I've got a broken amp. Can it be fixed?

Fritzell · 15th November 2005, 09:48 PM

Hey i got a Pioneer SX-205RDS from a friend which apparently didn't work. So i took a look at it and it turns out that the output transistors seems to be ok (2SA1939 and its complement. One of 'em were replaced with a BD246)

Anyway, it also turns out that the signal never made it to the amplifier stage. The +-40V is ok, but when it's turned on, the +-12V sinks to zero. My guess is that something is broken at draws awfully lot of current so it sinks down the voltage from the regulators (?) so if the input op-amps and other devices dont get their voltage right it won't work

Iv'e removed some of the IC's and tried without them and checked if they cause this, also felt on top of some, none become abnormally hot.. the input selector digital switch gets a little hot and has something that looks like a little burn mark on the top

so i solderer off the voltage supply to it, but it didn't change anything.

Also another thing that i noticed was that when the power is on, their is +40V but the -40V starts to tick down.. -35, -30 and stops on -27 :S. Same thing if power is removed, +40V stays on the cap but -40v starts to tick down again :S

I would appreciate if someone would like to help me on this one, or could give me a tip if someone has experienced the same problem

Thanks alot
Fritzell

sss · 15th November 2005, 10:40 PM

first disconnect the ps.. check if it working ok (no burned diodes)
check the -+12V also while disconnected from the amp

Quote:

2SA1939 and its complement. One of 'em were replaced with a BD246

Fritzell · 16th November 2005, 02:08 PM

Hi thanks you for your input.

I shall test this.
Btw it's no good replacement with bd246?

anatech · 16th November 2005, 02:32 PM

Hi Fritzell,
Generally, if one output needs changing, change them all in the channel. Use proper complimentary replacements and it never hurts to match them if you can. On cheap units, just check the new ones and stick them in. Use new grease, or sil pads, and clean the heatsink mounting area. Do not over tighten the new parts.

I would also replace the driver transistors. Basically, replace one stage back from the last blown part. Service time is more expensive than the parts, especially if the repair comes back. The next job you can't charge for.

sss is right, isolate the supply and check it.

-Chris

15th November 2005, 09:48 PM	#1
Fritzell diyAudio Member Join Date: Jul 2004 Location: Sweden	I've got a broken amp. Can it be fixed? Hey i got a Pioneer SX-205RDS from a friend which apparently didn't work. So i took a look at it and it turns out that the output transistors seems to be ok (2SA1939 and its complement. One of 'em were replaced with a BD246) Anyway, it also turns out that the signal never made it to the amplifier stage. The +-40V is ok, but when it's turned on, the +-12V sinks to zero. My guess is that something is broken at draws awfully lot of current so it sinks down the voltage from the regulators (?) so if the input op-amps and other devices dont get their voltage right it won't work Iv'e removed some of the IC's and tried without them and checked if they cause this, also felt on top of some, none become abnormally hot.. the input selector digital switch gets a little hot and has something that looks like a little burn mark on the top so i solderer off the voltage supply to it, but it didn't change anything. Also another thing that i noticed was that when the power is on, their is +40V but the -40V starts to tick down.. -35, -30 and stops on -27 :S. Same thing if power is removed, +40V stays on the cap but -40v starts to tick down again :S I would appreciate if someone would like to help me on this one, or could give me a tip if someone has experienced the same problem Thanks alot Fritzell

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16th November 2005, 02:08 PM	#3
Fritzell diyAudio Member Join Date: Jul 2004 Location: Sweden	Hi thanks you for your input. I shall test this. Btw it's no good replacement with bd246?

16th November 2005, 02:32 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Fritzell, Generally, if one output needs changing, change them all in the channel. Use proper complimentary replacements and it never hurts to match them if you can. On cheap units, just check the new ones and stick them in. Use new grease, or sil pads, and clean the heatsink mounting area. Do not over tighten the new parts. I would also replace the driver transistors. Basically, replace one stage back from the last blown part. Service time is more expensive than the parts, especially if the repair comes back. The next job you can't charge for. sss is right, isolate the supply and check it. -Chris

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