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Old 20th Sep 2017, 10:33 am   #21
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

Electrically, I think it would be OK, if you ignore the fact that you won't be able to run it at 115V in the future...

But is it physically equivalent to the existing one? Have you measured the height and diameter and compared it to the original?

225VA is massively OTT for a class AB 40 watt per channel amplifier, and you might run into problems with the inrush current causing the mains fuse to pop occasionally. Personally, I'd use 120 or 160VA:

120VA: http://uk.rs-online.com/web/p/toroid...rmers/6719148/
160VA: http://uk.rs-online.com/web/p/toroid...rmers/6719151/

Both these have dual 115V primaries, so you can retain the 230/115V operation (though I don't like the arrangement they've used). Of course, single 230V primaries are equally available if you prefer.

Finally, before spending the cash, did you see my earlier comment about verifying the rest of the amplifier first of all? It would be a shame to put a new transformer in, only to find that you've got a faulty output stage that might not be an easy fix?
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Old 20th Sep 2017, 4:11 pm   #22
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi,

Thanks for all your help.

I went for the 160VA as it seemed to be a better physical fit.

I've checked the output stage and the rectifier and all is well.

Thanks again,
Tim.
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Old 26th Sep 2017, 2:31 pm   #23
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi all,

I'm having trouble with this, I've installed the transformer but the amp is unstable. I'm getting the correct D.C volts but am getting a massive hum and it becomes unstable once an audio signal is put into it.

I'm a bit worried I might not of wired the transformer properly, would someone be kind enough to have a look at the service sheet for the transformer and tell me how they'd wire it up?

http://uk.rs-online.com/web/p/toroid...rmers/6719151/

the schematic for the amp is readily available online.

Many thanks.
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Old 26th Sep 2017, 4:23 pm   #24
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Default Re: Arcam Alpha 5 mains transformer replacement.

If you have the correct DC voltages, then it's likely you've wired it up correctly, and the amp has a fault - and it's possible that is what caused the original transformer to fail, so don't leave it powered too long in this condition. I did recommend using a lamp limiter for exactly this possibility, but you seemed confident that the rest of the amplifier was OK (though you didn't say how you checked this out - from what you wrote, it's possible you've just done "cold" checks to make sure the big semiconductors weren't shorted?). If you haven't already, put a 100W lamp in series with the mains. And what sort of current is the amplifier drawing from the mains?

Going by the colour code given on the RS website, you should join black and yellow, and connect them to 0V of the amplifier. That leaves the red and orange leads to connect to the bridge rectifier.

Normally, I expect the order of the colours to be black, red, orange and yellow, so would join red and orange to make the 0V connection, but it doesn't matter which of these two options you pick. Why don't you tell us (or provide a photo) of what you've done?

The symptoms you describe could be caused by many things. What sort of test equipment do you have - have you got a 'scope?
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Old 26th Sep 2017, 7:04 pm   #25
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi,

I am using a lamp limiter. I do have a scope. The fault is common to both channels, and I checked the output transistors by taking them out of circuit and measuring their junctions with an avo 8.

It's just that I've never replaced a mains transformer without my grandad helping me. I'm at home now and can't remember off the top of my head how I've wired it but will let you know tomorrow.

Many thanks.
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Old 26th Sep 2017, 7:28 pm   #26
Ted Kendall
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Default Re: Arcam Alpha 5 mains transformer replacement.

o/c feedback loop somewhere?
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Old 26th Sep 2017, 7:33 pm   #27
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

Quote:
Originally Posted by vampyretim View Post
I checked the output transistors by taking them out of circuit and measuring their junctions with an avo 8.
This is what I feared...

I don't know how well you know the circuitry in these sorts of amps - so forgive me if I teach you to suck eggs - but in an audio amplifier, faults can occur anywhere that cause all manner of problems without actually damaging the output transistors.

Yes, output transistors can fail - usually taking out several of the preceding transistors and other components in the process - but they might not. This amp uses MOS-FETs, which are pretty rugged providing you don't blow the gate.

With that in mind, the first step is always to power up the amplifier using a current-limited bench power supply so that you can establish that the DC conditions are correct, and that it amplifies like it should (using a 'scope). Then you can move back to the internal PSU and see if it can supply the current needed into a dummy load.

Using a lamp limiter is the next best thing in lieu of a bench power supply, but with this, the rails to the whole of the amplifier will be bouncing up and down, which could add a number of confusing symptoms.

Talking of symptoms, could you be more explicit about "becomes unstable"? What do you see on a DC-coupled 'scope? Does the signal need to reach a certain level before the instability happens? Oh, and until it's fixed, do be careful with your test speakers, as this amp doesn't loudspeaker protection beyond the output fuses.
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Old 27th Sep 2017, 1:23 pm   #28
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi,

I joined the black and yellow wires and put them to the 0V and I joined the blue and green wires on the primary and put that to the 115V, Violet went to neutral and brown went to 230V.

Before installing the transformer I gave the secondary (from the centre tap) a dummy load of 1k and calculated I should get 25 mA and got something like 27 through my AVO on both halves of the secondary from the centre tap.

So I'm happy with the transformer and the D.C conditions seem to be fine.

By unstable I mean the thing is whistling and howling it gets worse with the volume pot being at full I.E the loudest. The 1k signal I've fed it is very faint but as soon as I turn my sig gen up it starts motor boating.

I need to invest in a bench supply that's next on my list!

Thanks for taking the time to help me, I've never had a fault like this before, I don't even know what could do this (negative feedback issues?). I just wanted to make sure it wasn't something that I had done to it.

