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Old 22nd January 2004, 06:15 AM   #1
JhonDoe is offline JhonDoe  Philippines
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Default Unknown Power Amp

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.
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Old 27th January 2004, 02:46 PM   #2
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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
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Old 28th January 2004, 07:08 AM   #3
JhonDoe is offline JhonDoe  Philippines
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Quote:
Originally posted by Centauri
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.
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Old 28th January 2004, 01:43 PM   #4
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What's the number on the transistors ?

Cheers
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Old 29th January 2004, 05:24 AM   #5
JhonDoe is offline JhonDoe  Philippines
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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.
Attached Images
File Type: png xsistor.png (1.7 KB, 51 views)
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Old 29th January 2004, 07:26 AM   #6
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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.
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Old 29th January 2004, 01:05 PM   #7
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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
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Old 30th January 2004, 02:38 AM   #8
JhonDoe is offline JhonDoe  Philippines
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Quote:
Originally posted by priscared
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.

Quote:
Originally posted by Centauri
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..
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Old 30th January 2004, 04:45 AM   #9
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Quote:
Originally posted by JhonDoe


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
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Old 30th January 2004, 06:05 AM   #10
JhonDoe is offline JhonDoe  Philippines
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Quote:
Originally posted by Centauri



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.?
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