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Old 11th February 2014, 06:53 PM   #1
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Default Yamaha A-1000 Oscillates

Can any body help me with this accursed A-1000 integrated amp? I replaced all the output, driver and pre-driver transistors in both channels (with the same parts, good subs) along with various burnt flameproof resistors, cleaned all the glue and corrosion off the board around the small filter capacitors, replaced corrosion-damaged jumpers and resistors in that area. Now both channels make full output, no DC offset, BUT-one channel oscillates at about 4 MHz whenever I bias it even partially toward normal-more than 2 mV across emitter resisters and there it goes! I have replaced or jumped out all the small value caps in the NFB network, checked the NFB resistor, also everything in the Zobel network. I also resoldered everything in sight. Only odd thing is that the -22V supply is only 16.4V-but this supply is common to both channels. This is a matter of pride now!
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Old 11th February 2014, 07:11 PM   #2
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what were the original output transistors, and what are they now? you said "good subs" but that doesn't mean the junction capacitances are anywhere near what they were in the original outputs. one other possibility, since it oscillates in response to changing the bias is that the cap across the bias network, if it's an electrolytic, is dried out.
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Old 11th February 2014, 07:50 PM   #3
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I used the same devices in both channels, but only one channel oscillates-the other biases up normally. There is no cap across the bias pot or transistor.
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Old 12th February 2014, 07:56 AM   #4
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unclejed is right

IF the amp was blown often miller compensation caps either in bias , VAS , or drivers might also present some failure with oscillation results ( styroflex caps do that not ofetn but actually they do )

cob of transistors might be an issue good subs is the thing you said and your word for it probably will bot be enough ...If cob is increased in any of the outputs or drivers from production aspects previous stage will have a harder work to do

Increasing miller compensation in drivers or other areas might cure the thing but it doesn't come for free sonics of the amp together with bandwidth will be compromised.

Still this method can be used to observe and present indication where the oscillation comes from.

Kind regards
Sakis
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