Tim.
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Old 27th Sep 2017, 4:11 pm   #29
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Default Re: Arcam Alpha 5 mains transformer replacement.

When I worked at Arcam the standard fix for a non working amp just off the production line was...

1. Remove all transistors (in the non working bit, pre amp, left-right power amp)
2. Check and replace any wrong passives
3. Install a new set of transistors

This was a lot quicker (and cheaper) than doing a proper diagnosis.
 
Old 27th Sep 2017, 7:04 pm   #30
mhennessy
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Default Re: Arcam Alpha 5 mains transformer replacement.

Yes, it looks like the transformer connections are good.

I must say that I don't like the colours on that RS transformer. Normally, brown would be the "top" of one winding, and blue would be the "bottom" of the other, meaning that on 230V, the colour codes are logical, and you just have to join the other two colours (normally purple and grey IIRC). Having brown and blue at opposite ends of a 115V winding is not good IMHO.

Anyway...

By "DC conditions", I don't just mean the PSU rails. Every point of the amplifier needs to sit at the correct DC voltage. Start with the output - which should be 0V - and work back as needed. Easier said than done in a power amp with a dozen or so transistors that are all DC coupled

Once the DC conditions are correct throughout the whole of the amplifier, then we can think about applying a signal and seeing what's going on. Following the signal through with a 'scope is a good plan, as is dividing the unit into two (pre and power amp) to determine where the problem is. If it's motorboating, is that because the power amp is drawing too much current, or is it because of some other fault? These sorts of faults are always rather challenging - even for experienced engineers - so don't be too disheartened if it's not an easy fix. And if the worst comes to the worst, you're not all that far from me
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Old 28th Sep 2017, 4:30 pm   #31
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Default Re: Arcam Alpha 5 mains transformer replacement.

There's something horribly wrong with this thing, I wish the schematic gave some voltages. This isn't the nicest schematic I've ever worked from.

Whenever I try to plug something into the preamp I get the lamp on my limiter lighting up like a short.

I don't know how to go forward without a better schematic.
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Old 28th Sep 2017, 4:55 pm   #32
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Default Re: Arcam Alpha 5 mains transformer replacement.

Volume up or down?
 
Old 28th Sep 2017, 5:47 pm   #33
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Default Re: Arcam Alpha 5 mains transformer replacement.

sounds like it's oscillating. I would start by taking the board out and making sure there are no cracked joints or tracks or components that wobble when they shouldn't. It's a big board and is easily damaged. Make sure the low voltage supplies are spot on. Did the replacement transformer give you the main bus voltages correctly? Make sure these aren't wildly out.

Once its right you'll be rewarded with a great sounding amp.
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Old 28th Sep 2017, 7:22 pm   #34
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Default Re: Arcam Alpha 5 mains transformer replacement.

Thanks guys, I'll check over the board tomorrow. I can see on the scope that it's oscillating but it's common to both channels. The volume up does make it worse.

I'll report back tomorrow.

Thanks again,
Tim.
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Old 28th Sep 2017, 10:13 pm   #35
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Default Re: Arcam Alpha 5 mains transformer replacement.

You may have a bad earth via the print or a missing earth that's not connected.
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Old 28th Sep 2017, 11:13 pm   #36
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi
I'm tending to agree with Vidjoman. I find it odd that BOTH channels are affected. Either an earth off or too many earths causing a loop. Or could it be simply that the lamp limiter is doing too good a job, reducing the voltages as the current increases? If you're confident the PSU is OK then maybe investigate the preamp and its supply - hum or an earth fault here would cause trouble in both the output stages.
Glyn
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Old 29th Sep 2017, 12:13 am   #37
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Default Re: Arcam Alpha 5 mains transformer replacement.

Quote:
Originally Posted by vampyretim View Post
I can see on the scope that it's oscillating
What sort of frequency?

Pictures might help
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Old 29th Sep 2017, 5:23 pm   #38
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Default Re: Arcam Alpha 5 mains transformer replacement.

Am I being really stupid?
If you look at the 0 point on the secondary, shouldn't that be connected to chassis like the earth point on the mains input?

I'm not getting any continuity from earth mains to the rest of the amp. The earth mains seems to only be connected to the metal case.

I hate this amp!

Any help would be appreciated!
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Old 29th Sep 2017, 7:20 pm   #39
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Default Re: Arcam Alpha 5 mains transformer replacement.

This is not uncommon. If you join the signal ground to mains earth, you run the risk of getting an earth loop when you connect it to other earthed equipment.

However, the schematic seems to imply that the two earths are joined, so it's worthy of further investigation along the way. But for now, use one of the black loudspeaker terminals as your 0V reference point, not the chassis. That's good practice for any bit of kit - as I say, it's really not uncommon for signal earth and mains earth to be separate.

Any joy on the question I asked?
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Old 29th Sep 2017, 8:03 pm   #40
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Default Re: Arcam Alpha 5 mains transformer replacement.

"Lamp limiters" are really not a good idea on solid-state gear that uses Class-B bias; they can result in a whole slew of wobbly incorrect-biasing issues that drag you down nested rabbitholes of truly Alice-in-Wonderland recursiveness.

Job One for me would be to get it DC-stable: short the input terminals and fit a resistive-load [3, 4 or 8 ohms as appropriate] to the output. Then get it happy to sit there for hours [thermal runaway can take time to happen...] with the right static voltages on the various emitters/collectors/bases.

Only then allow it to see any audio signals!
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