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Alesis RA500 Repair

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spongebob View Drop Down
Young Croc
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    Posted: 03 May 2014 at 3:39pm
Hi All,

I have an Alesis RA500 here which is having a spot of trouble.

With an input signal and speaker connected to either channel, if I adjust the gain pot I get nothing for the first 1/3 turn, loud crackling for the next 1/3 turn then nothing for the final 1/3 turn.

If I leave the gain at around 1/5 of the volume on either channel, after a few seconds I get low level audio through the speaker (it sounds clean) but then it peters out after around 10 seconds.

With no speakers connected, both channels show signal on the front signal LED bar display and can be attenuated with the front gain knobs

I've taken the amplifier apart and it appears atleast one of the large 10000uf 80v PSU capacitors is bulging (not majorly, but I can see a bump). Additionally, around these two caps there is what appears to be a small amount of orangey white solidified type foam on the PCB (it looks like a splatter on the PCB, perhaps leaked from the bottom of the cap?)

This specific amplifier has been back to Alesis atleast once in its life for replacement of these capacitors.

Given the above, does it sound like replacing these two capacitors would resolve the issues?

Thanks!

Edited by spongebob - 03 May 2014 at 3:40pm
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kedwardsleisure View Drop Down
Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2014 at 4:13pm
probably half the caps in there need replacing, they lead a hard life as the amp tends to run warm. Any bulging cap should be replaced on sight if it's the cap itself (not just the plastic top cap which can dome all on its own)
K. Edwards Electronics Engineers

North Staffordshire

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JayeJSAV View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JayeJSAV Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2014 at 5:23pm
Quite a few of the larger caps have a slight dome in the top anyway, but it's always worth checking to see if they have pushed away from the PCB at the bottom, as they can leak from either end. :)
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spongebob View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spongebob Quote  Post ReplyReply Direct Link To This Post Posted: 04 May 2014 at 11:47am
Could someone please help me source 2x quality:

10000UF/80V,+50/-0%,85?,F30*62mm

85 is 85c - although going on Mr Edwards comments maybe putting in some higher temp rated ones might help?

Thanks
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kedwardsleisure View Drop Down
Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 04 May 2014 at 6:05pm
always a good idea, 85deg are alot cheaper than high temp ones

are you pushed for space?
K. Edwards Electronics Engineers

North Staffordshire

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spongebob View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spongebob Quote  Post ReplyReply Direct Link To This Post Posted: 04 May 2014 at 11:41pm
The two caps are side by side on the PCB, so anything larger will be a struggle :( Taller would work though
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