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Old 27th January 2013, 10:46 AM   #1
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Default Power supply question Arcam Alpha 8

A quick question following on from from my hum / earthing problems....

If one diode in a bridge rectifier has failed open circuit, what voltage drop would you expect to see? Would 34 volts when expecting 37 be about right?

Also, would you expect to be able to detect any ripple with a multimeter on, say, its 5 VAC range?

Finally, I need to double check that I should actually be getting 37 volts. My amplifier is an Alpha 8 but the only service manual I can find is for an 8r. My maths suggest there should be just over 26 volts AC coming out of the transformer. Does that make sense?

Thanks
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Old 27th January 2013, 11:13 AM   #2
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The Trafo is nominally 25-0-25V

The 8R also has an auxilliary 0-9V winding.

Last edited by KatieandDad; 27th January 2013 at 11:24 AM.
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Old 27th January 2013, 11:13 AM   #3
Mooly is offline Mooly  United Kingdom
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The DC voltage across the reservoir cap would depend on the current drawn with an OC diode so no definitive answer to that.

Detecting ripple with a meter also has problems because some meters react differently to the underlying DC present so again no real answer for that.

If you measure the AC voltage applied to the bridge and multiply by 1.414 and then subtract around 1.4 volts to allow for the diode drop you will get a fair answer.

Its very rare for power diodes (as in a bridge) to go open circuit but I suppose it could happen. It would either be a known issue with the diodes used and that model (and as such a common and recognised fault) or else a 10000:1 chance of a "normal" one off failure.
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Old 27th January 2013, 11:41 AM   #4
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Quote:
Originally Posted by KatieandDad View Post
The Trafo is nominally 25-0-25V

The 8R also has an auxilliary 0-9V winding.
That is what I had assumed with the 9V going the the remote volume circuit and "r" standing for remote. But apparently not according to my conversation with Arcam!

Mine doesn't have an "r" on the front but does have a remote volume.

I will check the AC later in the day. If it is 25 then that should give about 34 DC which is what I'm getting but the service manual I've got suggests 37!
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Old 27th January 2013, 11:50 AM   #5
Mooly is offline Mooly  United Kingdom
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Service manuals often have inaccurate voltage markings on them. Its an industry wide problem. Measure and be sure.
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Old 27th January 2013, 11:54 AM   #6
jaycee is offline jaycee  United Kingdom
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It is also dependant on your mains voltage. Some areas are lower than others.
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Old 27th January 2013, 12:01 PM   #7
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Originally Posted by Mooly View Post
Service manuals often have inaccurate voltage markings on them. Its an industry wide problem. Measure and be sure.

I will check.

The Alpha 7 (covered by the same service manual) should be about 34 volts. Assuming that my 34 is in correct proportion to the AC and therefore not a diode fault, should I set the bias to the higher levels they indicate for the lower supply voltage.

They suggest 3 to 3.5 mV with 34 Volt supply and 2.3 to 2.8mV for 37.

Thanks again.
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Old 27th January 2013, 12:04 PM   #8
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I'd leave it alone as it was built and tested in the UK.
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Old 27th January 2013, 12:09 PM   #9
Mooly is offline Mooly  United Kingdom
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I'd have to study the manuals to be sure...

Unless you suspect a problem then it probably is best to leave as set. The adjustments once set shouldn't drift. And tbh supply voltage shouldn't come into the equation for bias. The correct bias depends on the circuit configuration and would be the same whether running on 25, 35 or 60 volts
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Old 27th January 2013, 12:33 PM   #10
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I'd have to study the manuals to be sure...

Unless you suspect a problem then it probably is best to leave as set. The adjustments once set shouldn't drift. And tbh supply voltage shouldn't come into the equation for bias. The correct bias depends on the circuit configuration and would be the same whether running on 25, 35 or 60 volts

It was wrong, in that the right channel was 2.8 (top end of suggested for the 8r) but the left was only 1.9. I've adjusted both to 2.7 but can't detect any audible change.

In addition to the earthing type problems I described in the other thread I feel the amp has somewhat thin, lifeless sound compared to my Alpha 5 in the same setup.

I can't detect any sign of the amp having ever been worked on apart from the output mosfets being obviously hand soldered. However, Arcam tell me they are put in by hand so that doesn't prove that they have been replaced.

Last edited by Uncertain; 27th January 2013 at 12:35 PM.
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