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Old 17th March 2012, 12:04 PM   #1
tomst is offline tomst  United Kingdom
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Default arcam alpha 5 amplifier right channel

Hi
I have a Arcam alpha 5 amplifier and at low volume the right channel is noticeably quieter than the left channel. once the volume is turned up the difference evens out. It happens with the direct switch on or off and is noticeable on all inputs. so far i have replaced the scratchy volume pot, cleaned the the direct switch contacts and set the the quiescent current. My thought is that one of the transistors on i's way out and it is not charging a capacitor properly until more voltage/current is passed through. Also there it an audible but not loud pop/thud when turned on and off. but this is on both channels and from the wiring diagram it looks like it only has one muting circuit that both channels go through. I have the service manual attached the amp side of the circuit diagram however the service manual only shows the left channel. I have a Multi meter and have an understanding of circuits and components but i am struggling to find what is causing the problem.
Thanks Tom
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Old 17th March 2012, 01:12 PM   #2
jaycee is offline jaycee  United Kingdom
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The muting FET Q15 has probably gone bad. The right channel will be the same as the left, probably with the component numbers +100

It's probably worth also doing a full replacement on all electrolytic capacitors at this stage too.
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Old 17th March 2012, 02:25 PM   #3
tomst is offline tomst  United Kingdom
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Thanks for the reply. I think the FETS are ok. They have continuity source-drain when turned off and show 15V gate to drain/source when amp is on.
q15 resistance amp off drain-source 3.4Ohms amp off
q115 resistance amp off drain-source 1.8Ohms amp off
q15 resistance amp on drain-source 2.8Ohms
q115 resistance amp on drain-source 1.8Ohms
if i put a ohm meter over c206 and set the scale to 200k it behaves very differently to c207. c207 increases resistance at a steady rate as expected to 28.6kohms and c206 stops at 6.6, 6.7 kohms. (I really should get a capacitor tester).
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Old 17th March 2012, 03:42 PM   #4
jaycee is offline jaycee  United Kingdom
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OK. Well on an amp this age, you are best doing a blanket electrolytic capacitor replacement anyway. Some of them are used as signal coupling capacitors and might well be causing the volume problem you're having.

This shouldn't cost your much. CPC components (CPC | CPC - Over 100, 000 products from one of the worlds leading distributors of electronic and related products.) should stock the parts you'll need - they have good quality Panasonic capacitors.
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Old 3rd May 2012, 09:16 AM   #5
stenks is offline stenks  Serbia
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Have you checked the output transistor current on R1 and R22.
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Old 3rd May 2012, 12:47 PM   #6
stenks is offline stenks  Serbia
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I have the same problem and try to solve it, I am convinced that the problem with the potentiometer, the problem I also noticed on the amplifier Arcam Alpha1. The difference between the two channels is small but perceptible to the silent playback later to equalize the difference, in my opinion somewhere in the 2 w becomes uneven playback. I partially solved the problem adjusting the quiescent current, and now the difference is smaller.
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Old 3rd May 2012, 02:32 PM   #7
jaycee is offline jaycee  United Kingdom
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The quiescent current would not affect the volume directly. If you have not renewed the electrolytic capacitors in an amp of this age, you should do so.
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Old 3rd May 2012, 04:30 PM   #8
stenks is offline stenks  Serbia
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Of course, but I was a single channel function blurred after adjustment sounds much brighter. Quiescent current measured in this channel was 2 mA and it seems to me that it should be 4 mA, have you resolved your problem I'm pretty convinced that it is still a problem with a potentiometer, did you replace electrolytes?.
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Old 3rd May 2012, 04:38 PM   #9
stenks is offline stenks  Serbia
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I'm not a good look you gave the answer, have you had a similar experience with this amplifier?
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