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Old 29th Sep 2017, 8:19 pm   #41
vampyretim
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Default Re: Arcam Alpha 5 mains transformer replacement.

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Originally Posted by mhennessy View Post
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?
Thanks, it does as you say show on the schematic as though they're joined but I fear I've lost a day chasing my tail.

I have had quite a day with it today and did get the scope on what I thought was the amp oscillating but what I was picking up wasn't getting affected by the volume pot even though the oscillation I can hear is. So l don't really know what to make of it.

I will upload a picture tomorrow. The waveform that I was seeing was quite clipped I didn't check the frequency but will do so tomorrow.

I measured the low voltages going to the preamp by clipping my negative probe to the negative speaker terminal and both + and - voltages were correct.

When I put the mains transformer in I pulled up a bit of the print on the 0v centre of the secondary and have had to fix the print with a bit of wire, I have done it neatly and it's electrically as it was but I'll upload a picture of it tomorrow just to get a second opinion.

Thanks again for all your help.
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Old 29th Sep 2017, 9:24 pm   #42
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Default Re: Arcam Alpha 5 mains transformer replacement.

OK - but in the meantime, could you at least give us an idea of the oscillation? Was it low-frequency stuff (scope trace flickery), or high-frequency stuff? Precision isn't needed; just an approximate idea.

If it's a low-frequency oscillation, perhaps the lamp is flickering alone with it? That's the point being made in posts #36 and #40. Or, perhaps it's HF, which causes the lamp to come on and stay on (which I think is what you're describing)?

Amplifiers are often very difficult to troubleshoot once you move beyond simple remedies. It's even harder when attempting to do it remotely - which is why I'm pressing for more detail in your reports. With problems like this, we need to be slow, methodical, and very, very logical - one step at a time...

I'm assuming you've given the amplifier a really good visual inspection? Dry joints and cracked PCB print can send you on a merry dance. No point trying to apply logic until the "daft" things have been eliminated. I use a x4 loupe for this sort of work.
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Old 30th Sep 2017, 1:56 pm   #43
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi,

I hate it when this happens but, it's now working and I don't know why. I don't know what I've done to it but it sounds great.

I don't know what to make of it.
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Old 30th Sep 2017, 1:57 pm   #44
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Default Re: Arcam Alpha 5 mains transformer replacement.

What size of hammer did you hit it with?
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Old 30th Sep 2017, 2:30 pm   #45
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Default Re: Arcam Alpha 5 mains transformer replacement.

Lol, I've now gone to full power but without knowing what was wrong with it I haven't really solved anything and it may well come back.
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Old 30th Sep 2017, 3:09 pm   #46
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Default Re: Arcam Alpha 5 mains transformer replacement.

Good result, my memory fades a bit but I think the PCB was earthed only by the fixing screws, that could be the reason. Another good British amplifier, well designed and no "audiphool" rubbish in the circuit. However the Delta 90 had two unconnected rings of PCB track to "make it sound better" on advice from Peter Belt (a well known audiophool).

Arcam did (and may still) push posh cables in the 80's supplying a cheap interconnect (so at least you could use your newly acquired bit of kit) with advice to spent 20% of ones system on "better" ones. The post ATE fail test gear at the time I was there in the late 80s' was built in house on veroboard. ATE (well semi ATE) was done with two Beavers, who remembers them?
 
Old 30th Sep 2017, 3:49 pm   #47
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Default Re: Arcam Alpha 5 mains transformer replacement.

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Lol, I've now gone to full power but without knowing what was wrong with it I haven't really solved anything and it may well come back.
'full power' as in you've removed the lamp limiter ?
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Old 4th Oct 2017, 4:26 pm   #48
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Default Re: Arcam Alpha 5 mains transformer replacement.

Yes.

BTW does anyone know where I can get the spring loaded fuse for the power supply from? I can't find the spring loaded type anywhere It says on Maplin's site that they have them but when I went into their store they didn't have them, annoyingly.
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Old 12th Oct 2017, 8:09 pm   #49
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi,

The fault came back. I have finally found the source of the problem. For some reason the preamp isn't earthed even though all the screws were back as they should be I have soldered a wire from earth mains to the preamp board and I can now get a linear signal.

And although the hum is considerably quieter I can still hear it a bit which is annoying me. I just don't understand how the hell this could even happen.

Has this ever happened to anyone on here before?
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Old 13th Oct 2017, 6:50 am   #50
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Default Re: Arcam Alpha 5 mains transformer replacement.

Yes, it's the Gods having a laugh. Quite a few amplifiers have/had weird ground schemes with ground being made by screws to chassis, connecting bars or witchcraft.

Havn't seen a spring loaded fuse, can you not just solder a spring to a fuse or similar.

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Old 13th Oct 2017, 2:46 pm   #51
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi,

I have installed slow blow 1A fuses that keep getting taken out by the start up surge, it says in the service data to use a spring loaded fuse only.

