Hi everyone
I have a broken A500 amp that someone else was giving away. The left channel works perfectly, but the right channel outputs a low hum that does not change when the volume knob is turned.
I have tried different sources/inputs and all give the same problem. I'm fairly confident I'd be able to fix it if I could diagnose exactly what is wrong with it. I'm guessing that there isn't anything wrong with the power amp stage as this is producing a hum, so I'm guessing that there could be something wrong with the preamp?
I don't know that much about amplifiers but I'm pretty good with electronics in general, I have a multimeter and I can solder and I understand resistors, capacitors, inductors etc (doing a physics degree). I was just wondering if someone with experience with this sort of thing could point me in the right direction of where the fault might lie.
Thanks!
Sam
I have a broken A500 amp that someone else was giving away. The left channel works perfectly, but the right channel outputs a low hum that does not change when the volume knob is turned.
I have tried different sources/inputs and all give the same problem. I'm fairly confident I'd be able to fix it if I could diagnose exactly what is wrong with it. I'm guessing that there isn't anything wrong with the power amp stage as this is producing a hum, so I'm guessing that there could be something wrong with the preamp?
I don't know that much about amplifiers but I'm pretty good with electronics in general, I have a multimeter and I can solder and I understand resistors, capacitors, inductors etc (doing a physics degree). I was just wondering if someone with experience with this sort of thing could point me in the right direction of where the fault might lie.
Thanks!
Sam
First thing, measure the DC voltage on the channel thats buzzing. If it's any more than a few hundered millivolts then the power amp has a fault.
If you want to rule out the power amp, you can disconnect the signal input to it - the power amp is on a smaller board which connects to the preamp through a 3 pin cable - you can feed audio straight from a volume controlled line out source (eg your computer soundcard) here and check it.
It's usually the power amp that goes wrong on these amps - they blow up because of insufficient heatsinking, and people turn the volume up at a party, and *bang*, the ouput transistors go.
If you search on here you should find the service manual as PDF - if not send me a mail and i'll send it to you.
If you want to rule out the power amp, you can disconnect the signal input to it - the power amp is on a smaller board which connects to the preamp through a 3 pin cable - you can feed audio straight from a volume controlled line out source (eg your computer soundcard) here and check it.
It's usually the power amp that goes wrong on these amps - they blow up because of insufficient heatsinking, and people turn the volume up at a party, and *bang*, the ouput transistors go.
If you search on here you should find the service manual as PDF - if not send me a mail and i'll send it to you.
Whoa you're absolutely right, on the broken channel (actually the left channel is broken, the right channel works) there is a +35 volts between the black and red outputs. Does that mean a busted output transistor?
Both of the internal LEDs come on, don't know if that means anything?
I downloaded the schematic and I will try and make sense of it but I am not an electronics engineer unfortunately so I will probably need a bit of guidance.
Sam
Both of the internal LEDs come on, don't know if that means anything?
I downloaded the schematic and I will try and make sense of it but I am not an electronics engineer unfortunately so I will probably need a bit of guidance.
Sam
I've had a good poke around with a multimeter at the PNP and NPN of the broken channel.
The PNP on the broken side conducts e-s whereas the working channel PNP doesn't. I'm guessing the broken transistor has short-circuited as it failed?
Any other pins I need to test across? Is this failure likely to have taken out any other components with it?
Sam
The PNP on the broken side conducts e-s whereas the working channel PNP doesn't. I'm guessing the broken transistor has short-circuited as it failed?
Any other pins I need to test across? Is this failure likely to have taken out any other components with it?
Sam
What usually happens is the transistors have an inbuilt emitter resistance, and this fails. It was a design flaw of this particular transistor.
Usually, just the output transistors fail. However it is worth testing the bias trim pot (RV201 for the left channel) and the capacitor across it (C238).
Sometimes the VAS transistors or the current source feeding it get damaged. These are Q214 Q219 and Q218. All pretty easy to find parts.
Personally I would just replace the output transistors first (you can get them from Rapid or Profusion).
One thing you should do is to make up a cable that contains a bulb socket in series with the Live wire. Search this site for details on that. When you first power up the amp after replacing the transistors, install a 60W bulb in this cable and power it through that. The bulb acts as a current limiter and will glow brightly if there is a fault.
