Cambridge Audio A1 V3 - One Channel Missing

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T

Trailblazer

Member
Joined 2021
#1
Good Morning Everyone.

This is my first post, so hope i have it in the correct forum.

I have the above titles amp which has lost a channel.

I have traced a sine wave input as per the circuit diagram above. but i lose voltage to -inA.

I can trace the 10mV from the output of the Op-amp to where it enters the LM4766, but i Lose it at one point.

i have checked continuity and i have replaced CC33 to no avail.

i have another chip on order just in case a signal is being brought down within the chip, but other than that i can't find anything wrong.

The volume and balance control are working ok, and ive removed caps and tested them with an ESR meter and all ok.

Any help would be appreciated.
Circuit diagram came from a person on this site.

Thank You


https://lms-digital.co.uk/sites/choco/images/1234.jpg


https://lms-digital.co.uk/sites/choco/images/13363601773_1192eab047_o.gif
 
#2
The DC voltage on +IN and -IN and also the output pin of the chip should all be close to zero volts.

If you can pick up the AC sine on +IN then you should also see the same level of signal on -IN and an amplified version on the output pin.

Are you using an oscilloscope to trace the signal?

Make sure the two MUTE lines are correct as measured at the chip (be careful measuring). They should be as they appear to be in parallel.

Don't change parts randomly 🙂 its a recipe for disaster. Find the fault through measurement.
 
#3
Thank you for your reply.

I have not replaced any parts yet. Only checked each part.

What I have done with a couple of caps is switched two out of of same value to see if the fault changes.

I don't have a scope unfortunately using volt meter with millivolts scale and using a sine wave generated input set to about 10mV. This can be changed but gets to loud at speakers.

I have 10mv in at +-B terminal. Ok
But nothing on -A in. +InA is 10mV

I lose the voltage at cc33. One side it's there but the other is not but the cap is not faulty.

Can I short out cc33 to bypass it to force voltage to -in. Since it looks like a dc blocking capacitor

My electronics is a little rusty since my college days

Thanks
 
#6
Many chips have a mute capability but these are not always used by the design the chip ends up in. These look to be in permanent unmute so it would be very unlikely to be an issue there... it could only be broken print really if one channel worked and the other didn't.

CC33 aids stability and on no account must be shorted or removed. The chip might burst into destructive oscillation if you did that.

10mv AC is very very low to attempt to read on a normal DVM and DVM depending the reading may be affected by any low level DC there.

If you want to test the working theory with the good channel vs the bad channel then 10mv input (measured on C9 or C10) should give 0.34 volts AC on the speaker output line.

The gain is R26/R21 + 1 which is 34.

Measure the output on R29.

Are you sure there are no speaker feed fuses anywhere?

Are the DC conditions correct? There should be zero volts DC at R29.
 
#7
Had a look at the schematic (which does not open on my hand held device).

It is unlikely that a single channel of this dual power amp iC fails, but it may
be, for instance after shorting the output.

If you are sure that signal voltage is present on one side of CC33, but not on
the other (as opposed to CC32 of the second channel) EC7 or the power chip
U3 itself may be gone.
 
#10
Gentlemen.

Thank you for expert advice.
Tested everything and all read fine.

The IC turned up this afternoon. Fitted it and voila I have sound on both channels. Very happy man.

Cost me £5 for amp another £9 for ic so £14 I get a nice amp.

Thanks again...
 
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