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Old 17th March 2013, 07:34 PM   #11
Mooly is online now Mooly  United Kingdom
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One step at a time

With the bias pot shorted the current in the output stage should be zero. With NO speakers attached there should be NO voltage across R52 and R69 (the 0.33 ohms) That's reading across them and not from ground to them.

If the bias pot alone were the problem the amp should essentially work as normal now and yet its still very distorted... yes ? Have you tried swapping the speakers over... I'm wondering if there has been a DC offset at some point and a speaker is damaged... just a thought.

Lets see what all that reveals first.

(Make sure the mica insulating washers are back in place before doing anything else)
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Old 17th March 2013, 08:41 PM   #12
jaycee is offline jaycee  United Kingdom
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The pot looks like it's a 100 ohm linear trimpot made by Bourns. Can't be sure exactly which one... possibly a 3296Y.
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Old 17th March 2013, 10:06 PM   #13
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Thank you both for all your help and patience & sorry, get carried away

I have taken everything apart again, ensured the insulation plastic pieces are in the correct places, cleaned up the messy thermal paste (definitely put a little too much on the first time round) and re-assembled taking my time over it to ensure every screw and cable goes exactly where it should.

Re-tested with and without the shorting C47 and still get the heavy distortion. I tried with both speakers, both work on the right channel and both distort on the left channel - although the distortion it makes does sound very very similar to a blown speaker, i can confirm that they are not blown. Have also tried every input (DVD, CD, AUX etc.) and on Speaker A and Speaker B.

Thanks jaycee, will have a search and see what i can find.

Mooly : Following on from what you said, i have tested the voltage between the two contact points of R52 and R69 and you are correct, with no speakers attached but powered on, it reads 0v. I also tested between R52 and R69 which reads the same (couldn't quite figure out whether across meant between the two or individually but not to casing ground so tried both).

Thank you again
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Old 17th March 2013, 10:40 PM   #14
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If the schematic is anything to go by, i'd simply put this amp in the bin.
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Old 17th March 2013, 10:51 PM   #15
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THis link will give you a choice of different multiturn pots. Sounds like the pot is the problem to me.
Hope that helps.
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Old 17th March 2013, 11:11 PM   #16
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Quote:
Originally Posted by michaelkiwanuka View Post
If the schematic is anything to go by, i'd simply put this amp in the bin.
really? I know it isn't particularly high end but got very good reviews for the price when it came out & surely i may as well give a few things a try before condemning it.



Thank you Harleyjon, will take a look and get one ordered soon - may as well fix part of the problem .
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Old 17th March 2013, 11:19 PM   #17
jaycee is offline jaycee  United Kingdom
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To be fair, the circuit isn't bad.. what's usually the problem with Cambridge gear is the "Made in China" cost cutting.
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Old 17th March 2013, 11:36 PM   #18
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Cambridge Audio is pretty fair stuff. There was a problem a number of years ago but that has been sorted out. I don't mind fixing any audio but CA Amps tend to be reasonable. Not as nice to repair as Quad or Arcam but a lot better build quality than the Amstrads of this world. No offence intended.
Replacing the pots and setting the bias to the correct Iq after 10 minutes with no signal, I feel will fix your issue.
Keep an open mind and take it one step at a time. Compair side to side etc and you will get there.

Last edited by JonSnell Electronic; 17th March 2013 at 11:38 PM. Reason: Addition
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Old 18th March 2013, 01:05 AM   #19
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Originally Posted by nickalmond View Post
really? I know it isn't particularly high end but got very good reviews for the price when it came out & surely i may as well give a few things a try before condemning it.
The reason I recommended you bin it is your amplifier is oscillating, which is the reason for the symptoms you've described.

This is not surprising from the haphazard compensation schemes used in the schematic.
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Old 18th March 2013, 01:26 AM   #20
jaycee is offline jaycee  United Kingdom
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I've never seen these oscillate before... usually the failure mode is "I had a party with the volume up, it got hot and now it doesn't work" - ie, dead output transistors + drivers from the inadequate heatsink. The other failure is that the main power supply capacitors pop because they're cheap crap. Cambridge may well have addressed both issues by now.
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