Usually, just the output transistors fail. However it is worth testing the bias trim pot (RV201 for the left channel) and the capacitor across it (C238).
Sometimes the VAS transistors or the current source feeding it get damaged. These are Q214 Q219 and Q218. All pretty easy to find parts.
Personally I would just replace the output transistors first (you can get them from Rapid or Profusion).
One thing you should do is to make up a cable that contains a bulb socket in series with the Live wire. Search this site for details on that. When you first power up the amp after replacing the transistors, install a 60W bulb in this cable and power it through that. The bulb acts as a current limiter and will glow brightly if there is a fault.
Thanks jaycee that was very informative, I greatly appreciate your help on this.
I'll make the bulb thing, although surely if there is I simply test for a large AC or DC voltage on the output, that would also indicate whether it is broken or not?
How should I test the bias trim pot? What is indicative of failed or working? What sort of resistance should it have? I think I can figure out how to test a capacitor although a bit of further advice on this would be great.
Thanks again for your help, I'll order some replacement SAP16s today and see how it goes.
Sam
I'll make the bulb thing, although surely if there is I simply test for a large AC or DC voltage on the output, that would also indicate whether it is broken or not?
How should I test the bias trim pot? What is indicative of failed or working? What sort of resistance should it have? I think I can figure out how to test a capacitor although a bit of further advice on this would be great.
Thanks again for your help, I'll order some replacement SAP16s today and see how it goes.
Sam
The bulb is more to limit the current through the circuit so that it doesn't cook parts if there is still a fault. The bulb lighting brightly is merely a handy side effect 🙂 If there is a fault causing loads of current to be drawn, the bulb lights and drops most of the voltage over it, thus limiting the damage.
The trimpot is quite easy, just measure that it has resistance still. If it is open circuit, it is faulty. If it reads more than the 100 ohms maximum of the part, it is faulty. If it tests OK, set it to minimum resistance before replacing the SAPs
After replacing the SAPs do a DC check (voltmeter between speaker outputs) and check that youre reading no more than about 50mV. More than that suggests a fault. Compare with the good channel.
If all is good, and the bulb isn't glowing (it may glow for about a second on first power up - that's the power supply capacitors charging), you now need to set the bias on the channel you just repaired.
The service manual does explain it, but it's a bit too terse - adjust the trimpot so that 13mV appears between the S and E pins of one of the transistors - either will do. You do this with no speakers or source connected. You should adjust for 13mV and allow the amp to warm up for about half an hour, then adjust again. You might want to tack solder two wires to the S and E pins rather than trying to hold multimeter probes to them.
The trimpot is quite easy, just measure that it has resistance still. If it is open circuit, it is faulty. If it reads more than the 100 ohms maximum of the part, it is faulty. If it tests OK, set it to minimum resistance before replacing the SAPs
After replacing the SAPs do a DC check (voltmeter between speaker outputs) and check that youre reading no more than about 50mV. More than that suggests a fault. Compare with the good channel.
If all is good, and the bulb isn't glowing (it may glow for about a second on first power up - that's the power supply capacitors charging), you now need to set the bias on the channel you just repaired.
The service manual does explain it, but it's a bit too terse - adjust the trimpot so that 13mV appears between the S and E pins of one of the transistors - either will do. You do this with no speakers or source connected. You should adjust for 13mV and allow the amp to warm up for about half an hour, then adjust again. You might want to tack solder two wires to the S and E pins rather than trying to hold multimeter probes to them.
Well I'm in the process of changing the faulty transistors and there seems to have been a thin piece of plastic inserted between the metal transistor back plate and the heat sink. This is present on all the transistors. Surely this can't help thermal conductivity, but presumably it was put there for a reason?
Any ideas? Should I replace the plastic insulator pieces or not bother?
Sam
Any ideas? Should I replace the plastic insulator pieces or not bother?
Sam
Nooooooo
I put the new transistors in, turned it on and the balance knob made sparks and a fizzing noise :-(. Why oh why didn't I make up one of those bulb tester wire things?
Is it completely destroyed now?
Sam
I put the new transistors in, turned it on and the balance knob made sparks and a fizzing noise :-(. Why oh why didn't I make up one of those bulb tester wire things?
Is it completely destroyed now?
Sam
Ok I figured out what happened.
I accidently connected two plugs the wrong way round when reassembling the amplifier after removing the power amp circuit board.
