Recently my brothers CA 340a has started acting up, the left channel has a significant volume drop and what sounds like a 50Hz hum. There is no hum when it is used as a pre-amp (via the tape out), and there is a hum on headphones.
So can rule out the speaker conditioning bit and the input selector. However there is a lot of circuit between those two. Upon opening the amp up i found that R10 (see schematic) was burnt out. Have replaced it but problem persists. My guess is a problem in the left chip, how could i go about checking it/ any other suggestion.
I'm a mechanical engineer so all this electronics confuses my greatly have to convert each component into its hydraulic equivalent.
So can rule out the speaker conditioning bit and the input selector. However there is a lot of circuit between those two. Upon opening the amp up i found that R10 (see schematic) was burnt out. Have replaced it but problem persists. My guess is a problem in the left chip, how could i go about checking it/ any other suggestion.
I'm a mechanical engineer so all this electronics confuses my greatly have to convert each component into its hydraulic equivalent.
jaycee
diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK There is only VI limiting and no protection against DC. I agree, this is a lousy thing for Cambridge to have left out.
The Sanken SAP transistors are basically power Darlington transistors with built in temperature sensing diodes, and a built in emitter resistor of about 0.22 ohms. They were discontinued and replaced by the STD series - which is the same device, without the emitter resistors as they were prone to fail.
4 ohm loads, I would not recommend it. I don't know about the P500, but the A500 (integrated amp version) has a very poor heatsink made of a metal L-angle and bent sheet metal for fins. If you want to run 4 ohms, consider replacing the heatsink with a proper solid aluminium extrusion.
http://www.diyaudio.com/forums/solid-state/181955-cambridge-p500-question.html
If the 340a has the same Sanken output transistors, the answer may lie as above.
Some of the Cambridge amps used LM3886 chips which are pretty rugged.
diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK There is only VI limiting and no protection against DC. I agree, this is a lousy thing for Cambridge to have left out.
The Sanken SAP transistors are basically power Darlington transistors with built in temperature sensing diodes, and a built in emitter resistor of about 0.22 ohms. They were discontinued and replaced by the STD series - which is the same device, without the emitter resistors as they were prone to fail.
4 ohm loads, I would not recommend it. I don't know about the P500, but the A500 (integrated amp version) has a very poor heatsink made of a metal L-angle and bent sheet metal for fins. If you want to run 4 ohms, consider replacing the heatsink with a proper solid aluminium extrusion.
http://www.diyaudio.com/forums/solid-state/181955-cambridge-p500-question.html
If the 340a has the same Sanken output transistors, the answer may lie as above.
Some of the Cambridge amps used LM3886 chips which are pretty rugged.
Thats weird, R10 is on the input from the CD player to chip that selects the inputs, something must be very wrong if a .25Watt 100r resistor is burned out.
First I'd look at the output of the CD player and see if that is putting out way too much voltage, it should be 0VDC and a couple of volts AC with music playing. If the source is the problem we don't want more carnage. The chip itself should not be demanding significant current from the CD player, I don't know if a faulty device that the chip feeds could demand too much current from the source and the chip allow this. The NE5532 opamps are low current things and should not demand big currents if working, I think they'd be fried by currents large enough to kill that resistor.
You could bypass that selector chip and send a safe signal to C25 and C14 testing if the amp works from the input buffer onwards, this should narrow down the problem area. Any testing of a busted amp should be down with expendable speakers or headphones, whilst being sensible and unpluggin the amp when adjusting anything.
First I'd look at the output of the CD player and see if that is putting out way too much voltage, it should be 0VDC and a couple of volts AC with music playing. If the source is the problem we don't want more carnage. The chip itself should not be demanding significant current from the CD player, I don't know if a faulty device that the chip feeds could demand too much current from the source and the chip allow this. The NE5532 opamps are low current things and should not demand big currents if working, I think they'd be fried by currents large enough to kill that resistor.
You could bypass that selector chip and send a safe signal to C25 and C14 testing if the amp works from the input buffer onwards, this should narrow down the problem area. Any testing of a busted amp should be down with expendable speakers or headphones, whilst being sensible and unpluggin the amp when adjusting anything.
If you connect a signal directly to the amp use a resistor, 100r .25watt, so that if the amp is demanding insane current, the signal source won't be damaged. The resistance value is not that crucial, whereas the wattage it can take is.
Do the output devices on the heatsink smell burned?
http://www.neoentertainmentgallery.com/neox/images/products/100811122623-340inside.bmp
Do the output devices on the heatsink smell burned?
http://www.neoentertainmentgallery.com/neox/images/products/100811122623-340inside.bmp
The 340A is nowhere near the same amp as an A500. The 340A uses LM3886 chips as the power amps. These are usually fairly rugged but can die if abused.
R10 being burnt out points to something like a mains power surge having happened, but you say that the preamp stage is ok? Tape out on this amp doesn't actually seem to go through the preamp, so that could be a red herring.
R10 being burnt out points to something like a mains power surge having happened, but you say that the preamp stage is ok? Tape out on this amp doesn't actually seem to go through the preamp, so that could be a red herring.
Ok so not pre-amp but input selector ok. Source direct doesn't make any difference so tone controls unlikely to be the problem. Dry joint i can't smell any burning but it has been months since the problem occurred just haven't got round to looking at it till now.
