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Old 18th January 2013, 09:15 PM   #21
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Quote:
Originally Posted by Harleyjon View Post

The bias pots should be set to give a reading of between 3.0 and 3.5mV across points IQ1L and IQ1L also IQ1R and IQ1R (0R22) or 26mA. I favour no more than 12mA but that is my personal view. Fets are quite tolerant and tend to act as a thermionic valve does;- The hotter the Drain, the lower the Gain. When a valve is in over-current mode the anode glows and the electron flow is reduced accordingly. Hence the pleasing logarithmic sound produced.
Too much quiescent current and they will near class AB and be very inefficient. Too little quiescent current and they will introduce cross over distortion.

The hum on one channel is caused by EMF Radiation from the the mains transformer. Re-orientation would cure this but that is not practical in this case.
I hope that helps and is useful information for you.
Thanks but I'm a bit lost.

Would one of you kind people be able to explain about checking / setting the bias in a bit more detail. Keep in mind I only have a standard digital multimeter!

Should I go ahead and get the capacitors to modify the inputs as the Arcam chap suggested?

Overall, what is my best way forward?

Thanks again.
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Old 18th January 2013, 09:24 PM   #22
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I would say at this point that the current is not critical. If it is not running hot without a large signal, I would suggest that the quiescent current is, as we say, near enough. But to check it, there is a 0.22 Ohm resistor in each pair of output FETS. Carefully measure, with no signal, across the resistor and adjust the voltage to as near as you can get 3.0mVolts. That's it.
As for the capacitors, they should be fitted already. Replacing them may help but they probably won't be faulty. I would check their position, if they have been res=oldered as they may be in the wrong place. Ask your tame Arcam Man for a photo/drawing of the layout. That should help.
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Old 18th January 2013, 09:30 PM   #23
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Quote:
Originally Posted by Harleyjon View Post
As for the capacitors, they should be fitted already. Replacing them may help but they probably won't be faulty. I would check their position, if they have been res=oldered as they may be in the wrong place. Ask your tame Arcam Man for a photo/drawing of the layout. That should help.
Sorry, perhaps I wasn't clear back in post no 15.

The capacitors he was suggesting are a mod which they have done to only a very few of these amps. What I described was his recollection of how it was done, he couldn't find any written notes!

Last edited by Uncertain; 18th January 2013 at 09:32 PM.
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Old 18th January 2013, 09:35 PM   #24
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No, I am sure it isn't you. I find these links, confusing.
The only thing I would suggest, regarding the interference is to check the capacitors, C303 and C304 have not been re-soldered and therefore possibly put in the wrong position and fit ferrite cores to the cables that are giving you issues.
I hope that helps.
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Old 18th January 2013, 09:40 PM   #25
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Originally Posted by Harleyjon View Post
No, I am sure it isn't you. I find these links, confusing.
The only thing I would suggest, regarding the interference is to check the capacitors, C303 and C304 have not been re-soldered and therefore possibly put in the wrong position and fit ferrite cores to the cables that are giving you issues.
I hope that helps.
I'll check and report back in a day or two.

Thanks.
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Old 18th January 2013, 10:10 PM   #26
Bare is offline Bare  Canada
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I understand Frugality all too well, however this is growing beyond silly.
Have the Factory repair it? Likely the thing has been ineptly Bodged.. and You bought it..
Factory repair/refits usually come with a warranty.
Buy Cheap... buy twice :-)
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Old 19th January 2013, 07:21 AM   #27
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Originally Posted by Bare View Post
I understand Frugality all too well, however this is growing beyond silly.
Have the Factory repair it? Likely the thing has been ineptly Bodged.. and You bought it..
Factory repair/refits usually come with a warranty.
Buy Cheap... buy twice :-)
I thought this forum was called DIY Audio?

Everybody else has done their best to be helpful!

As I mentioned in post 15, Arcam are far from keen to work on it and no longer stock any proprietary spares for these units (although presumably standard electronic components wouldn't be a problem). As I live less than five miles from their factory it would otherwise have been a serious possibility.