Near the balance knob it has a socket labelled "Cn102" and one labelled "Cn104".
According to the schematic, one carries +30v,gnd,gnd,-30v and one carries Rchout,gnd,gnd,Lchout. I think I got these the wrong way around :-(. Unfortunately this probably means I dumped +/-30v into the balance knob on one side and +/-30v into the Rch/Lch inputs on the power amp.
Presumably this means that just about everything is likely to be fried? Please tell me this is fixable?
Sam
I accidently connected two plugs the wrong way round when reassembling the amplifier after removing the power amp circuit board.
Near the balance knob it has a socket labelled "Cn102" and one labelled "Cn104".
According to the schematic, one carries +30v,gnd,gnd,-30v and one carries Rchout,gnd,gnd,Lchout. I think I got these the wrong way around :-(. Unfortunately this probably means I dumped +/-30v into the balance knob on one side and +/-30v into the Rch/Lch inputs on the power amp.
Presumably this means that just about everything is likely to be fried? Please tell me this is fixable?
Sam
Schematic is available here: http://78.105.125.218/stuff/CambridgeAudioA500AmplifierServiceSheets.pdf
Looks like I put +/-30v straight into the output of the unity gain amplifier. I think this is probably very likely to fry it but if someone can suggest something otherwise I'd be very happy.
Sam
Looks like I put +/-30v straight into the output of the unity gain amplifier. I think this is probably very likely to fry it but if someone can suggest something otherwise I'd be very happy.
Sam
Yes, you will have caused quite some damage there 🙁
The pot will be fried and you'll need a new one. As you suspect the opamps will probably be stuffed too.
The power amp might be okay.. you could test that by feeding some attenuated source (cd walkman or pc output for example) into it directly. You may have damaged the 1uF nonpolar cap (C201/C226) and/or Q204/212... these arent high cost parts though.
The bulb tester would have saved the smoke and sparks yes. Work on it in 2 parts - first look at the power amps. If they work, check the preamp seperately - you can get away with feeding it's output into a pair of headphones as a test, make sure the opamps arent putting out full 15V though due to damage first.
I can see how you have done this tbh - the service manual doesnt have ANY indication of which connectors connect to what!
The pot will be fried and you'll need a new one. As you suspect the opamps will probably be stuffed too.
The power amp might be okay.. you could test that by feeding some attenuated source (cd walkman or pc output for example) into it directly. You may have damaged the 1uF nonpolar cap (C201/C226) and/or Q204/212... these arent high cost parts though.
The bulb tester would have saved the smoke and sparks yes. Work on it in 2 parts - first look at the power amps. If they work, check the preamp seperately - you can get away with feeding it's output into a pair of headphones as a test, make sure the opamps arent putting out full 15V though due to damage first.
I can see how you have done this tbh - the service manual doesnt have ANY indication of which connectors connect to what!
I just noticed Post 8.
These are mica insulators. They are there because the metal backplate of the transistor is also connected to the Collector pin of the transistor. This means that, without insulation, you would have a short between the collectors of the NPN and PNP transistors - which would directly short out the power supply.
The mica's should be in place, with thermal paste (the white stuff) being used to transfer the heat (thin layer of paste on BOTH sides of the mica strip).
I am guessing you did put it back, or you wouldve had one hell of a bang.
These are mica insulators. They are there because the metal backplate of the transistor is also connected to the Collector pin of the transistor. This means that, without insulation, you would have a short between the collectors of the NPN and PNP transistors - which would directly short out the power supply.
The mica's should be in place, with thermal paste (the white stuff) being used to transfer the heat (thin layer of paste on BOTH sides of the mica strip).
I am guessing you did put it back, or you wouldve had one hell of a bang.
I connected an ipod directly to the power amp input and both channels worked!
So it looks as though only the balance knob and preamp are melted. Is it possible I could get replacement parts for these?
How can I tell if the 1uF nonpolar cap on the power amp board is busted?
Thanks for your patient help Jaycee
Sam
So it looks as though only the balance knob and preamp are melted. Is it possible I could get replacement parts for these?
How can I tell if the 1uF nonpolar cap on the power amp board is busted?
Thanks for your patient help Jaycee
Sam
Interesting...
After studying the circuit diagram for a while, and now knowing the power amp works ok, it seems as though the only damage that can have been caused is from the application of +30v,gnd,gnd,-30v to the CN104 socket.
