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Old 28th September 2002, 05:31 AM   #1
Jean is offline Jean  United States
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Default need some trouble shooting help

Hello !

I need some help in figuring out why my amplifier is blowing fuses.

I fired up one channel of P3a today for the first time. At first everything seemed ok, led lit up. I measured DC output and its about 65mvolts. There was no DC output on the input. Then I connected old 8 ohm speaker to the output and all was ok. It didn't suck in or out, I could hear normal humm when I touched the input wire. I measured voltage between two 5 watt resistors and it was under 100 mv DC.

So then I went ahead and connected signal to the input and all I could hear was crackling like its out of power. I went ahead and then adjusted vr1 and thats when negative rail fuse blew and speaker sucked in. I went ahead and disconnected the AC
power to the power supply. I checked all the components for visual damage and didn't find anything, only one of the 5watt resistors got warm and one blown fuse.

I pulled both fuses and checked the power supply again and it was ok. I am using 25-0-25
500va transformer, bridge and two 10000 uF caps. I put the fuses back in and turned it on
again. Led lit up for a second and then it blew the other fuse (positive rail).

What should I check next ?

Tia
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Old 28th September 2002, 07:31 AM   #2
Jean is offline Jean  United States
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Ok I think I found my mistake, will update in 15 minutes once I fix it.

NOPE, I just replaced 4 BC 546s w/o any luck
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Old 28th September 2002, 02:26 PM   #3
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Default darn rail fuses

Happened to me a couple of time
the rails fuses just gave away leaving me with crossover cooked to perfection I now believe that the problem was the speaker cable I was using, there was too much capacitance thru it.

Now when you turned power down before connecting audio source? what was your source?

You should not check the drivers BD139 BD140
and the OP tansistors
those were the ones ceasing to function in my amp.

Hope you find the problem
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Old 28th September 2002, 05:04 PM   #4
Jean is offline Jean  United States
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I replaced BC 546s so far, I will replace the rest of transistors today.

My source was a cheap cd player setup nearby, the load on the amp was 8 ohm speaker really close to the amp too (10" or so).

Thanks
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Old 28th September 2002, 06:12 PM   #5
sangram is offline sangram  India
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Have you insulated the transistors correctly from the heatsink?

Is it running ok when you are bringing up the voltage through the Variac?
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Old 28th September 2002, 06:16 PM   #6
Jean is offline Jean  United States
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Output 2SA and 2SC pair are insulated from each other, I don't have a Variac. I believe that I over adjusted the VR1 and thats what caused the failure. Would that normally blow output pair or the BD 139/140 pair ?

I am still swapping components
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Old 28th September 2002, 06:24 PM   #7
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Default blow output pair or the BD 139/140 pair ?

Often both.
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Old 28th September 2002, 07:43 PM   #8
Jean is offline Jean  United States
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Ok I just replaced all transistors, set the VR1 to maximum resistance (I took it out and measured it), replaced R11 220ohm resistor and put in 2amp fuses.

I turned it on and LED is not coming up now, after about 5 seconds R7 560 ohm resistor started smoking ...

Ok I am now replacing it and the LED.
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Old 28th September 2002, 08:13 PM   #9
Jean is offline Jean  United States
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Alright replaced both, turned the amp on LED is on now and nothing is smoking. There is over 20 volts DC on the output. What's next ?
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Old 29th September 2002, 09:21 AM   #10
sangram is offline sangram  India
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Sorry. Can you point me to a schematic? 20 volts means something seriously wrong.
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