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Old 18th March 2013, 03:38 AM   #21
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The reason I recommended you bin it is your amplifier is oscillating, which is the reason for the symptoms you've described.
A few seconds thought will tell you that any amplifier which works fine for some years and then begins to show increasing
overheating with deteriorating sound quality, has developed a fault - most likely a component or latent assembly fault.

That fault will be fixable and the condition effectively eliminated, whether it has thermal instability, damaged output devices,
oscillation or any typical fault. The 640 may not be one of D. Self's designs but we can assume it was sufficiently competent
to invest large sums in global commercial distribution. I see no reason to scrap what was said to be a satisfying, somewhat
expensive product, only because it no longer works properly.

The essence of DIY is, of course, to provide that fix for yourself even though depending on others for the methodology.
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Old 18th March 2013, 07:48 AM   #22
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That's good that the speakers are OK

Well I must admit this doesn't fit the normal fault patterns seen. My instincts at the moment say this is still a problem with those compound output devices. If one of the internal resistors went OC you could get weird effects.

I can see there is doubt over the bias pot. Whether faulty or not, with C47 shorted the amp should work but lets try and prove that. If you short C15 on the good channel, is the sound still OK (there will be some distortion at low levels but it should not be terrible)

The constant current sink of the driver stage should be checked as well. Measure and compare readings between channels of the voltage across R55, R56 and D9.
D9 should have around 3.3 volts across it and R55 around 2.6 volts. R56 I would guess would have around 0.4 of a volt (but compare with the other channel)

Not sure if this has been mentioned before but also check that R52 and R69 (the 0.33 ohms) have not gone high in value. Check with the amp OFF of course. Also check that with the amp ON, that the correct DC voltages (approx. plus and minus 42 ish volts) are present on the left hand sides of R96 and R98. First rule... always check the supplies are correct
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Old 18th March 2013, 08:28 PM   #23
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jaycee; "To be fair, the circuit isn't bad.. what's usually the problem with Cambridge gear is the "Made in China" cost cutting."
This doesn't surprise me re. being made in China - have gone for a Denon this time around, not quite as pretty but seems a lot better build quality on the face of it (not sure how the internal components compare).

Quote:
Harleyjon; "Replacing the pots and setting the bias to the correct Iq after 10 minutes with no signal, I feel will fix your issue."
Thank you, will get a replacement ordered based on yours and jaycee's advice earlier. Will do some more research before i go ahead to make sure i do it correctly.
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Ian Finch; "The essence of DIY is, of course, to provide that fix for yourself even though depending on others for the methodology."
Thank you for your support There is a sense of satisfaction when you can get something working yourself so i am eager to continue with the repair and hopefully, some day i will be able to provide help to others.

Quote:
Mooly; "I can see there is doubt over the bias pot. Whether faulty or not, with C47 shorted the amp should work but lets try and prove that. If you short C15 on the good channel, is the sound still OK (there will be some distortion at low levels but it should not be terrible)"
Hello again . I think i must have 2 separate problems - first being the pot which is causing the over-temp and an issue with the distortion - its just seems strange that the audio was working fine until i adjusted the pot (bar the occasional cutting out of the left channel when it got hot). So maybe they are related, maybe i am just thinking wishfully .


Quote:
The constant current sink of the driver stage should be checked as well. Measure and compare readings between channels of the voltage across R55, R56 and D9.
D9 should have around 3.3 volts across it and R55 around 2.6 volts. R56 I would guess would have around 0.4 of a volt (but compare with the other channel)

Not sure if this has been mentioned before but also check that R52 and R69 (the 0.33 ohms) have not gone high in value. Check with the amp OFF of course. Also check that with the amp ON, that the correct DC voltages (approx. plus and minus 42 ish volts) are present on the left hand sides of R96 and R98. First rule... always check the supplies are correct
Will get the results and post back. Thank you again. Everyone here has been fantastic.


Thought i would also quickly upload a recording of the audio changing from right channel to the left channel. Same files just of different sites if anyone has a preference.
Upload Files, MP3, Music. Free File Hosting To Share Files
http://www.filedropper.com/ca640-distortion


Think this post is long enough now so time I actually get testing things again . Thank you. Nick
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Old 18th March 2013, 08:46 PM   #24
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The increased temperature could have been caused by a faulty pot, that then, over time, caused something else to fail because of the excessive heat. This could be a pot, a capacitor or even a dry solder joint etc that gets worse from larger expansion/contraction forces experienced from the increased temperature.

I too am skeptical towards the bias pot causing the distortion, as shorted out, it isn't part of the circuit any more, but we shall see. If it is a bad solder joint, replacing the pot might fix it, if only because everything got moved around a bit physically. You can always try tapping components with a plastic pen, when music is playing, and see if that makes any difference.
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Old 18th March 2013, 10:17 PM   #25
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You can always try tapping components with a plastic pen, when music is playing, and see if that makes any difference.
Haha, i have already tried this method last night - not wanting to damage anything further though i just gave various components a slight wiggle with the back end of a pencil - i assumed that if it was a bad solder joint, i might get something cutting out.

