Arcam Alpha 3 Int. Amp. what caused this!

dynoPort · 2nd April 2010, 06:42 PM

Hi all, I bought this off ebay 'untested'. Looked great untill I took the lid off!
please see photo; before I just replace them, what could have caused these resistors to become charcoal?

dynoPort · 2nd April 2010, 06:53 PM

There are also some strange goings on inside tho they look original?

AndrewT · 2nd April 2010, 07:04 PM

Hi,
This damage can only be due to massive overpowering.

It is very likely some other components are damaged as well.
Semi conductors blow first to protect fuses is a humorous way of saying the semiconductors do not tolerate overload very well.
Resistors tolerate overload a lot better than fuses.

I suspect that link has been added to bypass a burnt out trace. That would high current indeed.

jaycee · 3rd April 2010, 04:14 AM

The mod on R24/R124 is an official factory mod to reduce turn-on thump.

The wire from R127 to R206 does seem like it's a replacement for a damaged +V trace. R206/202/203 provide power to the LED.

I would say there is a lot of damage here. If i had to take a guess, the output transistors and VAS failed, and the VI limiter blew the resistors up. Fortunately, the parts in this amp are very cheap. The most tricky parts are the FST239/240 transistors - these were replaced by ZTX653/753 IIRC.

If you are very keen on repairing this amp I would replace all of the power amp transistors. Rapid stock all of the transistors used.

I've uploaded the service manual here

dynoPort · 6th April 2010, 04:53 PM

The power transistors just failed? could a shorted out speaker cable cause this? and what is VAS

dynoPort · 6th April 2010, 05:09 PM

VAS: Voltage Amplifier Stage?

still doesn't mean much to me

Mooly · 6th April 2010, 05:21 PM

Another Alpha 3 lol
Arcam Alpha 3 Left Channel broken...

jaycee · 7th April 2010, 02:45 AM

VAS does indeed mean Voltage Amplification Stage. It's Q8/Q108 in this amp.

A shorted speaker cable can indeed cause all this damage.

2nd April 2010, 07:04 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, This damage can only be due to massive overpowering. It is very likely some other components are damaged as well. Semi conductors blow first to protect fuses is a humorous way of saying the semiconductors do not tolerate overload very well. Resistors tolerate overload a lot better than fuses. I suspect that link has been added to bypass a burnt out trace. That would high current indeed. __________________ regards Andrew T.

6th April 2010, 05:21 PM	#7
Mooly diyAudio Member Join Date: Sep 2007	Another Alpha 3 lol Arcam Alpha 3 Left Channel broken... __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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3rd April 2010, 04:14 AM	#4
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	The mod on R24/R124 is an official factory mod to reduce turn-on thump. The wire from R127 to R206 does seem like it's a replacement for a damaged +V trace. R206/202/203 provide power to the LED. I would say there is a lot of damage here. If i had to take a guess, the output transistors and VAS failed, and the VI limiter blew the resistors up. Fortunately, the parts in this amp are very cheap. The most tricky parts are the FST239/240 transistors - these were replaced by ZTX653/753 IIRC. If you are very keen on repairing this amp I would replace all of the power amp transistors. Rapid stock all of the transistors used. I've uploaded the service manual here

6th April 2010, 04:53 PM	#5
dynoPort diyAudio Member Join Date: Oct 2004 Location: Cymru/Wales	The power transistors just failed? could a shorted out speaker cable cause this? and what is VAS

6th April 2010, 05:09 PM	#6
dynoPort diyAudio Member Join Date: Oct 2004 Location: Cymru/Wales	VAS: Voltage Amplifier Stage? still doesn't mean much to me

7th April 2010, 02:45 AM	#8
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	VAS does indeed mean Voltage Amplification Stage. It's Q8/Q108 in this amp. A shorted speaker cable can indeed cause all this damage.

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