Problem with an amp, need help

[melkaone]

Hi all
I have a Carlsbro Cobra 90BG amp and since a few days, something strange is happening.
This head broke a while ago, so, i changed the bridge rectifier (with a little bit greater amperage), fuses and a friend of mine working in a music store said i should change the power transistor with brand new MJ15024.
Since that, i can't play more than 10 minutes before the sounds decreases and then stops. If i switch it off for 2 minutes and switch it on again, it's ok for 10 more minutes.
I'm thinking of a heating problem of the new transistor, so i plan to put some heatsinks on it, with computer adhesive thermal compound.
But if you have any idea, tell me, cause i'm sure that i don't have the solution
thanks

melka

[niftydog]

change the power transistor with brand new MJ15024.

is that the original part number?

Was it heatsinked to begin with?

You say it "broke" a while ago, did you find the cause or did you just replace those parts and hope for the best?

You could run it with the cover off (carefully!) and try to work out where the heat is coming from. (most likey the transistor). But, this is dangerous due to the mains voltage and high powered output stages.

[melkaone]

is that the original part number?

Honesly, i can't remember... I think i have a schematic somewhere, when i find it, i'll tell you. But i don't think they are the same as the originals.

Was it heatsinked to begin with?

no. theres only a small steel plate, where the transistor are fixed, going out of the amp.

You say it "broke" a while ago, did you find the cause or did you just replace those parts and hope for the best?

I was almost sure the bridge rectifier had a pb, a soldering point was burnt. So i changed it. The fuses, because as it is open and it's really easy, it's worth doing it . And i remember having killed some while i was testing the amp. For the power transistor, it's my friend who found the pb.

You could run it with the cover off (carefully!) and try to work out where the heat is coming from. (most likey the transistor).

I'll do that tomorow, right now, must go to sleep

thanks

[niftydog]

theres only a small steel plate

that would act as a heatsink, I assume that is still in place and connected to the transistor?

How's your soldering skills? There's a chance that it's a dodge solder joint. Especially if you didn't clean up properly around the burnt joint.

[melkaone]

well, i found what was the pb. I have two resistors (i guess, i never saw this kind, but, hey, i'm a newbie ) just before the + and - V pins of the preamp section of my amp. They're hot when the sound stops.
I'll post a photo asap
I'll go change those tomorrow

[R.G.]

well, i found what was the pb. I have two resistors (i guess, i never saw this kind, but, hey, i'm a newbie ) just before the + and - V pins of the preamp section of my amp. They're hot when the sound stops.

That may be a symptom, but it's not the problem. Resistors don't just get hot - other things may make them hot, but they don't just get hot. So if the two resistors getting hot is linked to the non-function, something is failing and letting them get hot as a consequence.

Resistors don't just fail in a manner that makes them get hot.

[Paul Marossy]

Resistors don't just fail in a manner that makes them get hot.

When that happens, doesn't that mean that they are being exposed to conditions that are greater than what they are designed for? In other words, it would be like using an 1/8 watt resistor in place of a 1 watt resistor, assuming this was the design condition.

Anyhow, like RG says, that is a symptom of what is going on. It sounds like there is large amounts of current going thru those resistors, causing them to heat up until they reach "infinite" resistance due to overheating or something...

[melkaone]

well, i think the pb comes from the bridge rectifier. I remember having changed it with one that was not the same as the original but i can't remember what it was. I now have a KBPC1010...
And I still can't find this schematic...
And the resistors that are overheating are 4W, if that could help...
thanks for your help

[melkaone]

huum, something that i find strange...
heres how is the amp

TRAFO          a |---------------------------------------| a  
AND             b |---------------------------------------| b  PREAMP
POWER AMP  c |-x1-- R480 4W (overheating) -x2-| c  SECTION
SECTION      d |-y1-- R480 4W (overheating) -y2-| d

a & b are the signal wires, no pb here
c & d are the v+ and 0v power. is it normal that between x1 and y1, the voltage is 90VDC? isn't it too much for an amp? After the two resistors, between x2 and v2, the voltage decreased to 30VDC. The 2 R480  are overheating, and when they get to a certain level (unknown), they stop fonctionning. They're not deficient. So the pb must comes from the bridge rectifier i changed or the power transistors that my friend changed.
Well, what do you think of this 90VDC thing?

NB : the originals trans were 2N3055, they were changed with MJ15024
http://www.ee.latrobe.edu.au/internal/workshop/store/pdf/MJ2955.pdf
http://www.ampslab.com/PDF/mj15024.pdf
some things bothers me : the higher amperage et voltage of the new transistors... is it important or just safer values?