While at a first glance I thought that must have been fatal, the CN104 socket appears to lead only to the preamp output (CN6), which I don't actually care about much. The preamp output is isolated from everything else as far as I can see, (except the +/-15v supply but I doubt that could have been damaged much).
Which leaves only the left hand side of the CN104 socket on the circuit diagram. RCH and LCH were replaced by +/-30v. The balance knob sparked and fizzed when I turned it on, so I'm guessing that a large current flowed through the balance knob (it was turned to one side, so resistance to ground would be lower on that side) to ground and melted it. The only other path for voltage to flow is through the volume knob, which was set to minimum (so maximum resistance between it and the rest of the tone control circuit I am hoping). Hopefully, from what I can see, this means that only the balance knob and possibly the volume control is damaged due to the large current flowing through one or both of them to earth.
Thoughts on this analysis? I will poke about with an ohmmeter and have a look at the balance knob and volume control.
Sam
After studying the circuit diagram for a while, and now knowing the power amp works ok, it seems as though the only damage that can have been caused is from the application of +30v,gnd,gnd,-30v to the CN104 socket.
While at a first glance I thought that must have been fatal, the CN104 socket appears to lead only to the preamp output (CN6), which I don't actually care about much. The preamp output is isolated from everything else as far as I can see, (except the +/-15v supply but I doubt that could have been damaged much).
Which leaves only the left hand side of the CN104 socket on the circuit diagram. RCH and LCH were replaced by +/-30v. The balance knob sparked and fizzed when I turned it on, so I'm guessing that a large current flowed through the balance knob (it was turned to one side, so resistance to ground would be lower on that side) to ground and melted it. The only other path for voltage to flow is through the volume knob, which was set to minimum (so maximum resistance between it and the rest of the tone control circuit I am hoping). Hopefully, from what I can see, this means that only the balance knob and possibly the volume control is damaged due to the large current flowing through one or both of them to earth.
Thoughts on this analysis? I will poke about with an ohmmeter and have a look at the balance knob and volume control.
Sam
Some notes:
When powered on, the input on the back of the amp seems to make no difference at all to what the speakers do, no sound is transmitted.
There is quite a loud thump noise on power on and power off, could be that the amp does this naturally though. When turning up and down the volume knob, it seems to generate a small dc current, the speaker cone moves in or out as it is turned then comes back to rest when it is stopped. As the volume knob is turned up, the hiss from the speakers definitely becomes louder, so I don't think the volume knob can be broken.
I suppose on second (third?) thoughts the amplifying op amps for the preamp (and accompanying capacitor?) could have been damaged. These look to be NE5532 chips. Are replacements for these easily obtainable?
While I can get my head around the electronics here, what I really lack is experience in what symptoms should I be looking for to try to find out where the problem lies.
I had a look at the preamp outputs (the wires that usually go into the power amp). When the amp is switched on (with nothing plugged in) they have 0 volts across them with the volume knob set at 0. As the volume knob is turned up, the DC voltage across the outputs increases up to about 1-1.5 volts. With the amp powered off, the resistance across the preamp outputs drops to a few ohms with the volume knob at zero, increasing up to about 3Kohms at full volume. I haven't checked with headphones, but knowing that the power amp works ok, no sound from the inputs is reproduced at all on the power amp outputs.
What should I make of this? Is all this normal?
Sam
When powered on, the input on the back of the amp seems to make no difference at all to what the speakers do, no sound is transmitted.
There is quite a loud thump noise on power on and power off, could be that the amp does this naturally though. When turning up and down the volume knob, it seems to generate a small dc current, the speaker cone moves in or out as it is turned then comes back to rest when it is stopped. As the volume knob is turned up, the hiss from the speakers definitely becomes louder, so I don't think the volume knob can be broken.
I suppose on second (third?) thoughts the amplifying op amps for the preamp (and accompanying capacitor?) could have been damaged. These look to be NE5532 chips. Are replacements for these easily obtainable?
While I can get my head around the electronics here, what I really lack is experience in what symptoms should I be looking for to try to find out where the problem lies.
