Unknown Power Amp

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Hello!!!..

My brother in law gave me a pair of speaker box, the other box got the power amp w/ tonecontrol inside. My problem now is the transformer is burn out it is 1ampere no marking for the for the output voltage it has a center tap. One of my friends told me that it maybe 12-0-12 output so i'd replace it with a 2ampere tranformer 12-0-12 output voltage.
The power amp is now working eccept when i try to increase the volume together w/ treble and bass the 4 power output transistor (2-for left 2-for right channel) and the bridge rectifier is get too hot.. enyone can tell me what will i do to cool them out.
 
Centauri said:
Seems like you have a problem with the amp that needs to be repaired, especially if the bridge gets hot. This is probably what caused the original power transformer burnout.

Cheers


Yup!!.. by this time i have no other clue to start on. i'd already replace suspected component (caps,resistors,pots) now i'm planing to replace the output power transistor but my problem i cannot find them in the local store i did serch their part # in the net but got nothing... I think this x-sistors are old enough.
 
The amp got 4 output Xsistors, 2 for evely channel..
The # is B825 all of them got the same # i guest except for the marking on the lower part of the transistor look for the drawings in my attachment.
 

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priscared said:
Besides my massive project at the moment, all the amps ive worked on a 40v, maybe too many amps into too little volts.
just a guess, i wouldnt try it in a hurry unless u really know, if it is 40v, expect to pay nuthin for a replacement.
daniel.


Centauri said:
As is common with Japanese transistor markings, the prefix '2S' has been omitted - these are 2SB825 transistors. My wholesaler has them listed - they should be easy to get.

Specs: PNP 60V 7A 40W TO220 case

Cheers

Thanks Centauri for the Info...

Do you think the supply voltage is not enough (12-0-12, 2A transformer) for this amplifier?

Do the lack of voltage and current is responsible for the Transistors and bridge diode to get very hot?

I notice that when I try to adjust the treble(clockwise) in tone control the transistors and bridge is started to gettin' very hot after just a 3-5 seconds. But when the treble is in 0 position the temp. is normal but the sound is not good..
 
JhonDoe said:


I notice that when I try to adjust the treble(clockwise) in tone control the transistors and bridge is started to gettin' very hot after just a 3-5 seconds. But when the treble is in 0 position the temp. is normal but the sound is not good..


This sounds like supersonic oscillation. For some reason the amplifier is becoming unstable and producing full power tone above hearing range. Could be earthing problem or faulty capacitor somewhere. Connected between speaker output line and earth, there should be a series resistor/capacitor network - make sure these are ok (resistor usually around 4 to 10 ohms and cap around .047uF to .1uF).

Ideally, a variable bench power supply and oscilloscope would be useful here - maybe you know someone with the gear to help?

Cheers
 
Centauri said:



This sounds like supersonic oscillation. For some reason the amplifier is becoming unstable and producing full power tone above hearing range. Could be earthing problem or faulty capacitor somewhere. Connected between speaker output line and earth, there should be a series resistor/capacitor network - make sure these are ok (resistor usually around 4 to 10 ohms and cap around .047uF to .1uF).

Ideally, a variable bench power supply and oscilloscope would be useful here - maybe you know someone with the gear to help?

Cheers


I forgot to mentioned, I'de tested it too w/out speaker and input(only supply voltage). sould it be the problem is in the tone control crkt.?
 
When the tone control is turned full bass, then it is lowering the total gain of the amplifier at high frequencies, thereby stopping oscillation. I would suspect problem is not actually in tone circuit, however, depending on circuit, faulty power supply bypass capacitor on power rails to tone circuit could cause it. Testing without speaker should not effect it as long as the network described in my last post is present and working - this puts a load on the amp at high frequencies, and as such is important.

A schematic of the amp would be extremely useful.......

Cheers
 
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