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Old 12th August 2010, 10:00 PM   #1
DcibeL is offline DcibeL  Canada
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Default Wanted: Schematic for Crest VS900

Hello,
I'm very interested in a schematic for a Crest VS900. I have one that is stuck in protection, I took it apart and found some water damage around the power supply and protection circuit area. Looks like the water corroded some connections to the point that the shorted together and caused a fault.

I'd really like to get this beast working again, if you have a schematic please contact me.
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Old 12th August 2010, 10:29 PM   #2
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Hi,

Sometimes it's easy
http://www.crestaudio.com/media/sche...matic_set1.pdf
Check for leaky caps, electrolytic on pcb, typical failure on Crest VS amps.
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Old 12th August 2010, 10:53 PM   #3
DcibeL is offline DcibeL  Canada
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Thanks very much. A google search didn't get that.

The amp is visually quite immaculate. If you look at page 15 of the schematic you linked, from what I can tell the end of R9 that isn't connected to the negative supply shorted to the center tap of the supply (audio ground). R51 is a little cooked, and there is some discoloration due to heat around R92 which may be normal.

Thanks again, I'll keep poking at it.
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Old 12th August 2010, 11:17 PM   #4
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The big supply caps are mounted upsidedown by bolts trough PCB. The vents are pointing down on the PCB, certainly a bad design decision.

If the 'leaky cap syndrome' is in it's early stage PCB can be cleaned. If it has gone far and PCB has absorbed electrolytic it's more or less toast.

A preventive measure is to run these amps upside down in order to get the caps in proper position.

Good luck with your repair.
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Old 12th August 2010, 11:40 PM   #5
DcibeL is offline DcibeL  Canada
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Caps are not leaking, they are fine. This is what I have determined:

Amp was damaged by water. The protective paper thing under the main PCB got wet which caused corrosion to build up on it over time eventually causing a failure. I described the visual problem above, parts mentioned are part of the protection circuit.
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Old 13th August 2010, 04:08 PM   #6
AKN is offline AKN  Sweden
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Hi,

Ok, sorry, I'm so used to see leaky caps on Crest amps so I just took for granted that it was the fault in your case. Typically the protective paper under pcb soaked up the leaking electrolytic and caused shortcuts almost everywere.
Water there is ok :-)
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Old 14th August 2010, 10:08 PM   #7
DcibeL is offline DcibeL  Canada
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Yeah I took out the caps and they are very clean at the terminals. Definitely looks like water got in. Haven't had time to look at it any further, so I'm still not sure what the matter is with this amp.
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Old 26th May 2011, 02:00 AM   #8
DcibeL is offline DcibeL  Canada
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Just thought I'd update this to say that I got the amp working. I found a little transistor in the protection circuit that had cracked, but was not easily visibly when looking at it. I also now have a oscilloscope at my disposal, so it was easy to track down.

The transistor was Q7 if anyone is interested. I replaced the obvious components around it as well just for good luck. This amp is going to a friend of mine now, who will use it to power a couple subwoofers for his theatre.
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