I had a look at the preamp outputs (the wires that usually go into the power amp). When the amp is switched on (with nothing plugged in) they have 0 volts across them with the volume knob set at 0. As the volume knob is turned up, the DC voltage across the outputs increases up to about 1-1.5 volts. With the amp powered off, the resistance across the preamp outputs drops to a few ohms with the volume knob at zero, increasing up to about 3Kohms at full volume. I haven't checked with headphones, but knowing that the power amp works ok, no sound from the inputs is reproduced at all on the power amp outputs.
What should I make of this? Is all this normal?
Sam
The power amp does thump on turn off. This is normal. Most good power amps have a power on/off mute circuit to prevent this (as well as protect against output failures damaging speakers!) but the A500 doesnt.
You've already confirmed the power amp works 🙂 If the 1uF input caps were toasted you would have got poor or no sound from the power amp.
It's likely you cooked the balance pot. There's probably a nice hole in the track now. You could jumper it out if you weren't too bothered, but it should be easy to get a 100K linear potentiometer. You'd need to check the size though. At a guess it's a 16mm type.
NE5532's - can get those anywhere, for about 35p 🙂 The question is which one got fried. If the Tone Defeat switch was in, U105 got it. If it was out, U102 got it. It might be best to replace both of them. Make sure the power supplies for them are still OK - there are seperate regulators per chip. There should be -15V on pin 4 of the NE5532, and +15V on pin 8. I doubt the regulators will be damaged but it's best to check.
You've already confirmed the power amp works 🙂 If the 1uF input caps were toasted you would have got poor or no sound from the power amp.
It's likely you cooked the balance pot. There's probably a nice hole in the track now. You could jumper it out if you weren't too bothered, but it should be easy to get a 100K linear potentiometer. You'd need to check the size though. At a guess it's a 16mm type.
NE5532's - can get those anywhere, for about 35p 🙂 The question is which one got fried. If the Tone Defeat switch was in, U105 got it. If it was out, U102 got it. It might be best to replace both of them. Make sure the power supplies for them are still OK - there are seperate regulators per chip. There should be -15V on pin 4 of the NE5532, and +15V on pin 8. I doubt the regulators will be damaged but it's best to check.
Hi again Jaycee, thanks for the help
There are two metal shiny shell things near the balance controls, I'm guessing that under these are hiding some op amps. I'll see if I can prise them off with a screwdriver or something and replace the NE5532s underneath it as well as checking the 15v supply.
I'll see about just shorting the balance controls to the preamp for now, a pot for balance isn't something I use often and I can always fix that later after I actually have sound coming out of this thing.
If any part of the power amp board is broken, will the sound quality difference be immediate and obvious?
Thanks again for the advice, I wouldn't have got anywhere at all without you and my understanding of how these work is getting better all the time. I'll post back after I've had a go at replacing the op amps and the balance pot.
Sam
There are two metal shiny shell things near the balance controls, I'm guessing that under these are hiding some op amps. I'll see if I can prise them off with a screwdriver or something and replace the NE5532s underneath it as well as checking the 15v supply.
I'll see about just shorting the balance controls to the preamp for now, a pot for balance isn't something I use often and I can always fix that later after I actually have sound coming out of this thing.
If any part of the power amp board is broken, will the sound quality difference be immediate and obvious?
Thanks again for the advice, I wouldn't have got anywhere at all without you and my understanding of how these work is getting better all the time. I'll post back after I've had a go at replacing the op amps and the balance pot.
Sam
don't prise them up with anything, just unsolder them, they can be left off if you wish, also buying some sockets for the opamps would be a good idea, no risk of heat damage then!
thanks
thanks
Well I took out the balance pot and just connected a straight wire from the rh and lch inputs to the volume knob. Still nothing with the direct button pushed in or left out. I guess this means all the op amps are likely to be broken. I will replace them all.
EDIT: After taking it apart it appears that there are only two ne5532 chips (one for each channel) the direct button must simply switch in or out the tone control circuitry, not switch between ne5532 chips.
Here's something interesting - at full volume the resistance across the volume knob is ~2k for one channel and ~125ohm for the other. Is this normal? Does it mean the volume pot got fried too?
Sam
EDIT: After taking it apart it appears that there are only two ne5532 chips (one for each channel) the direct button must simply switch in or out the tone control circuitry, not switch between ne5532 chips.
Here's something interesting - at full volume the resistance across the volume knob is ~2k for one channel and ~125ohm for the other. Is this normal? Does it mean the volume pot got fried too?
Sam
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