Cambridge 640A - Keeps cutting out

amazingtrade

amazingtrade

Mad Madchestoh fan
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My Cambridge amp keeps tripping its power protection. Its worth about £150 working. It cuts out when the source is not swithced on but is selected after a few hours or after about an hour of playing music. Once it has cut out it keeps cutting out constantly until I remove the power lead for a few hours.

I am guessing it could be the paste on the heatsinks?

How can I go about fixing this?

Anybody got any ideas of what the problem is?
 
Might be worth taking the case off and leaving it off whilst in use to see if that prolongs the time before it trips again, might give an indication if it is heat related before digging any deeper. Damm annoying when this starts to happen.
 
Sounds like a thermal issue. First check inside for clogs of dust. Also check the speaker connections and make sure there are no shorts. If there is just a tiny hair of wire shorting it might drive the load but over-heat very quickly.
 
Forgot to mention it does it via the headphones or speakers otherwise a speaker cable short would have been my first suspect. It is the safety cut off which just cuts out. I am annoyed with myself I have only had it two and a half years and I forgot to take out the extended warranty.

I need to find the right torx screw bit to get the lid off, I was searching all night. Got too many jobs today so may open it up tonight if I get time. The amp is currently on my bench so everything has to be rewired anyway.
 
I can't even get into it, just bought a load of star bits but even T10 is too big :(

I am just going to wire everything back up and hope the best. I don't even know exactly when I bought it. Either late 2007 or late 2008 I just cannot remember.

2007 would be a lot more likely.
 
When you do get the lid off, run the amp for about 1 hour with no signal and feel the heat sinks. At idle it shouldn't get particularly warm. If it becomes so hot that the thermal protection trips, the bias current in the output stage is probably set too high.
 
Thanks been to B&Q and they only had T10s too. I will try and go to Maplin tomorrow. Its a nice little amp which fits my needs perfectly and as I am getting my new car soon I can't really afford to be buying new kit.
 
If you are considering taking it back to the retailer then maybes think twice about breaking any "quality" seal that is on the amp - I have fell for this before where I have had a look to see if I can fix something then taken it back and been told to sod off cos I have tinkered :(
 
There isn't any quality seal on it :). I really can't see Richersounds wanting to know after two years either and I don't even know if I have the receipt.

If it was a £1k amp then yes but it was only £150 and that won't go in my favour as the judges seem to take into account purchase cost when deciding if it was fit for purpose.

I should have taken that trader to court who sold me my first Fiesta though, selling a car with shot piston rings is clearly not fit for purpose even if it did cost £400.
 
I managed to get the T6 screwdriver and opened it up. Everything looked fine and the heatsink was getting very hot. I figured that 3inch clearance on the shelve may not be enough so I move my turntable and put the amp on top. It now has the entire room to radiate the heat and it has not cut out once.

It coped with 80 minutes of loudish (10 o oclock) rock music without cutting out.

I am hoping it was simply over heating. I shall watch corrie via it tonight and that will be a real test.
 
This is where I had my amp before

P1050202.jpg


I simply moved to where my turntable is. I suspect what was happening is the heat was hitting the top shelf and then bouncing back inside the amp. The lid of the amp feels so much cooler now.

PS Ignore the speaker stands, we are in the middle of putting the house on the market so most my furniture is in storage.
 

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