"Protection" circuits.

[Mr English]

I poke about...literally...to see what's going on. What I can't understand is this:

I am looking deeply into a Pioneer A-9. This has fuses, a protection circuit and all the rest.

So why does one of the output transistors have a little hole and a burn mark, whilst all the fuses are intact?


Granted, the red protection light is on all the time, but I can't understand why no fuse has blown. Does a protection circuit protect the fuses? And, if I may be so bold, what protects the protection circuit?

Just realised, having written this, that I haven't checked the actual fuse ratings. I was therefore thinking of taking this thread out, but on second thoughts I'll leave it in.....just to remind people that the basics can easily be overlooked!

In fact, most of the important things can be easily overlooked......when was the last time (if you have a partner) you bought him or her flowers or chocolates (or both!), for example?

If you don't have a partner, go buy yourself some flowers and chocolates!

If you can't afford flowers and chocolates for yourself......don't worry - you are not alone!

 

[M Jarve]

In the A-9, the relay and protection circuit is for the benefit of the speakers. If the amp starts passing unacceptable levels of DC voltage it will cut off the speakers, hopefully before they're damaged.

The fuses protect from over current. A blown transistor should not be passing any current, so, it kind of just looks pretty.

 

[Mr English]

Pretty in a punk kind of way!

Thanks for that....my current mission is to find out what made the transistor blow in the first place. Maybe the burnt-looking resistor, which should have had a higher power rating.
No rush....the learning curve should be a pleasant place along which to glide (up or down?).

What keeps me going is that I know I will find out. Not by myself, unfortunately!

 

[markn2wae]

Here is a possible scenario.

I assume the fuse is BEFORE the main filter capacitor(s).

There is an energy "reserve" stored in the power supply capacitors, then you have the OP transistors, protection circuits then the speakers.

What may have happened is that one OP transistor shorted out, sucked a large surge of current from the capacitors, blew open and stopped the current surge, and left the protection circuit "on" to protect the speakers.

Because the capacitors can supply a large current for a small period of time the surge did not build up enough (at the fuse) to blow the fuse (It's a known fact that for the most part, transistors can fail before a fuse will)

BTW, fuses are there to protect only ONE component, the manufacturers BACKSIDE from fire lawsuits, if they go without anything else failing, you really lucked out!

Mark T.

 

[M Jarve]

BTW, fuses are there to protect only ONE component, the manufacturers BACKSIDE from fire lawsuits, if they go without anything else failing, you really lucked out!

Mark T.

And other forms of end-user foolishness. I learned my lesson about moving around rack shelves, with the amp still on it, while it's still playing (a tube amp no less), when I suffered only a blown fuse (and my heart stopping for a full 10 seconds) for my utter stupidity. If that fuse had not blown, things could have been... ugly.