Arcam Alpha 3 - desolation row? - diyAudio
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Old 12th September 2012, 09:35 PM   #1
tmbfca is offline tmbfca  United Kingdom
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Default Arcam Alpha 3 - desolation row?

Bought this off ebay a few years ago. Changed a fuse and got it working. It seemed immensely powerful but almost no bass response - took it to a boffin who swapped over the volume and bass pot, saying they had got reversed - one linear and one logrithmic, apparently. He thought it was ok but still no bass through my old Celestions. It has since been to France and back, unused but one channel not working now.

I can't get the old schematic up from earlier posts - complete idiot at electronics but would like to make some use of physics degree from 30 yrs ago therefore any help most welcome. Female, btw, so can you please tell me whether to plug in before testing?

Alternatively any willing helpers in the Shropshire area could be well fed with cake etc..... or even filty lucre, dammit!
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Old 12th September 2012, 11:07 PM   #2
jaycee is offline jaycee  United Kingdom
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My guess for no bass is just that age has affected some of the capacitors in there used to couple signals from one stage to another.

A non working channel could be a few things - output transistor failure, muting FET failures, or even just cracked solder joints.

What's the serial number on the unit ?
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Old 12th September 2012, 11:12 PM   #3
tmbfca is offline tmbfca  United Kingdom
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Oh thanks for replying - there's a sticker showing AL2 042869 on the back, but if you need me to look inside it'll be tomorrow before I can.
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Old 13th September 2012, 12:07 AM   #4
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The electrolytic capacitors usually go with age.
The capacitance drops blocking bass frequencies.

I dont know what your skill level is but if you change them remember they have a polarity and only go in one way.
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Old 13th September 2012, 11:11 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nigelwright7557 View Post
The electrolytic capacitors ....................... if you change them remember they have a polarity and only go in one way.
the caps can go in two ways, one way blows up, the other way works. Be very careful you understand how to determine the correct way.
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Old 13th September 2012, 12:17 PM   #6
Mooly is online now Mooly  United Kingdom
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Swapping the pots sounds a little odd ?

The Alpha III I believe has a tone defeat button so the first step is to see if the "no bass" is present with the tone stage out of circuit.

But first you have to fix the amp...
There's a good thread on that here,
Arcam Alpha 3 Left Channel broken...

Always use a bulb tester for faultfinding (100watt filament bulb in series with mains).
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Old 13th September 2012, 04:25 PM   #7
Mooly is online now Mooly  United Kingdom
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OK... so one channel is duff
First things to check are the two main power supply rails to the faulty channels output stage (page 18) and confirm that the +37 and -37 volts approx supplies are present.

Next check the DC voltage on the speaker fuse (middle of diagram at right). It should be pretty near to zero volts DC. Is the fuse blown ? Any DC voltage at either end of the fuse indicates a fault.

Lets see what that shows. Another possibility is that is the unit has been transported then it could have suffered physical damage.

We check the basics first though. Supplies present and 0.00 volts on the output is a pretty good confirmation that things are OK in the output stage.
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Old 13th September 2012, 06:53 PM   #8
tmbfca is offline tmbfca  United Kingdom
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Thank you for all your help - I will need a week or two to get my head and hands around all this - I'll come back with progress report in due course, if I haven't blown myself up
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Old 14th September 2012, 06:36 AM   #9
Mooly is online now Mooly  United Kingdom
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No problem
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