Cambridge Audio 640A Overheating / Distortion

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Hoping someone can help me diagnose the problem with my Cambridge Audio 640A v1 amp - I have had it for about 4 years now and recently (6 months ago) it began severely overheating until the protection would kicked in after about 15minutes of it being powered on. To temporarily 'solve' this, i had a 12v fan running through a temperature sensor to cool it down. This ran fine up until last month when the left channel began intermittently cutting out - to resolve i simply wiggled the volume control a little and it sprang back into life. Not wanting to damage my new speakers (B&W 684's) i decided to buy a new amp which is working perfect.

I am hoping to get the Cambridge unit up and running again as it was a great amp during its first few years of use and would like to do the work myself to hopefully learn a little more about them.

I had opened the unit and found the heatsink for the faulty channel was rising to around 80 Degrees Celsius before cutting out - with or without an audio input. I took the PCB and heatsinks out, checked over all the contacts, removed some thin plastic sheets that sat between the heatsink and the thermal paste (not sure if this was a wise thing to do), cleaned it up and re-applied new thermal paste. All contacts seems in good condition.

All this made little difference to the heat output so after reading for a couple of days on various websites, I decided to open the unit up again and have another 'play'. I adjusted the BIAS pot to a lower value which made a seemingly huge difference to the temperature of the left channel (faulty) heatsink - I left the amp on for several hours and no overheating occurred (still a little hotter than the heatsink for the right channel).

I connected a cheap hifi speaker to the working channel and audio played perfectly as expected, followed then by the faulty channel and got huge distortion throughout the entire range when playing back any audio. Working backwards, I turned the amp off, adjusted the BIAS pot back to what i believe was its original position, reconnected the speakers and still the same heavy distortion along with the overheating.

This is where i am up to and not sure if it is simply a case of replacing the pot or if there is now a bigger problem - also not sure how to correctly test the pot.

The only additional note i can add is that I tested the voltage across two other pins of the chips noted 'SAP15' (sorry, don't know their correct name but please see first image) - the voltage jumps around between 2v and 6v through the working channel but stays fairly idle at around 2v on the faulty channel.

Apologies for the long post but i really appreciate any help anyone can give me. Happy to do further testing - have a decent multimeter to hand and fairly good at soldering. I should also note that I only have a basic knowledge of electronics so apologies if it seems like i am asking daft questions.

Many thanks,

Service Manual;


Plastic Pieces Removed;

Brilliant. Thank you, will pop the plastic pieces back in.

The pot is a multi-turn one, yes. I counted 10 full turns which reduced the temperature significantly to stop the protection module kicking in. It is currently back in what i believe is its original position (counted 10 turns in the opposite direction).

Thank you :)
It sounds to me like the faulty channel is oscillating. Does R44 (left) or R50 (right) look burnt?

Also, yes, the "plasitc" bits are actually mica insulators, as the back of the power transistors are connected to the power. If you bolt them straight to the heatsink, you will create a short circuit. Hopefully, you haven't powered the amp up without these in place!

To refit the mica washers you will need to clean the white heatsink paste off of everything (the insulators both sides, the heatsink and the back of the transistors), and apply a thin layer of thermal paste between the heatsink and insulator, then between the washer and transistor. I use a toothpick to spread it out. You need a THIN layer only - no more than a pea sized blob.
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Hi jaycee. Thank you for your reply. I have checked both R44 and R50 and both look perfect.

I have indeed powered it on without the insulators (oops!) - it was turned on approximately 5 times for a period of about 30seconds. Both speakers connected to the left and right channel. The right channel continued sounding perfect. The light channel had the distortion. I will make sure the insulators are replaced before it is next powered on for testing.

Could the problem with overheating be anything to do with the BIAS pot or am i way off here? As i understand it, the BIAS limits the amount of current sent through on 'idle' - as in the original thread, the left side was overheating and therefore suggests to me that too much 'power' is being sent through. Reducing the pot results in less heat. If this is even a remote possibility, i would be happy to try replacing it and re-post my results.

Thank you again for your reply, its much appreciated.
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If you fit a link (to short out) across C13 that will force the bias current to zero (assuming the outputs are OK).

Have you measured the DC offset yet across the speaker output ? Should be zero volts DC give or take a few 10's of millivolts.
Hi Mooly.

Thank you, that will give me something to go off - will short C13 and check the left channel again.

I did measure the DC offset earlier on today after reading another post on this forum - with no speakers connected, i had 4.7mv through the faulty left channel and 11.8mv through the right working channel.

Thank you.
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The offset is surprisingly low for a DC fault... which is good :)

Maybe your problem is the pot (have you measured it when turned to minimum resistance, it should read as a short), or perhaps one of the diodes built into the output devices is faulty. Shorting C13 should give zero quiescent current and the outputs should be cold. The amp will still work and play but with crossover distortion present.
You got lucky - the anodising on the heatsink insulated them enough. If you remove the transistors, you should always apply fresh paste.

The bias pot adjusts the idle current. Think of it like the idle speed on a car engine. It should be set to allow just enough current through the transistors to get rid of crossover distortion.

Too much bias will result in the transistors getting hot, but doesn't explain the distortion. I would say something else is wrong rather than just simply a fault in the bias circuit. These amps are not really very robust and can go faulty especially when driven hard.
Hello again :). With your help, it appears i am making some progress - I have shorted C47 with a piece of wire (C47 being on the left channel) and temperature of the heatsink is right back down to a more than acceptable level which i guess indicates that the BIAS pot has some form of fault?

The heavy distortion however is still present on the same channel (only started after i adjusted the BIAS and removed the heatsink). I will take the PCB back out again and re-seat all the connections and the heatsink just to make sure i haven't missed anything.

Would it at all help if I recorded the distorted sound?

Also, do you know where i can find a replacement pot? It doesn't provide much information on the pot itself - would Cambridge Audio be the best place?

Thank you again :)
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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)
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 :)
If the schematic is anything to go by, i'd simply put this amp in the bin.:mad:

:( 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 :).
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.
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:( 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.:mad:
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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