Arcam Alpha- fuse keeps blowing

ash_dac

Member
2005-01-27 8:22 pm
UK
Hi,


I purchased an original Arcam Alpha( untested ), and replaced the mains cable connections.

The secondary transformer connections had been re-done (looked like the connections had broke in the past, and had been resoldered with paper clips). Anyway, I sorted the connections out but the fuse still blows on turn on.

I've taken out the transfomer, and tested that the coils conduct electricity. Any more tests i can perform on it?

In terms of fixing the thing I have only got these ideas?


1. Output stage fried (driving into a short)
2. Transformer dead?
3. Short somewhere on PCB
4. PSU capacitors have failed

any more?

To detail the problem again:-It is meant to have a 500ma fuse in there, even tried a 2A one and that went pop. The fuse is connected to the primary transformer core.

If i can't fix it I will harvest the bits :)



Kind regards,


Ashley.
 
Dunno how much relevance this is, but my d290 had similar problem the other day..
One of the rectifier Diodes in the power supply failed..
seems someone at the Factory installed an errr.. incorrect one.. Did last 10 years despite that though
Lift them and Meter them.. might be an Easy fix.
 
Ashley,

If it just says Arcam Alpha on the front of the amp then I assume it must be the original Alpha instead of the later Alpha 5/6/7/8/9/1 ?

Try and measure the resistance of the transformer as I previously described. Also let me know what happens if you disconnect the secondary of the transformer from the rest of the amp and power it up. This will enable you to isolate the problem between the mains input and the transformer.

Jon
 

ash_dac

Member
2005-01-27 8:22 pm
UK
Jon,


Disconnected the secondary: still blows fuse!

I don't have a multimeter (brother destroyed his last week) so I had to improvise using a 5V dc 500mA supply . The primary core can be wired for both 240/120V.

Thes are the details on the pcb:-

black BRN+Red Orange 240V

Black+Red Brwn+Orange 110V/120V


Upon testing (note with a 5V dc 500mA power supply for safety) all primary wires seemed to conduct electricty with each other. I don't think this is right. I thought there were two 110V primary's that would connect to make a 240V combined primary.


Kind regards,


Ashley.

Note:- Fiddling with a PSU is potentially dangerous.If in doubt seek a qualified electriction for guidance.
 
Ashley,

>Disconnected the secondary: still blows fuse!

This is positive, at least you've isolated the problem !

>Upon testing (note with a 5V dc 500mA power supply for safety) all primary wires seemed to conduct electricty with each other. I don't think this is right.

True if the transformer has twin primaries, connected in series for 240v and parallel for 120v but if it has a centre tapped primary (centre for the 120V) then they *would* all be connected.

If you have a dc supply then why not connnect it across the primary side and connect a bulb (with current limiting resistor) between the 0V of the supply and each wire of the secondary. This will tell you if the primary and secondary are shorted - since DC should not flow across the transformer.

You'll find this much easier with a multimeter !!

Jon
 

ash_dac

Member
2005-01-27 8:22 pm
UK
Jon,

What replacement toroid should i get ?

The old one in the alpha had a centre tap on the secondary, the amp is rated at 30W i think, and there are +15V, and -15V regulators in there)

Ps:- I connected a 12v dc 500ma supply up to the rectifiers, linked up a cd player, speakers, and low level sound came out. I therefore think the circuits have been saved.

How does a transformer primary fail? (drawing too much current?)

(I assume the fuse stops the unit bursting into flames when the transformer fails)



Kind regards,

Ashley.
 
Ashley,

>What replacement toroid should i get ?

I'm afraid since I don't have any information on this amp this would be complete guess work on my part so send Arcam an email [email protected] and ask them. I've found them to be very helpful, if you ask them for a service manual they should send you one. They may even be able to sell you a replacement transformer.

As I said I don't know about the transformer in the Alpha but the toroid in my Delta290 has a centre tapped secondary that after being rectified produces +/-44V, this is used for the power amp and is also regulated to produce +/-15V for the pre-amp stage. I expect the supply rails will be similar in the Alpha, probably in the 25-40v range. There is also a further secondary which produces something like 8v (ac) and this is rectified for the digital section.

>How does a transformer primary fail? (drawing too much current?)

Normally a transformer would fail in one of two ways. There is a break one of the coils of wire, creating an open circuit - not the case in your situation. Or the insulation breaks down and this creates a short circuit, reducing the resistance, more current flows, the fuse blows (hopefully!). The short can either be on the primary coil, the secondary coil or even between primary and secondary. If the primary and secondary were to become shorted then the mains voltage would flow on the secondary side and this would likely blow the amp!

I believe you disconnected the secondaries (therefore removing the load from the transformer) and the fuse still blew, this would imply a short on the primary side. This may be due to the transformer but there is another possibility. Sometimes a manufacturer will fit capacitors between live and neutral and from live/earth and neutral/earth. These are intended to remove mains noise. If a cap between live and neutral were to fail short circuit then this would also blow the fuse and the fuse in the amp will probably blow before the fuse in your plug. If a cap between live or neutral and earth were to fail (and these should fail open circuit) then you would pass current down through the earth and the RCD in your fuse box should trip instead.

Hope this information is of some help.

Cheers,

Jon

p.s. I've sent you a PM about an Alpha you might use for spares.
 

ash_dac

Member
2005-01-27 8:22 pm
UK
Jon,


I put a 120va 2X18V toroidal transformer in today. I connected the two secondaries together to produce a centre tap.

The amp now works, and sounds alot better than my Nad 3130 (the bass has much more punch):D

I have put in a slow blow 250mA fuse that seems to be ok. I'm not sure exactly what fuse to put in. Is there are calculation i need to do?


Again, many thanks for your help in repairing this amp.


Kind regards,


Ashley.
 
Ashley,

Glad to hear you've got the amp working again!

>I have put in a slow blow 250mA fuse that seems to be ok. I'm not sure exactly what fuse to put in. Is there are calculation i need to do?

If 250mA works and you don't drive the amp too hard then you could stick with that. Better to have a fuse with a rating too low than too high!

You've fitted a 120VA transformer, so if V=240 then A = 0.5 (500mA). Alternatively you could work out what fuse to use if you know the output power of the amplifier. Use this to calculate the current in the secondary and work back given the primary/secondary turns ratio to calculate the primary current and therefore the required fuse. By my calculations 30W into 8 ohms, two channels, 3.75amps, turns ratio 240/(18+18), gives 0.5625amps.

Cheers,

Jon