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Old 4th November 2008, 11:14 PM   #1
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Default Crest CA9 spare input boards?

I have one of these and the input board for channel B is destroyed beyond usability... Does anyone have a spare?

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Old 4th November 2008, 11:22 PM   #2
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Something seems to be missing from those pictures...

Here's the broken channel:

Output board, it's mostly soot. Just one burned trace.
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Destruction:
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Once upon a time this was a preamp board:
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Output board cleaned up a bit:
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Old 4th November 2008, 11:40 PM   #3
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Copied from another thread:
----

I've started looking at the CA9 amp now. The blown channel has a lot of the preamp board destroyed. The output board has blown emitter resistors on one side (one of them has a big crater in it!). One trace has burned off but the board seems repairable, the charring is only superficial. A lot of resistors and transistors are gone but that is not a big problem.

The preamp board is in worse shape. It has a long trace that is burned off, and this has charred a large part of the board. The damage is especially bad around the connector connecting ouput module to preamp board. The track that has burnt off is the track that connects the DC protection resistor (5K, R95 in this schematic) to the intra-board connector.

I don't see what else could have made it burn off than an arc from the output trace to the ground plane so probably it was something that fell inside, moisture/dirt or plasma from the exploding emitter resistor on the output board that set it off. I don't have the pictures on this computer but will post them later. The solder points look a bit strange, they are matte. Some kind of oxidation or has it inhaled smoke from a smoke machine maybe?



I've started testing the other channel, so long without load. It runs but the clip limiter is not working. That part of the circuit is a little different, instead of Q22 (which the comments say were added in a revision) the internal transistor of the OTA is used. Something seems strange with the OTA chip, the internal transistor seems broken in some way. I'd guess it gets damaged by the current pulses when trying to charge C21 so this is probably why Q22 was added.

This also has the side effect of not only rendering the clip limiter inoperative, the current limit circuit won't work either. Maybe this is what made the other channel blow?

Shorted output & no current limit => Output transistors blow => Exploding emitter resistors starts an arc on preamp board from the output node now sitting at 120V DC to the ground plane/case. The arc travels from the far end of preamp board to where the connector is. Result: One charred preamp board.

http://www.crestaudio.com/media/sch...le%20Preamp.pdf

I'm going to modify the OTA circuit to include Q22 in the working channel. Do you know if there are other modifications that should be done? Maybe I should contact Crest, I'd guess they have sevice manuals/service bulletins for it.
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Old 4th November 2008, 11:53 PM   #4
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Schematics of the rest of the amp:
http://www.crestaudio.com/media/sche...matic_set1.pdf

The amp is a class G (lifts linearly, not as a step) amp with +-60V and +-120V rails. Output power is 900W in 4 ohms, 600W in 8 ohms IIRC and 1000W in 2 ohms (protection circuit is supposed to activate). The amp has a clip limiter built with an OTA that increasess feedback around one of the opamps in the preamp. The current limit circuit uses this circuit too - decreasing amp gain when the output devices are about to be overstressed.

There seems to be a bug though in this revision, the clip limiter on the non-fried channel is not working. Seems the OTA blows up, probably because the internal darlington transistor is overstressed when charging a cap. In a later revision an external darlington transistor was added.

Devices on heatsink in this picture:
http://img407.imageshack.us/img407/7...hnad021tl7.jpg
are, from left to right:
1 x Rail lift predriver
1 x Rail lift driver (2SC5242)
1 x Rail lift output (2SC5242)
1 x Low rail diode (A motorola in a very strange case)
2 x Rail lift output (2SC5242)
4 x Output devices (2SC3281)
1 x Driver (this is also a 2SC3281)
1 x Predriver
1 x Vbe multiplier (TIP-series darlington)
1 x VAS.

VAS and both predrivers are 2SA1006 and its complementary.
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Old 4th November 2008, 11:57 PM   #5
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I will look and see if i have a spare board. Maybe...not sure
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Old 6th November 2008, 04:17 AM   #6
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I do have a spare Pre-amp board that matches your photo. It seems i have stolen two caps from it however at some point. I marked the board 470uf 50V so you will need to replace them. and i have no idea if the board works but it is clean so you should be better off then that charred hunk you have!!!

Make me an offer.
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Old 6th November 2008, 03:17 PM   #7
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you can email me directly at

tetech2 a t doitnow d o t com
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