Arcam Alpha 3 Left Channel broken... - diyAudio
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Old 21st April 2008, 07:17 PM   #1
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Default Arcam Alpha 3 Left Channel broken...

I have an old Arcam Alpha 3 which has served me well for the best part of 15-odd years now, but unfortunately it's not on it's way out.

Now I'm relatively electronically adept, but more of a specialist in the digital realm than analogue audio stuff, so while I can safely wield a multimeter and test various things, I'm not exactly sure what I'm really looking for with this (possibly worth getting it to a specialist, or just replace it, I know!).

Anyway, the problem is this...

It started off with a very quiet scratchy/crackly noise on the left channel (the one that's blown), which was annoying so I switched the amp off for a while.

After turning it back on the following day, all was working fine, but after a while a pop/thud was heard, and then the output from that speaker was a low-volume hum - the same sort of hum that you get with an earth conflict. After a few seconds, another pop and the music continues... an so on.

I've tested the various combinations of inputs and headphone/speaker output, and the problem remains the same - so the problem lies somewhere in the power amp circuitry.

Following on from this, the amp has now started emitting the usual 'something electronic is getting hot' smell, although on inspection of the board and components, there's nothing obviously burnt yet... but from that point I've left it off until I can work out what to do (I don't really want to power it up and damage anything else beyond repair if I can help it).

I have obtained the service manual and schematics from Arcam, and this is where my knowledge breaks down. While I can stick a multimeter on the various test points labelled on the schematic, I'm not exactly sure what I'm looking for in order to determine what's gone/going wrong.

Does anyone have any hints/ideas, or am I really going to be better off taking it to a specialist?

Wayne
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Old 22nd April 2008, 03:03 AM   #2
Leolabs is offline Leolabs  Malaysia
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Start with checking PCB solder joints first.
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Old 22nd April 2008, 10:28 AM   #3
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A close inspection of all the solder joints all appears fine. None look to be dry.
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Old 22nd April 2008, 11:28 AM   #4
Mooly is offline Mooly  United Kingdom
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If you can post the circuit, can help more !
Regards Karl
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Old 22nd April 2008, 12:32 PM   #5
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It's too large to attach, but I've uploaded the service manual including the circuit schematics here: Alpha Service Manual

Wayne
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Old 22nd April 2008, 04:23 PM   #6
Mooly is offline Mooly  United Kingdom
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Hi, Downloaded manual, will have a look and see what can come up with. If you can do this SAFELY, solder a couple of wires to a 60watt filament mains bulb and solder this in place of the mains input fuse FS201 on your diagram. Make sure the bulb is well out of the way so you can't touch it.
Regards Karl
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Old 22nd April 2008, 04:40 PM   #7
Mooly is offline Mooly  United Kingdom
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With no fuse, just the bulb wired in switch on, what happens ?
The bulb should settle to a glow, if something is getting hot try and identify. Measure the supplies to the power amps +&- 37 volts, what are they ? Is R41 and R42 getting hot on the bad channel
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Old 22nd April 2008, 04:53 PM   #8
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Interesting... any particular reason for the light bulb?
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Old 22nd April 2008, 05:06 PM   #9
Mooly is offline Mooly  United Kingdom
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The bulb limits the current that can flow, it's crude but very effective, if you had a short circuit output transistor for example it would limit any large current flows that could burn out components and even vaporise any fine PCB tracks. It's damage limitation. I am puzzled when you say that the fault was intermitant to begin with. As Leolabs says, really check for dj's, are any component legs "blackened" through oxidisation. If your not sure just run an iron over any suspect looking ones. If the solder does not flow freely round the joint this could well be the initial cause. If it's permanently duff now it is worth just measuring on ohms the output transistors to check for shorts. You have to start somewhere and if you can not identify problems with ohmeter then you are going to have to switch it on and measure and observe. If something goes pop it's not the end of the world, in fact it can be much easier than trying to guess what may or may not be wrong.
Regards Karl
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Old 22nd April 2008, 05:46 PM   #10
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Novel approach for limiting the current flow. I like it.

The intermittency was how it all started; that was until I smelt the electronic component burning up smell, at which point the amp was switched off. I have no idea if it's still intermittent or whether whatever component was breaking down is now completely gone.

So back to the job in hand... The light bulb dims down to a glow so faint I can't even see it. Voltage between TP1 and TP2 is 74.4V which is what I would expect for +/- 37V.

Resistors R42 and R41 are fine, temperature-wise, but resistor R40 is too hot to touch (there I was casually dapping my little finger on all the various resistors and transistors until I hit that one). Yowch.

The equivalent resistor R140 for the right channel is perfectly comfortable to touch.

Regards,

Wayne
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