To apply a signal to the input buffer can i just use a set of crocodile clips/probes (with a 0.25w resistor in the line) onto the legs, and what counts as a safe signal?
Thanks for the safety advice am using a pair of sony speakers i got for 50p from a car boot sale and am just about sensible enough to unplug it from the mains before doing anything.
To apply a signal to the input buffer can i just use a set of crocodile clips/probes (with a 0.25w resistor in the line) onto the legs, and what counts as a safe signal?
Thanks for the safety advice am using a pair of sony speakers i got for 50p from a car boot sale and am just about sensible enough to unplug it from the mains before doing anything.
If R10 really is burned then you can be sure the IC (the TC9163) is as well.
The only way that can burn is via something nasty being applied to the input socket. If you isolate C14 and C25 positive (L/H connections on diagram) and link them together (the positives) you can then apply a signal that should drive both L and R power amps correctly.
The only way that can burn is via something nasty being applied to the input socket. If you isolate C14 and C25 positive (L/H connections on diagram) and link them together (the positives) you can then apply a signal that should drive both L and R power amps correctly.
haven't got round to trying your suggestion mooly but a couple of developments. C45 has leaked goo, i think i must have missed this rather than it being a new development but not 100%.
In order to see if this was a new development i decided to power it up and see if it sounded the same as last night. Decided to use the DVD input for some reason and surprisingly this sounds absolutely fine, as did the MD/HD slot but the issue persists on CD, Aux and tuner. Maybe as suggested the problem lies in the TC9163.
In order to see if this was a new development i decided to power it up and see if it sounded the same as last night. Decided to use the DVD input for some reason and surprisingly this sounds absolutely fine, as did the MD/HD slot but the issue persists on CD, Aux and tuner. Maybe as suggested the problem lies in the TC9163.
Not sure that the capacitor legs will be exposed enough for a crocodile clip, one of the legs of both R38 and R40 join those capacitors at the input, that might be a better bet than the cap leg, depends of the components used.
Looking at the photo of the 340a that heatsink does not look great, it has way less surface area than DIYers and higher end amps would use with a LM3886. There will be tons on that chip in the chip amp section, as they have were popular in DIY chimp amp building for years. Car boots are a good source for old computer heatsinks that can be drilled and used for audio diy, audio DIY suppliers do gouge on heatsinks.
A new LM3886 will cost about a fiver as would the selector chip, the buffer opamp are very common and are cheaper. The semi conductor companies do free samples, that are freely available for free.
Looking at the photo of the 340a that heatsink does not look great, it has way less surface area than DIYers and higher end amps would use with a LM3886. There will be tons on that chip in the chip amp section, as they have were popular in DIY chimp amp building for years. Car boots are a good source for old computer heatsinks that can be drilled and used for audio diy, audio DIY suppliers do gouge on heatsinks.
A new LM3886 will cost about a fiver as would the selector chip, the buffer opamp are very common and are cheaper. The semi conductor companies do free samples, that are freely available for free.
The goo might be glue 🙂 It's very common in electronic manufacture to find caps secured with it. If glue it will be hard and perhaps rubbery. A leaky cap is gunge that tastes 'orrible.
You'll know 😀
OK, just looking at the circuit. C45 is 1000uF reservoir cap and could have failed. You need to be sure and replace if any doubt whatsoever. Also before fitting the new IC check that the outputs from the 3 legged voltage regulators are OK. That's very important.
I'm thinking another scenario. Has the cap failed and that in turn caused the reg to fail and so the IC. Check !
You'll know 😀
OK, just looking at the circuit. C45 is 1000uF reservoir cap and could have failed. You need to be sure and replace if any doubt whatsoever. Also before fitting the new IC check that the outputs from the 3 legged voltage regulators are OK. That's very important.
I'm thinking another scenario. Has the cap failed and that in turn caused the reg to fail and so the IC. Check !
Glue then i think (haven't tasted it though), did think this originally but as none of the others were so second guessed myself.
If the cap is bad it might be prudent to replace any identical ones, I've seen computer PCBs with multiple bad caps and the result of multiple failures is a mess. No need for fancy parts, high temp low esr parts should be better for PSUs.
It would be odd for just one cap only to be glued. Unless 100% sure (remove it and look) then you have to replace it. Given the cost and the damage that can be caused....
cambridge audio use cheap crappy capacitors, so it wouldnt surprise me if it popped. I'd replace them.
Any good sources of single caps? If not i will wait till doing my next order (planning on making my first gain clone) and order one with that.
Maplin maybe.
Worth keep checking the CPC website as they do occasional "free delivery on orders over £10".
Worth keep checking the CPC website as they do occasional "free delivery on orders over £10".
Cricklewood do fairly low del prices, they have a low esr high temp range. Rapid are good, their low ESR suntan caps are nicely priced and I think sound better than some audio rated parts. Hifi collective do boutique parts but can be okay value on small orders, they probably have Nichicon fine gold in an appropriate size. Ebay can be surprisingly good for caps, great for transistors. Your local electronics shop will order a bit for you, charge a high unit price but probably not the delivery cost you'd pay rswww or farnell. Someimes diy forums will have guys that trade bits, as enthusiasts you tend to get good deals, Mike/pink floyd at rock grotto has done caps and various bits.
- Status
- Not open for further replies.