I'm having real trouble finding the right size the closest I found was

http://uk.rs-online.com/web/p/cartridge-fuses/0412481/

But I need 20mm not 32.

The amp works brilliantly now just need to get the correct fuse.

Thanks for everyone's help.
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Old 13th Oct 2017, 3:15 pm   #52
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Default Re: Arcam Alpha 5 mains transformer replacement.

Antisurge fuses aren't necessarily made with a spring. Some have a tightly coiled fuse element, and some have what looks like a blob of solder mid way. Examples of both can be found on eBay if you enter 1A antisurge in the box.
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Old 13th Oct 2017, 4:16 pm   #53
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Default Re: Arcam Alpha 5 mains transformer replacement.

Thanks.
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Old 13th Oct 2017, 4:31 pm   #54
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Default Re: Arcam Alpha 5 mains transformer replacement.

If it's humming and taking out fuses, it's possible that it still has a fault. After all, we don't know why the original transformer failed...

Does the output stage run warm? After perhaps an hour of operation at low volumes, how hot is the rear panel?

On the other hand, the fuse might be failing because the replacement transformer is a larger unit than the original. The larger the transformer, the larger the inrush current. If Arcam fitted a 120VA transformer originally, the 1A fuse might have been OK with that, but not the 160VA you've fitted. So, you might have to increase the fuse size. Perhaps try a 1.6AT.

I'd echo what Bill says in post #52 (I think I might've already said this in a PM IIRC?). What's important is the letter after the rating ("T" or "F").

Good luck,

Mark
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Old 13th Oct 2017, 6:14 pm   #55
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi Mark,

It's now hum free. But I think you're right about the replacement transformer having a bigger inrush current.

I will leave it on a low volume for a couple of hours tomorrow but nothing is running hot even though I've been driving it quite hard.

Many thanks,
Tim.
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Old 13th Oct 2017, 7:39 pm   #56
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Default Re: Arcam Alpha 5 mains transformer replacement.

Cool running is a good sign, so hopefully a slightly bigger fuse will do the trick
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Old 14th Oct 2017, 6:21 am   #57
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Default Re: Arcam Alpha 5 mains transformer replacement.

I know what you mean now, the "spring" is inside the fuse.

You could try a thermistor before the tfmr which means you can keep a lower rated rated fuse.

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Old 14th Oct 2017, 7:41 am   #58
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Default Re: Arcam Alpha 5 mains transformer replacement.

A weedier transformer, wound with thinner wire would have had substantially higher copper resistance in both primary and secondary. This would have not only reduced the inrush current of the transformer, but also that of charging the reservoir capacitors. Fitting a beefier transformer will increase inrush currents more than would be expected from simply the change in power rating.

So long as the heat tells you nothing is wrong, you need the smallest value T characteristic fuse which doesn't blow.

just where in the mains cycle you turn on has a big effect, so there is an element of roulette in whether fuses blow if they're a bit marginal, so you need to check the value you've chosen over many starts. Too few and the statistics can have you believing that some fuses look tougher than higher rated ones.

In product development, a good test is to short each secondary of the transformer in turn, to check that the fuse blows correctly and saves the transformer. If wrong, this can be an expensive test, but it's good practice and done by high class (engineering not phoolery) manufacturers.

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Old 14th Oct 2017, 12:17 pm   #59
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Default Re: Arcam Alpha 5 mains transformer replacement.

In my experience, inrush protection methods (e.g., NTC thermistors or chunky resistors bypassed by an equally chunky relay) aren't needed on 160VA transformers. When you get to the 400-500VA region, then yes, definitely.

So yes, bigger fuse should do it.

I've just dug out an old prototype amplifier I threw together years ago. It uses two 160VA transformers, and the fuse is 3.15A. Obviously, it was only an experimental test bed, but since that project work was concluded it has been used a lot as a general purpose bench amp as well as a standby hi-fi amp. In all that time, the fuse has never been replaced.

So if T3.15A was OK for two, I'd say that T1.6A or perhaps T2A should be fine. If a T2A fuse fails, then that warrants further investigation. It's possible the power does funny things while the rails are establishing.

Just in case, it's worth saying that you should buy fuses from reputable sources

For a mains fuse, the ceramic bodied types are definitely preferably. Occasionally, I've opened a fuseholder to find a pile of shattered glass held within...
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Old 15th Oct 2017, 9:28 pm   #60
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Default Re: Arcam Alpha 5 mains transformer replacement.

Hi guys,

I played the amp for about 4 hours yesterday and nothing got too hot. The amp sounds great.

I also installed a 2A time delay fuse as l didn't have a 1.6A and the fuse was fine on start up.

I'm going to call that a victory and want to thank everyone for their help, I feel I've learnt quite a bit.

Thanks again,
Tim.
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