One thing you could do to eliminate the question of cracks in the board would be to do a resistance measurement for continuity from the output to the relay input. Are there any fuses or speaker switches in this - does the headphone socket switch off the outputs when the phone jack inserted in the socket.
Well, I've been testing over the last week and I've not been able to re-create the problem!
I'm beginning to wonder if it could be something as simple as the binding posts. I'm using a quite thick cable to connect to the speakers, I'm not sure what gauge it is - the speakers themselves have nice meaty posts, but the ones on the back of the amp are a bit flimsy and the posts kind of bend over when tightened. I imagine they're designed for thinner cable.
I'm wondering if perhaps after I took the amp apart to play with the relay I didn't tighten the posts enough, or maybe the cable has been working itself loose or 'settling/compressing' after staying connected for some time which would explain my problems in the past.
So I think I'm going to leave it for now, but I plan to get some better quality speaker cable and get some banana plugs. If I get any more problems after that, I'll order the relay (nope, still haven't ordered it yet 😀) and invest in some solder wick and examine the traces while I have the amp apart.
Thank you to all for taking the time to help me out, especially you Ian, really appreciated! I will be sticking around on the forum to broaden my knowledge on amps!
I'm beginning to wonder if it could be something as simple as the binding posts. I'm using a quite thick cable to connect to the speakers, I'm not sure what gauge it is - the speakers themselves have nice meaty posts, but the ones on the back of the amp are a bit flimsy and the posts kind of bend over when tightened. I imagine they're designed for thinner cable.
I'm wondering if perhaps after I took the amp apart to play with the relay I didn't tighten the posts enough, or maybe the cable has been working itself loose or 'settling/compressing' after staying connected for some time which would explain my problems in the past.
So I think I'm going to leave it for now, but I plan to get some better quality speaker cable and get some banana plugs. If I get any more problems after that, I'll order the relay (nope, still haven't ordered it yet 😀) and invest in some solder wick and examine the traces while I have the amp apart.
Thank you to all for taking the time to help me out, especially you Ian, really appreciated! I will be sticking around on the forum to broaden my knowledge on amps!
I realize this is a very old thread, but I have something to share.
I have a CA 640A (in fact I have three 640's, two 540's and a 340) and have experienced the same symptom as the OP on one of my 640Av2.
I use this amplifier as one of a pair in the lounge TV system, one stereo amplifier per channel. I have had intermittent low output and distortion from one channel (always the left channel). The amplifiers are used mostly for TV duty, or occasionally vinyl/turntable.
I have found that when the distortion/low output occurs, turning the volume knob (remote) does not clear the problem. However, when I have been playing records, the distortion does not occur for a long time. I have put this down to the relays which determine inputs, rather than the loudspeaker ones.
If I get distortion, I just cycle through the inputs until it clears, and that always fixes it. HTH
I have a CA 640A (in fact I have three 640's, two 540's and a 340) and have experienced the same symptom as the OP on one of my 640Av2.
I use this amplifier as one of a pair in the lounge TV system, one stereo amplifier per channel. I have had intermittent low output and distortion from one channel (always the left channel). The amplifiers are used mostly for TV duty, or occasionally vinyl/turntable.
I have found that when the distortion/low output occurs, turning the volume knob (remote) does not clear the problem. However, when I have been playing records, the distortion does not occur for a long time. I have put this down to the relays which determine inputs, rather than the loudspeaker ones.
If I get distortion, I just cycle through the inputs until it clears, and that always fixes it. HTH
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The input switch is probably dirty - a squirt of contact cleaner in it ought to help
Hi, I've whittled your post down to what I think are the main points and it does seem that your conclusion could be right, as long as the output relay isn't also switching whenever you cycle through the various inputs. Either switch could still be at fault if both are operating at the same time. Note that the input switch is a surface mounted microprocessor (U4) not relays, so its operation is silent and the various states are indicated by the LEDs. If you hear a relay click, that can only be the speaker relay.
You could test this by swapping input sources externally - say, with a spare integrated amplifier or preamp that has a line-output, like the tape-out sockets and has its own input selector. Obviously, if this doesn't clear the fault, its very likely the signal switching arrangements which are not mechanical relays as previously referred to, but a surface mounted microcontroller IC, U4. That IC obviously works as a switch if it cycles through its connections and the LEDs light up accordingly. The usual suspect with microprocessors is the crystal/ceramic resonator which stops working or is erratic but if the LEDs are working normally, that's unlikely.
I suspect it could still be the speaker relay, otherwise the microprocessor circuitry rather than the micro chip itself but there are a some multi-pin connectors and ribbon cables on the main and microprcessor boards there, that may have some corrosion/grime preventing reliable contact. With power off, you might gently remove and reinsert the plugs as a cursory check. Any dust or grime on the finely divided micro's tracks could also cause problems so use a fine brush to clean up around it. You may find the service manual useful too - CAMBRIDGE AUDIO 340A SERVICE MANUAL Pdf Download | ManualsLib
Oooops! - I forgot that yours is a different Cambridge model - still has the micro, though. There are a number of revised versions and this is only one: Amended Input Circuit Schematic (Jan 2004) - Cambridge Audio 640A Service Manual [Page 40] | ManualsLib
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The 640A V2 utilises 2 switching relays for the speaker outputs.
A is default and B is switched via the remote.
To test connect your speakers to B, if the distortion is no longer present then you have confirmed the issue is being caused by worn oxidized contacts inside the relay and it will need to be replaced.
The speaker protection relays click on and off when inputs are selected except for tape monitor.
That's is the sound you can hear input switching is done electronically via the input selection IC U11 on the input / tone board.
A is default and B is switched via the remote.
To test connect your speakers to B, if the distortion is no longer present then you have confirmed the issue is being caused by worn oxidized contacts inside the relay and it will need to be replaced.
The speaker protection relays click on and off when inputs are selected except for tape monitor.
That's is the sound you can hear input switching is done electronically via the input selection IC U11 on the input / tone board.
No big deal but I think U1 is the microprocessor on the input/tone board of 640A, either version. There's no U11 anywhere among the service manual's PCB schematics linked in #29, so perhaps just a typo.
U11 is the input selection IC TC9163AF.
U1 is the micro located on the front panel board.
V2 service manual
U1 is the micro located on the front panel board.
V2 service manual
Quite so - My error. I couldn't refer to a 640A V2 manual but the similar 540 V2 manual certainly shows quite a different layout with new features and parts numbering to the first version. Its odd that it does have notes and variations for other unspecified model(s) highlighted on it, so I assume this manual is still applicable. CAMBRIDGE AUDIO AZUR 540A V2.0 SERVICE MANUAL Pdf Download | ManualsLib