Amp works perfectly, but has 4 blown transistors??? Wtf?!?

[Suicide Bobb 5,000+ posts]

I have a Hifonics Thor Viii (awesome, I know), and I have been running it for a while to power some speakers. It is a great little amp, and very powerful. The only issue I have had with it is the RCAs sometimes need to be wedged up in order to work, but that is probably because they are not held in by the screw that is supposed to be there //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

After I took it out of my car, I decided to peek at its guts. Lo and behold, 4 blown transistors! Two of them seem to have had the outer two leads clipped off by wire clippers, and the other seem to be blown because of the ugly residue under the leads (and the middle lead of each is clipped as well). They are all in a row. I cannot see the top part of the transistors as they are hidden, but in front of each one is a code. Starting with the seemingly blown transistors, closest to the speaker terminals, they read "Q306; Q312; Q209; Q208. For further clarification, the 306 and 312 seem to be blown, while the 209 and 208 seem to have been clipped by a human.

How the **** is this thing still working perfectly????? I have asked a few professionals, and it stumps them as well. And yes, I have hooked it up since, and it works the same as before. This amp is confusing the hell out of me.

-Dylan

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[Suicide Bobb 5,000+ posts]

Here are some pics

 

[Louisiana_CRX 5,000+ posts]

its probably only half ass working if you get it fixed properly you'll be amazed......

 

[Suicide Bobb 5,000+ posts]

Yeah, I guess...Usually when a mosfet is blown, in my experience, it either won't turn on, or it will only turn on for a second.

 

[Suicide Bobb 5,000+ posts]

How much do you think I could get for the amp if I fixed it?

 

[Spooney 10+ year member]

clipping the leads removes the damaged transistors from the circuit. techs do this all the time when trouble shooting to see if there are any other issues(distorted output,etc) .If the amp is put back to normal it will be awesome.

 

[Suicide Bobb 5,000+ posts]

Ok, great. Now it all makes sense! Any idea where I can pick up those transistors?

 

[Spooney 10+ year member]

I can't tell what they are in the pics but you should replace all of them though in the channel or channels that are blown. It will increase reliability. If you only replace the blown ones the amp will most likely fail again as the ones that are still in the amp and working have seen the same abuse that destroyed the others. They are probably just barely holding on right now. You should make sure to check the emitter resistors as well. They are the larger resistors that would be connected to the emitter leg of the output transistors. They are usually really low ohm ratings like .1 or .22 ohms . They help insure each output transistor is seeing an equal load. If they are out of tolerance one transistor may be working harder than the others and it will heat up and eventually fail.

 

[Spooney 10+ year member]

I should add that just because those transistors have that yellowish residue on their leads it doesn't mean they are blown. Actually they are probably still working as the leads haven't been clipped on them and the amp still works. They have been out through hell though. The yellow residue is a sure sign of heat and abuse. If you look at the solder on the legs you can see it looks kind of bubbly like it got so hot it started to unsolder itself. Somebody tried to mke that amp do something it wasn't designed to do. They need to be replaced along with any other output transistors in that channel(this is assuming they are outputs and not ps fets or rectifiers) . I can't tell as I can't see the part number but they most likely are not mosfets if this is an old class ab amp. They are probably bjt's(Bipolar Junction Transistors).

 

[dookie-face 10+ year member]

That amp is Zed made. Send it back to Zed Audio, they can repair & upgrade it for you.

"Zed Audio services and upgrades all amplifiers which we have manufactured."

Zed Audio Corporation

zedaudio@aol.com