorted speaker wires or excessive moisture.

The amp bias and the +/-77 voltage was fine before they blew. It happened on power up. According to my Peak Atlas DCA-55, 4 of the 12 outputs test bad.
 
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Good question!

Speculation: If a component (like a bias trimmer) goes bad it can turn the OP TR on FULL, constantly. The circuit is designed to handle crescendo peaks, but not for extended periods.

Failures in both channels at the same time could be an extended surge from the P/S.

Most failures will occur at power on (surge) unless they are related to heat buildup. When is the last time that you saw a light bulb go out after it had been 'burning' for a while?
 
Heat - auxillary component failure - poor connection - design - usage (abuse) ... a lot of reasons. Once something goes, then outputs are probable victims.

Normally (at least in audio) they don't die a natural death. (although with multiple outputs, if they aren't matched, the one that gets pushed the hardest will usually go first). QC and improved production techniques make modern devices hardier than older ones.
 
Heat and overloading usually. Run a 4 ohm load on something only rated for 8 ohms, or dead-short the speaker terminals at high volumes will do it nicely. In the case of something with multiple output transistors, if one dies for some reason, each of the rest will be asked to carry more of the load. You can end up with cascade failures when that happens, the transistors just start blowing one by one as they get increasingly overworked.
 
Thanks Steve!

In this case the bias started high (40-50mV) on power up and slowly came down (+/-18mV). The bias trimmers adjusted linearly. No spikes noticed.

Most of the damage was on one channel. 1 exploded cap, a pair of outputs shorted all ways, and 1 output read something like "B-E resistor shunt". The other side just had one the Peak thought was a geranium.
 
The cascade effect seems probable to me. The amp itself appears to be a bit of a cream puff. It doesn't "look" abused for whatever that's worth. It was a recent acquisition so I don't know it's history but it doesn't appear to be toasted inside. I did note that some of the small heat sink transistors were heating up just idling. Perhaps that was a good indication of something going wrong with the outputs?
 
Transistors have a Safe Operating Area, defined by a graph in the data sheet, showing current vs. voltage across the transistor. The SOA is derated by higher temperature, so if the bias is high and the amp is running hot, it will take less abuse to blow the transistor. Sometimes an amp is unstable, due to poor design, component failure or changes someone made to the amp. The amp will oscillate at rf frequencies. You won't hear it but it will cause the outputs to overheat and detonate. This is why it's always a good idea to look at the output with a scope after replacing the transistors and getting the amp operational.
You can also have open or intermittent components in the bias circuit, dodgy resistors or bad transistors that sometimes stop working like a transistor and cause a high DC offset.
If the amp has been serviced before you can have counterfeit transistors that blow with a load on them.
 
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There's some more to think about. Is an amp more likely to oscillate on power up, or is it more likely something that builds up as it works?

I plan on digging into the amp further this weekend. No 'scope yet unfortunately so I'll have to depend on testing on a component level. My concern is I don't fully trust the methods I have to test transistors. I think I'll need to replace all of the Toshiba MT-200(?) outputs with On Semi TO-3Ps c/o another thread I found. The shot gun method is what I'd prefer to do with the rest but I think finding good substitutes for some might be difficult.
 
Oooh, I like that question!

When I was taking pictures I turned off the speakers and cranked it up to show the lights. I had a hum for a few minutes after that. Perhaps I damaged it right there and that's why they eventually blew?
 
I suspect mistakes while checking bias/offset, followed closely by drunken parties.
At least that's what I read in the papers...

Tom
 
Sriskie said:
Could doing so stress the amp if everything is not quite right and lead to failure?
Click to expand...

Running it full tilt with no load is not a good idea ever. You can hear the outputs "singing" when you do this. That indicates power is being dissipated in the devices. Further, referring again to the SOA of the transistors, as the voltage across the transistor goes up (as when you crank the output) the allowable current goes down. When the amp is not loaded, the output voltage can go higher than it would if the amp is loaded. It is possible the output transistors were damaged internally by a previous abuse and what you did pushed them over the edge.
 
dr*audio said:
Running it full tilt with no load is not a good idea ever. You can hear the outputs "singing" when you do this. That indicates power is being dissipated in the devices. Further, referring again to the SOA of the transistors, as the voltage across the transistor goes up (as when you crank the output) the allowable current goes down. When the amp is not loaded, the output voltage can go higher than it would if the amp is loaded. It is possible the output transistors were damaged internally by a previous abuse and what you did pushed them over the edge.
Click to expand...


Thanks for the detailed explanation. Either way I'll be sure not to make the same mistake again.
 
Sriskie said:
Why Do Outputs Blow?
Generally speaking, what is it that causes output transistors to blow? Do they tend to die a natural death? Bad drivers? Emitters? Problems with the rectifier circuit? Assuming there wasn't shorted speaker wires or excessive moisture.

The amp bias and the +/-77 voltage was fine before they blew. It happened on power up. According to my Peak Atlas DCA-55, 4 of the 12 outputs test bad.
Click to expand...

It doesn't necessarily need to be load, wrong adjustments or hard usage and abuse. Power transistors can eventually develop a fault just under normal wear and tear.
A big factor that can lead to this is simply the heat they generate. Also other components which are in circuit with them can develop a fault, which in turn can cause them to fail.
 
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