I think your theory regarding the overheating causing a secondary problem is quite plausible and i understand that bypassing the pot should get it working providing this is the sole problem - as it 100% fixes the heat problem, i can safely assume that the bypass is working and therefore i require a new pot. But yes, you are completely right in that there must be a secondary issue somewhere.

Is there any other tools that i could buy to help test this further or is just a matter now of comparing left vs right channel voltages until i stumble upon something that doesn't quite look right?

Here are some further figures;
R55 : 2.69v
R56 : 0.42v
D9 : 3.25v
R96 : 2.74v
R98 : 2.3v
R52 : 0v
R69 : 0v

Mooly - i am impressed, the figures are as near as identical to what you mentioned - i assume these are relatively universal figures?

The other channel show the exact voltages (give or take a few mv).

Please could i ask you to clarify the following;
"Also check that with the amp ON, that the correct DC voltages (approx. plus and minus 42 ish volts) are present on the left hand sides of R96 and R98"

I wasn't quite sure what was meant by measuring the left hand side of R96 and R98 so i measured across each one individually but as the above results show, neither are anywhere near 42v so i have come to the conclusion i must have done this bit wrong .

Thank you
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Old 19th March 2013, 07:27 AM   #26
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The distortion recording is strange in that it cycles between good and bad... very odd.

The DC voltages are fine (which makes this all the more unusual). The voltages are pretty easy to calculate given that the amp is made up of "basic building blocks".

Voltages on R96 and 98 will be correct, those readings sound like the voltage across them. I really meant the voltage measured from ground to those resistors. There will be full supply on one end (R/H) and slightly reduced supply on the other end of each.

So where does that leave us ? Ideally a scope should be used to just confirm that there is nothing really weird going on but without we have to guess a little bit.

I still think the output devices could be suspect and should be replaced, possibly along with Q32 and Q25.

Did R52 and R69 read OK on the ohms range on your meter ? (with the amp OFF)
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Old 19th March 2013, 04:50 PM   #27
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The distortion recording is strange in that it cycles between good and bad... very odd.
Sorry, think I should clarify, this was me changing the balance dial on the front of the amp between the working right channel and the faulty left channel to demonstrate the issue sorry if this caused confusion.

& sorry again, that's my lack of knowledge/understanding here giving you the wrong information. Just got back from work so will go test again . I will post back asap.
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Old 19th March 2013, 05:16 PM   #28
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Well this will teach me to work with electronics when I am tired - I think I just well and truly killed it (I hope not but feel for the worst). Whilst trying to measure R96 to Ground i *think* I have bridged R96 and LK10 which resulted in a pop and the circuit protection kicking in. I disconnected power, powered back on and it instant loops into protection mode and now getting a huge amount of heat to the smaller heatsink to the left near C58 and C57. Unplugged immediately after I noticed (5/6 seconds) but made some very strange noises through the speakers during this.

So, clearly I have made things a lot worst and now in a position where I can test very little which i feel really bad for as everyone has spent a lot of time with me which i now feel i have wasted.

What is the likelihood this can be 'easily' solved and get me back to the previous state?

There are no obvious components blown on the board but i assume i have blown a resistor somewhere close to either R96 or LK10.
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Old 19th March 2013, 06:05 PM   #29
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Originally Posted by nickalmond View Post
Well this will teach me to work with electronics when I am tired - I think I just well and truly killed it (I hope not but feel for the worst). Whilst trying to measure R96 to Ground i *think* I have bridged R96 and LK10 which resulted in a pop and the circuit protection kicking in. I disconnected power, powered back on and it instant loops into protection mode and now getting a huge amount of heat to the smaller heatsink to the left near C58 and C57. Unplugged immediately after I noticed (5/6 seconds) but made some very strange noises through the speakers during this.

So, clearly I have made things a lot worst and now in a position where I can test very little which i feel really bad for as everyone has spent a lot of time with me which i now feel i have wasted.

What is the likelihood this can be 'easily' solved and get me back to the previous state?

There are no obvious components blown on the board but i assume i have blown a resistor somewhere close to either R96 or LK10.
Do not worry! you are not the first one that made this mistake, I know for sure many people that are "technicians" have made mistakes like this. so do not feel bad about it. What I would do know since now it is more difficult to fix it for you, I would use it for parts and I would "build" one from scratch like something not too complicated like a Chipamp LM3875/LM3886 or another amp kits. it sounds complicated, but if so many newbies have done it...so can you!!!
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Old 19th March 2013, 07:05 PM   #30
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Oh heck

This doesn't sound promising. LK10 looks like it goes to the PIC (the microprocessor or uP) so applying -45 volts to a PIC input is probably going to zap it.

To prove it all would involve making the output stages "safe" such that nothing gets hot and then seeing if all the input and logic works OK. Possibly also forcing the "protect" data lines to some known logic level to fool the uP that all is OK.

The power amps can be fixed easily no matter what you have done... its the other damage to the processing circuitry that will be the problem. The uP would have to come from Cambridge as it is programmed specifically for this model.

All that is assuming the worst of course...
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