Cambridge Audio 640A v2.0 Service Manual

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#22
To clarify the common cause of the Cambridge Azur 540 / 640 MK1 & MK2 protection mode operating immediately when switching out of standby mode.

The Amplifier will turn off and the protection LED will be on with a brief flash off.

The user manual states CAP5 has detected a fault which requires the unit to be serviced.
DC has been detected on the output of the amplifier which could damage the
speakers. The amplifier is now unusable.
Please switch off and contact dealer.

MK1
Check R43 / R41 150 ohm 1/4 Watt left channel and R98 / R96 on the power amplifier board, the resistors are near to the power supply smoothing capacitors and may be covered in brown glue.

R43 is commonly found to be open circuit resulting in the loss of the plus 45 Volt supply rail to the left channel.

MK2
Check R51 / R6 150 ohms 1/4 Watt on the Power Amplifier board normally R51 will be found to be open circuit.

Also check R42 100K ohm on the vertically mounted protection board.

Note: The Amplifier service manual will often show more than one R43 for example if a CTRL F is done.

To confirm the actual resistor value for the resistor number ensure the correct board is being referred to.
 
#23
Audio Service said:
The user manual for the Amplifier provides the following information.

Protection LED constantly double Flashes

The internal temperature of the output transistors has reached the over temperature limit. The unit is not damaged although it should be left for 15 minutes to cool down before being switched out of standby.

But if the protection system is operating randomly and the Amplifier is not over heating then the following should be checked.

With power and top cover removed first make a visual inspection of the Amplifier main board the one with the heat sink.

Look for brown glue which may be covering multiple components, the glue will be near to the power supply smoothing capacitors.

There are also links near to the capacitors all the brown glue covering any components or links needs to be removed.

The glue becomes corrosive and conductive and can result in spurious operation of the protection system.

Also check the bias for each channel disconnect the speakers set the volume control
to minimum and the balance treble / Bass to mid point.

Next with a multimeter set to millivolts and power up the Amplifier measure across R113 and then R78 they are raised up from the circuit board near to the output transistors.

Your meter should read 13 millivolts if it high or low adjust the respective Bias trimmer PR1 / PR2 until correct.

After initial adjustment wait 15 minutes and repeat.
Click to expand...






Thank you.
Holding the power button while plugging in the power plug helped for about half an hour but then the amplifier cut itself again so I checked the bias and it was 12mv on both channels (no speakers or source connected) after adjusting it to 13mv it didn't engage the protection again (since yesterday)....that's a bit odd as I find it hard to believe that 1mv can engage the protection circuit.


Anyways it's all good now so many thanks for your advice.
 
#24
Setting the Bias to 13 millivolts would not fix the issue our seeing.

THI Temperature sensor is mounted on the heat sink, it connects to CN6 / CN4 connectors.
U8:B is the comparator, pin 6 is Temp Sense and pin 5 is the reference, pin 7 is the output PIC Temp.
PIC Temp connects to pin 3 of CN 6 then to the PIC micro pin 17 RC6 on the front panel.
If there is no issue with brown glue covering any of the components or links on the board then check the input(s) to the comparator.
Also check for dry solder joints on R63, R68, R70, and R71
 
#25
rob100961 said:
I have a Cambridge Audio Azur 540A Amplifier (recently bought), I'm not sure what version it is, but it has a fault. Where is the version written?

From what I can recall there's hardly any output through the speakers, whichever input is being used. More importantly R48 and R56 get "very hot", and are cooked - I think they may even smoke a bit. See attached picture.

I can't see what value they are, so if anyone knows I'd be grateful. I see further up this post it's recommended to get a higher wattage metal film replacements, so I'll make sure they are. Is it worth replacing anything else at the same time? I see other contributors have said it's a lot of work to get the board out, so would be good to do it in one hit 🙂

I'll try and download the manual too - hopefully the correct one.

Thanks, Rob
Click to expand...

@rob100961 could you please give me the R48 and R56 values ?

Thank you
 
#28
Hi,
I have 540a v2.0 w/ protection LED constantly double flashing. Found out failed U8:A (comparator), there was a shorted pin 2 to Vcc. As soon as I replaced it, nothing changed, still double flashing. Some time later, after long searches, I found several pins of PIC like cut-off, w/ infinite resistance. They were as follows: #17 (Temp), #18 (VI short), #25 (Speaker A relay). I believe that this is a final reason of double flashing. Then I ordered the new PIC and a programmer, but I will need firmware tor this PIC. Does anybody can help with it?
 
#29
while listening music with my azur 640a v2 the protection light went on suddenly (short flash once, so dc-fault).
long story shortly told: the culprit was a small resistor (100k) close to the psu-elcaps
in brown glue............
foto with new res.
 

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#32
hello guys. I recently bought a Cambridge 640a v2 amplifier and I have a problem with a flashing diode. When you turn it on, you hear a click and after a while it goes into standby. the diode signal shows fault requiring service DC. I checked R42, I'm posting photos of the measurements.
IMG_0855.JPG
IMG_0858.JPG
IMG_0857.JPG
IMG_0856.JPG

it gives different values with each measurement, could this be the reason?
 
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