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Old 19th Sep 2017, 10:21 am   #1
vampyretim
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Default Arcam Alpha 5 mains transformer replacement.

Hi,

I need a mains transformer for an Arcam Alpha 5 amp.

I'm a bit confused as the service manual only states that it's +/- 37V D.C when rectified but surely I want to know what the A.C on the secondary is. Am I being really thick? Is there a formula that will tell me what it should be if I have this D.C?

Many thanks.
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Old 19th Sep 2017, 10:39 am   #2
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

It will be a custom-made transformer, only available to Arcam, and made to their specification. Therefore, you would have to buy it from Arcam directly - hence they haven't put details about it in the manual.

But having said all that, a standard 0-25, 0-25 would do the job.

To go from DC to AC, you need to use root-2 (1.414). That's because the DC voltage is close to the peak value of the AC input, and the transformer windings are quoted in RMS. However, there are many, many variables, so treat that as "approximate, but close enough for this conversation".

Needless to say, the mains transformer is a safety-critical part.
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Old 19th Sep 2017, 10:45 am   #3
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

I've just looked at the service manual.

In addition to the main winding, there is an auxiliary winding to supply the control circuit. That's 9V AC. You'll need to supply that with a separate transformer. It only has to supply enough power to drive the volume motor, so a 6VA model should be more than adequate.

EDIT: Sorry - that's the Alpha 6 only. That's better - I thought my memory was playing tricks because I didn't remember the 5 having remove control.

I also spotted the note in the manual about the double-sided tape securing the transformer. That sounds like "fun"

Good luck,

Mark
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Old 19th Sep 2017, 11:00 am   #4
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Default Re: Arcam Alpha 5 mains transformer replacement.

Well, it depends on load, of course, but the theoretical factor is 1.414 x RMS secondary volts for the rectified and smoothed DC. Alternatively, you could get in touch with Arcam themselves - they tend to support their kit better than some, and it's not beyond the realms that they could sell you a kosher replacement.
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Old 19th Sep 2017, 12:03 pm   #5
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Default Re: Arcam Alpha 5 mains transformer replacement.

And knock off a couple of volts for bridge or a volt for full wave, silicon rectification.
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Old 19th Sep 2017, 12:15 pm   #6
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

As I say, "many variables". If the OP wants more background, here's a simple article I wrote for the DIY audio crowd ages ago: http://www.markhennessy.co.uk/articl...r_supplies.htm

Personally, I've always felt that given all the variables - some of which are rather difficult to predict (transformer manufacturers won't give anything but approximate guesses, if anything) - it's hardly worth aiming for any sort of precision when talking about power supplies like this. Rules of thumb are fine.

In industry, you pick a transformer and test it, then try another one based on the results of that. Repeat until you get the output power your Marketing department want, at the lowest possible cost. Test at low mains so that no hi-fi review can accuse you of over-stating the output power (which is why hi-fi amplifiers always exceed their specification). Job done!
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Old 19th Sep 2017, 12:18 pm   #7
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Default Re: Arcam Alpha 5 mains transformer replacement.

Quote:
Originally Posted by Ted Kendall View Post
Well, it depends on load, of course, but the theoretical factor is 1.414 x RMS secondary volts for the rectified and smoothed DC. Alternatively, you could get in touch with Arcam themselves - they tend to support their kit better than some, and it's not beyond the realms that they could sell you a kosher replacement.
I had an Arcam7 amp power unit off them a while ago , so you may get lucky.
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Old 19th Sep 2017, 9:32 pm   #8
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi Tim, it can always be reverse engineered and rewound if you cannot get a spare from Arcam.

Ed
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Old 19th Sep 2017, 10:15 pm   #9
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hardly worth the effort though - it's so close to a standard 25-0-25 toroid (or 0-25, 0-25 if you prefer). Even if they had a custom job done - perhaps 27-0-27, say - the difference in output power would be inaudible.

I suspect it actually is a standard 25-0-25. When you take regulation into account (typically 7 to 10% for a transformer that size), you'd get about 37V in practice. I wrote up one such example of this some 15 years ago (about 2 thirds down): http://www.markhennessy.co.uk/articl...xperiments.htm

The service manual mentions the addition of an electrostatic screen for later models. If you have an additional earthed wire emerging from the transformer (should be green/yellow wire if they've followed the usual convention), then that's what it's for. But a standard model that lacks one of these wouldn't cause any problems in practice.

