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Old 25th August 2010, 03:26 PM   #1
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Default Cambridge Audio P70 Integrated Amplifier

Hi guys..
I have a question.. Few days ago I bought an Cambridge Audio P70 Integrated Amplifier.
On power up you hear nasty bump and buzzing noise like what you get on phono input and volume set at 11 o clock.
That is noise I get on normal selector with volume set to minimum. On direct selector noise is still there but acceptable.
After a while (about 3 minutes) everything is fine and amp sounds great. I was (and still am) suspecting biasing storm at power up.
Since there is no relay on output, I even thought it could be normal phenomena (my Audio Research SP11 need a minute before relay switches line outputs).

Anyway.. I tried everything to get my hands on service manual. I even emailed Cambridge Audio but since old Cambridge Audio is sold to somebody else, they couldn't help me. Only thing I could measure is DC offset at speaker terminals ( 27.4 left and 8.5 mV DC right)
On power amp board there is only one trimmer per channel so I think it might be for bias adjustment and I have no clue what would be value and since biasing is different for almost all amps, I don't want to screw with it too much.
My question, does anybody know P70 and would he/she be willing to share their knowledge with me/us :P
Better yet, does anybody have service manual or at least specs for bias and DC offset.. I would be most grateful..
Thanx in advance..
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Old 25th August 2010, 07:20 PM   #2
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this type of amps often suffer from thermal issues ... high temps here and there is a very good reason to have leacky or drifted value on electrolytic caps ...

i could go sampling a few and then work from there ... if needed replace them all ..

a can of info is here vintage amplifier repair/upgrade manual

also there is a chance that the amp is repaired before and parts inside are not the same grade as the origiunal

i have repaired some of them in the past but never found service manuals for ... get some inside pictures they might help

kind regards sakis
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Old 25th August 2010, 10:59 PM   #3
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Thanx Sakis.. Well.. That's just it.. Amp never get hot anywhere. Even at all day work temperature is just above room temperature. I bought that amp from a older guy and he said that he wasn't using it for years. I don't even think it ever was open I'll have look at link you gave me and post some pics tomorrow..
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Old 26th August 2010, 06:22 PM   #4
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Pics from power and pre section..
Btw you can hear noise even trough mute circuit but power up plop not..
Attached Images
File Type: jpg Power amp .jpg (248.3 KB, 207 views)
File Type: jpg Phono.jpg (209.1 KB, 171 views)
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Last edited by Notorious; 26th August 2010 at 06:24 PM.
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Old 26th August 2010, 06:46 PM   #5
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all axial electrolytics are to be trushed ... in similar units i ha ve seen issuel like that but also resitirs that have drifted values ... around output and driver stage red electrolytics are to be trushed also /

i dont remember the selector switch configuration but notice that problems could lay there also
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Old 26th August 2010, 09:19 PM   #6
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Those capacitors with ROE on them are suspect IIRC, they fail quite easily. They are 220uF 6V capacitors I think. 220uF 16v electrolytics will do to replace them.
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Old 26th August 2010, 11:18 PM   #7
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It sounds to me like you've got a dry joint and when it warms up the circuit board expands very slightly and closes the joint. When you first switch on try tapping around the circiut board with something plastic, you'll probably notice some part is sensitive and that's where the dry joint is. Just dab a soldering iron around that area melting the solder making sure you don't short anything. you just need to melt the offending joint to cure it.
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Old 27th August 2010, 12:23 AM   #8
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I would also mention, that unless you're getting lots of hum, that the main smoothing caps probably don't need replacing.
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Old 30th August 2010, 07:24 PM   #9
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You've just got to work out what the problem is by a process of elimination.
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Old 30th August 2010, 07:39 PM   #10
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Quote:
Originally Posted by dave w p View Post
You've just got to work out what the problem is by a process of elimination.
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process of elimination is not good enough even for beginers, electronics is about facts ,process of elimination will end up in a ghost chase ...

drifted values in resistors ,leacky electrolytics,carefull inspection of all solderings and perfect cleaning of all switches pots and trimmers is a simple fact for every machine at this age

( process of elimination will end up in a leacky electrolytic that if replaced machine will work and fail again after a few weeks when some other electrolytic goes absolutelly dry ...)

with respect
sakis
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