In any case two of the problems seem to be "issues" rather than faults if you see what I mean. Two Arcam staff have talked about making modifications to the standard design to deal with the RF problem which it seems they had to do on a "handful" of amplifiers 15 years ago. They say it is an obscure combination of circumstances that causes this issue which I believe as there is no trace of it if I connect it up in my workroom.

They also say the very, very slight hum on the right channel is a design flaw and you only have to look at the inside to see why. A small rotation of the transformer may well improve this but it is very firmly stuck down so not easy to do.

Whilst I'm no expert on amplifiers (hence asking questions here and from their service department) I've done more than enough small soldering work in the past to be able to tell if a circuit board has been worked on. The only trace of any work is the pins of the output fets. I'm certain nothing else on the board has been touched.

So, what I have here is a fifteen year old "mid" price amplifier which was bought cheaply as the case is not pristine. A new front panel is not available. Even if Arcam were willing to do a "factory re-conditioned" job on the insides I suspect at UK prices it would not be economic. It is highly likely they wouldn't experience the RF issue so would have to modify it blind according to their current staff's recollections of what they did a "handful" of times in the past.

So, those are the reasons I'm asking for help and advice on a DIY forum and I'm sorry you think that is "beyond silly"!
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Old 19th January 2013, 08:53 AM   #28
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Quote:
Originally Posted by Harleyjon View Post
No, I am sure it isn't you. I find these links, confusing.
The only thing I would suggest, regarding the interference is to check the capacitors, C303 and C304 have not been re-soldered and therefore possibly put in the wrong position and fit ferrite cores to the cables that are giving you issues.
I hope that helps.
Those capacitors are present and show no signs of ever having been disturbed. They are very close to the input sockets. There are no unused holes in the PCB. I suppose they could be faulty but it seems unlikely?

The capacitors the Arcam chap has suggest fitting are, presumably, are to do much the same job but better? He wants me to fit the directly on the sockets which would be quite easy. They are also ten times the value of the factory fitted pair?
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Old 19th January 2013, 10:54 AM   #29
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Quote:
Originally Posted by Harleyjon View Post
I would say at this point that the current is not critical. If it is not running hot without a large signal, I would suggest that the quiescent current is, as we say, near enough. But to check it, there is a 0.22 Ohm resistor in each pair of output FETS. Carefully measure, with no signal, across the resistor and adjust the voltage to as near as you can get 3.0mVolts. That's it.
Can I just double check that we are looking at the same service manual?

The one I have says 2.3 to 2.8 mV for the 8r and 3.0 to 3.5 mV for the 7r.

My amplifier is an 8, not an 8r. I had assumed that the r simply stood for "remote" but apparently not according to the chap at Arcam. The 8r has a different badge on the front panel. He didn't say what, if any, differences there were in the power stage.

I can't find a service manual online for the 8, only one titled "Arcam Alpha One/ 7R/ 8R/ 8P Amplifier Service Manual
Issue 1 Serial Number 001 - (Paul Newton/ Haider Bahrani Sept Ď98)".

This won't be immediate as I need to find something capable of reading a few mVs accurately. The lowest range on my meter is 2 volts and I'm not confident of the calibration on my ancient scope. I have a few bare moving coil meters somewhere. Maybe one will be something like 10 mV FSD!

There is no sign of the amplifier getting hot so I doubt it is set significantly too high.
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Old 19th January 2013, 04:42 PM   #30
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Default Update - RF Interference cured

Well, I've fitted the capacitors and braid as suggested by the chap at Arcam. In fact I fitted seven rather than eight as the last phono socket is the pre-amp output so I wasn't sure whether one should be fitted there or not.

That seems to have cured the RF interference completely.

I still have hum issues. With just the Arcam CD player and Tuner connected it is very slight although with the Arcam Alpha 5 amplifier in the same setup there was none at all.

However, if I plug my Sony BluRay player in the hum increases significantly regardless of which input is selected. Again, when connected to the 5 there was no problem at all.

Whilst I understand the chap's explanation about why there might be "slight" hum on the right I feel I have more than is reasonable.

There are other devices all around the setup and it is not easy to isolate. However, they didn't affect the Alpha 5.

The units are all plugged into wall sockets on a ring main, not trailing four ways.

Any other ideas would be most welcome.

Thanks
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