I'd echo the comment to contact Arcam - they are very helpful. Even if they don't have a spare after 20+ years, they ought to be able to recommend a standard replacement. I bet they'll say 25-0-25

Obviously, the VA rating is easily determined by reference to the dimensions. The amp is only 40 watts per channel, so 120VA would do, or 160VA if there's space.
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Old 19th Sep 2017, 10:19 pm   #10
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Default Re: Arcam Alpha 5 mains transformer replacement.

Oh, and it's worth asking *why* the transformer has failed. Are we sure the power amplifiers are Ok? Easy enough to test with a dual bench power supply if the OP has access to one. If not, be sure to use a lamp limiter when powering it up for the first time with the new transformer.
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Old 19th Sep 2017, 10:25 pm   #11
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Default Re: Arcam Alpha 5 mains transformer replacement.

being domestic there may be a thermal fuse in the primary as well.

I do a bit of work for an Arcam dealer and they've never had problems getting bits yet. Very nicely made stuff
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Old 19th Sep 2017, 10:41 pm   #12
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Default Re: Arcam Alpha 5 mains transformer replacement.

It's not mentioned in the manual nor shown on the schematics. Obviously Arcam will give you an authoritative answer, but in my experience, it's rare to find a thermal fuse in a UK hi-fi amplifier. The tell-tale bulge would give the game away, but I can't see such a thing on any of the pictures available. To be honest, I can't remember that last time I saw a thermal fuse embedded in a toroidal transformer, though I have seen them screwed to the top washer in the past.

Because of the voltage switching arrangements, it would have to be in the neutral feed.
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Old 19th Sep 2017, 10:47 pm   #13
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Default Re: Arcam Alpha 5 mains transformer replacement.

mcMurdo has a point there. I have had to have replaced 2 mains transformers in my Arcam CD72T CD player, Both instances the internal thermal fuse had blown. The bugbear is that the darn things are epoxy or similar encapsulated so its not a simple matter of replacing the fuse. No issues with the electronics but I remember a web search at the time threw up a tendency for the mains transformer on these units to fail. Usually on switch on.
The other thing I distinctly remember was Arcams total silence when I tried to contact them so i can't unfortunately endorse other posters comments on Arcam's "Helpfulness". The repair technician did eventually obtain a new transformer but it took ages and he gave me the voltages and ratings of the unit on the basis of if it failed again it might be easier to try and fit a different one, even if it meant getting one wound specifically.
Sorry to seem a bit negative here but this was my experience. The good news is that the CD72T hasn't gone wrong since, touch wood. Not a bad bit of kit at all.
I'd be tempted to go for an aftermarket transformer unless the owner is set on an original. I wonder if it would be possible to do an overwind on a standard toroid to get the 9V AC for the control board? Or price up a custom wind.
Andy.
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Old 19th Sep 2017, 10:56 pm   #14
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Default Re: Arcam Alpha 5 mains transformer replacement.

Yes, but those transformers are not toroids - they are potted IE types. Yes, they occasionally fail. I've got three Arcam CD players (Alpha 9, CD72 and CD92) with those transformers, but so far, so good (touch wood!). Unless you switch them off at the wall, they run continuously, and the analogue stages remain powered when in standby. In the case of the Alpha 9 and CD92, that results in significant heat and cost because of the complex RingDAC module. Later models used toroids for the analogue stages - I think that Arcam had their fingers burnt with those transformers - no pun intended! They were quite complex, with 3 separate windings (two of which are CT), so using off-the-shelf replacement transformers would require 3 separate units. Luckily, there's a fair amount of space inside those units.

There's no need for the 9V winding with this one - that's only needed for the Alpha 6
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Old 19th Sep 2017, 11:15 pm   #15
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Default Re: Arcam Alpha 5 mains transformer replacement.

