Cambridge audio A500 amp with broken right channel - diyAudio
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Old 26th April 2009, 10:47 PM   #1
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Default Cambridge audio A500 amp with broken right channel

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
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Old 26th April 2009, 11:16 PM   #2
jaycee is offline jaycee  United Kingdom
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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.
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Old 26th April 2009, 11:29 PM   #3
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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
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Old 27th April 2009, 12:56 AM   #4
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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
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Old 27th April 2009, 09:46 AM   #5
jaycee is offline jaycee  United Kingdom
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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.
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Old 27th April 2009, 10:14 AM   #6
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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
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Old 27th April 2009, 10:30 AM   #7
jaycee is offline jaycee  United Kingdom
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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.
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Old 7th May 2009, 04:43 PM   #8
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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
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Old 7th May 2009, 06:06 PM   #9
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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
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Old 7th May 2009, 07:17 PM   #10
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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
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