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Old 10th August 2005, 07:39 AM   #1
Stevey is offline Stevey  United Kingdom
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Default Crest Amp Circuit Schematics

I have just got some freelance repair and service work from a local PA dealer and they have sent over a Crest Ca18 for repair.
Has anyone got a circuit Schematic for this amp ?. I mailed Crest asking about this but didn't get a reply.

If anyone can help this would be greatly appreciated.

Thanks
Stevey
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Old 13th August 2005, 05:13 AM   #2
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Location: hobart tasmania
Hi Stevey

Did you try contacting their
techncian jose@mwaudio.com

Some firms as you might understand are quite hesitant
with releasing schematics. Perhaps try and isolate the
fault to one area saying you have found such and such
a component measuring ?? could they steer you
in the right direction- ie try and see it from their perspective
- they probably want only their staff to repair their amps
but you might get their help if you can isolate where the fault
is occurring and converse in electronics terms.

Best of luck.

Cheers / Chris
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Old 13th August 2005, 11:33 AM   #3
Hendrik is offline Hendrik  Belgium
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Hello,

(this is my first post in this forum)

if you need schematics for a crest amplifier you better send a mail to your sales rep. I worked for crest's sales rep. in Belgium as a repair technician. If your sales rep. has the same attitude they will send you a service manual. What went wrong with the amplifier?


Regards,

Hendrik
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Old 13th August 2005, 11:33 AM   #4
Hendrik is offline Hendrik  Belgium
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I forgot to mention that most older crest pro amp's like the 3301, 4601 4801 and so on and the CA series can be cured by cleaning the molex board to board headers. Just clean the pin's with isopropanol and don't touch the pin's after cleaning. In some cases it is better to replace them.
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Old 14th July 2008, 06:42 PM   #5
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I have alot of crest scam's. I think I have the CA 18. I can email u a copy. Tony
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Old 15th July 2008, 12:08 AM   #6
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I think i have a CA18 schematic but i need the BIAS setup procedure if anyone has that.


Zc
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Old 15th July 2008, 02:21 AM   #7
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Stevey:

I have the schematic email me.

art
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Old 15th July 2008, 06:15 AM   #8
lineup is offline lineup  Sweden
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Default Re: Crest Amp Circuit Schematics

Quote:
Originally posted by Stevey
I have just got some freelance repair and service work from a local PA dealer and they have sent over a Crest Ca18 for repair.

Has anyone got a circuit Schematic for this amp ?
Stevey.
You can download Crest CA18 schematic from http://www.crestaudio.com/

It is a PDF on this page.
CA18 ( 640kb pdf)
http://www.crestaudio.com/schematics/

Regards Lineup
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lineup
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Old 15th July 2008, 11:13 PM   #9
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I called crest today to get the BIAS Procedure for the CA Series amps and was sent a document that they use for all bi-polar output device amps. It may be of use to anyone working on a Crest amp.

General Bias Adjustment Procedure Recommendation.

This is intended for units that have no manufacturer’s recommended bias set-up procedure. It is only a recommendation based on field-tested and other established recommendations. This procedure is intended only for standard bi-polar output stage amplifiers. It is not recommended for digital (switching) or FET (Field Effect Transistor) output stage type amplifiers that may have special procedures for bias adjustments.
Test equipment requirements:
● Stable, low-distortion sine wave oscillator capable of 1 volt RMS output at 20kHz.
● 20MHz oscilloscope.

● Distortion analyser.

● 8-ohm non-inductive load resistors of at least 50W power handling capacity. These will NOT handle the full output of most commercial amplifiers; they are for low level testing only. 500W or 1000W resistors are preferable. (The more power handling capacity they have, the better.) The non-inductive characteristic is a must for this adjustment.
● VariAC to establish recommended line voltage (100, 120 or 220/230/240VRMS depending on locale.)
Procedure:
Establish that the amplifier to be tested is in a stable operating condition.
Connect unit to load and sine wave oscillator and (if available) distortion analyzer. Set oscillator at zero output for the time being.
Apply line voltage and stabilize at recommended point for amplifier.
Set level controls (if present) to full level.
Apply 20kHz sine wave signal of sufficient level to amplifier to generate 2.828 to 5.656 V Peak to Peak on oscilloscope.
Adjust amplifier bias control until the crossover distortion is plainly evident in the oscilloscope display. (Bias is headed toward the “cut off” point of the output devices.)
Re-adjust the bias so that the crossover distortion just begins to disappear.
Go no further on the adjustment so as to prevent the amplifier from idling too hot and possibly being damaged by heavy signal levels. Reading the distortion on a distortion analyser is highly recommended in conjunction with the oscilloscope display.
If possible, lock bias controls with a dot of glyptal resin or ThreeBond adhesive.

Repeat for all channels that were repaired.

End of procedure.
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