Many domestic appliances with a transformer will need a thermal fuse to gain BEAB approval. It's often printed on the transformer label. But dont hold me to it! The days of curtain-burners are supposed to be over!
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Old 19th Sep 2017, 11:17 pm   #16
bikerhifinut
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Default Re: Arcam Alpha 5 mains transformer replacement.

Quote:
Originally Posted by mhennessy View Post
Yes, but those transformers are not toroids - they are potted IE types. Yes, they occasionally fail. I've got three Arcam CD players (Alpha 9, CD72 and CD92) with those transformers, but so far, so good (touch wood!). Unless you switch them off at the wall, they run continuously, and the analogue stages remain powered when in standby. In the case of the Alpha 9 and CD92, that results in significant heat and cost because of the complex RingDAC module. Later models used toroids for the analogue stages - I think that Arcam had their fingers burnt with those transformers - no pun intended! They were quite complex, with 3 separate windings (two of which are CT), so using off-the-shelf replacement transformers would require 3 separate units. Luckily, there's a fair amount of space inside those units.
That's interesting.
I'm going to have a look inside with the meter now! Only because I have this memory of the last time i was inside it (a fair few years ago now), that the CD72 switched the mains side. But as my memory aint what it was I'll give it a check. I'm not keen on gear that stays in standby mode if i can avoid it.
Hope Vampyretim gets a result.

A.
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Old 19th Sep 2017, 11:23 pm   #17
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Default Re: Arcam Alpha 5 mains transformer replacement.

Worth saying I havent had any parts off Arcam since Samsung took them over
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Old 19th Sep 2017, 11:56 pm   #18
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

I have the service manual for the CD72/92, but not the T models. There might have been a change with those, as I recall lots of complaints at the time because the "standby" button on the remote didn't actually put the CD player into standby (it would control an amp if you had one that responded to standard RC5 IR codes). The power switch in a 72 was just a standard low-voltage latching pushbutton switch. It has 4 poles. The later CD73 definitely had a "proper" mains switch (and a lovely big toroid!): http://i.**********/l19T1.jpg

In the case of the CD72, the analogue sections that remain powered amount to a couple of op-amps so I doubt it makes much difference to your bills. The 92 is much worse - I must measure it sometime. Not good, but it was the fashion back then when energy was so much cheaper.

Looking at my photos, there are no markings on the transformer beyond a basic paper label indicating the type. No indication that there's a thermal fuse in there. As I say, thermal fuses are comparatively rare in hi-fi gear - at least in my experience - as normal fuses are used in series with the primary. The trouble with thermal fuses is that they can fail with no good reason, and that writes off a rather expensive transformer. Far better to let it blow the mains fuse if the windings get hot enough to breach the insulation and cause a shorted turn. Given the use of metal boxes and self-extinguishing plastics, the risks of a fire are basically zero. But I understand that thermal fuses are required on appliances like heaters, hairdriers, tumble driers and toasters - in case they get covered or clogged up with crumbs or fluff or similar.

Thinking about it, the only place where thermal fuses were ubiquitous were in the wall-wart powers supplies (which don't have a conventional fuse). Some Philips gear used them, but they are replaceable (if you can get spares today).

I knew that Arcam had recently been acquired by Harman but didn't know that Samsung had Harmon. Small world! Harman have a lot of pro brands like Soundcraft and Studer as well.
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Old 20th Sep 2017, 12:14 am   #19
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Default Re: Arcam Alpha 5 mains transformer replacement.

Interestingly, the toroid in the CD73 does have a thermal fuse. It's clearly shown on the schematic and discussed in the text.

But this model has no other fuse. And I guess that's because they have a voltage selector on the rear panel, making this a customer option rather than the usual dealer change (as a different fuse is required according to the voltage). Hmm - mixed feelings about that one...

As well as the big primary-side power switch, this does have a standby mode as well. The more I think about it, I'm fairly sure the CD72T didn't get the standby mode, as it was a fairly minor revision to incorporate the updated Sony chipset...
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Old 20th Sep 2017, 9:27 am   #20
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Default Re: Arcam Alpha 5 mains transformer replacement.

Thanks for all the replies.

Do you guys think this would do the job then?

http://uk.rs-online.com/web/p/toroid...rmers/6719047/

Thanks again.
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