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Old 30th May 2010, 01:48 AM   #1
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Question A&R CAMBRIDGE ARCAM ALPHA problems.

Hi all

New user, first post.

Am having trouble with my Arcam Alpha. The right channel plays at very low volumn compared to the left and has slight distortion. I have run through the basic trouble shooting, eliminating source, speakers and cabling. The problem exists on the headphone output also.

I am a novice in regards to techinicals, but have a keen interest and know its time to learn. Am i right in thinking ,the fact it exists on the headphone stage means its a preamp issue? Or is its likely cause an output cap?

Any info appreciated.
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Old 30th May 2010, 03:30 AM   #2
amc184 is offline amc184  New Zealand
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I've got both an Alpha and an Alpha 2, they're great amps and good value here in New Zealand.

Quote:
Am i right in thinking ,the fact it exists on the headphone stage means its a preamp issue? Or is its likely cause an output cap?
No to both actually. In this amp the headphone and speaker outputs are one and the same, so it could be either a pre or power amp problem. The output stage in the Alpha is DC coupled, so no output cap.

The best way to troubleshoot an amp with one bad channel is to compare it with the other, working channel. What test gear do you have?
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Old 30th May 2010, 03:49 AM   #3
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[What test gear do you have?[/QUOTE]

Thanks alot for your reply.

I have a volt meter and access to an oscilioscope(tho working it is another thing)
There are no obvious issues with the circuitry, in fact everything "looks" in great condition.
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Old 30th May 2010, 06:48 AM   #4
amc184 is offline amc184  New Zealand
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Okay, start by tracing the signal through the amp and see where something's going wrong.

Plug in a source to one of the line level inputs, play it some music, leave the outputs unconnected. Measure the AC voltage to ground on each channel at various points to see where the signal is being attenuated on one channel. I would suggest:

- one side of resistor R9 / R109 (input after selector switch)
- testpoints 8 and 9 (after first buffer)
- testpoints 10 and 11 (after second buffer)
- base of Q2 (input of power amp)

By the way, how did the fault develop? Was it sudden or did it happen slowly? Were you working on it? Did it get dropped, posted somewhere or otherwise damaged?

Personally, the main problem I've had with this amplifiers is bad potentiometers. Unlike the rest of the amplifier, they don't age well, and the Alpha is built in such a way the impacts to the front of the amp can wreck the potentiometers without the damage being outwardly obvious.

Another thing; do you have the service manual for this amp? If not, PM me your email address and I'll send it to you.
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Old 30th May 2010, 07:06 AM   #5
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Thanks Anton. I appreciate the help. Im very keen on learning how to repair old amps, as im developing a strong interest in old hifi. I also have a perreaux 1221 to repair and a pioneer A70 with a intermittent power supply isssue. I got this amp from Dunedin the other day, in this condition. So im unsure of its history. No service manual. PM sent. Cheers
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Old 30th May 2010, 01:23 PM   #6
jaycee is offline jaycee  United Kingdom
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You may want to check for a muting FET, Q1/101 which is a J111 device. These sometimes go bad and cause attenuation and distortion. Pulling it completely will help you determine if this part is faulty. You will get some turn-on thump by removing this, so might be best to use some test speakers (old car speakers are good for this)

Does the signal sound scratchy when you adjust volume? If so replace the volume pot. Probably best to replace it in any case.

Finally, you may find bad output transistors. These are cheap enough to replace, but they may damage the driver transistors marked FST239/FST240. These are no longer available, but can be directly replaced by ZTX653/ZTX753 which is what Arcam used in later versions of the same circuit.
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