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Old 19th August 2008, 06:06 PM   #1
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Default Arcam Alpha blows PSU fuse

I just bought an Arcam Alpha (original model - 1984) as a working amplifier, and after 1 hours use the fuse blew.

(I know that there is a similar thread on the BB, but I think that this is a different problem).

I found that it had a 7.5A 250V fuse in the PSU fuse FS201.

If I fit a 500mA fuse in, which I think is the correct one, the amplifier starts, but fails in seconds.

While I've had the amplifier working, I've checked the output of the secondaries with an AVO meter, and it appears that the output is approx 25V each coil.

With the device turned off, I went around with an ohm meter, on the transformer, and there is low impedance (not 0) on the primary coil, low impedance on the secondaries (not 0), and nothing showing between the primary and secondary coils.

From what I've described above, I think that it's likely that the transformer may be okay?

I've emailed Arcam for a schematic yesterday, which I hope arrives soon, but in the meantime has anyone any suggestions on how to progress with diagnosing the faults, short of replacing things until I run out of patience, money, or it's fixed!

Best wishes,

Twosheds.
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Old 19th August 2008, 09:23 PM   #2
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Default Re: Arcam Alpha blows PSU fuse

Quote:
Originally posted by twosheds
I just bought an Arcam Alpha (original model - 1984) as a working amplifier, and after 1 hours use the fuse blew.

(I know that there is a similar thread on the BB, but I think that this is a different problem).

I found that it had a 7.5A 250V fuse in the PSU fuse FS201.

If I fit a 500mA fuse in, which I think is the correct one, the amplifier starts, but fails in seconds.

While I've had the amplifier working, I've checked the output of the secondaries with an AVO meter, and it appears that the output is approx 25V each coil.

With the device turned off, I went around with an ohm meter, on the transformer, and there is low impedance (not 0) on the primary coil, low impedance on the secondaries (not 0), and nothing showing between the primary and secondary coils.

From what I've described above, I think that it's likely that the transformer may be okay?

I've emailed Arcam for a schematic yesterday, which I hope arrives soon, but in the meantime has anyone any suggestions on how to progress with diagnosing the faults, short of replacing things until I run out of patience, money, or it's fixed!

Best wishes,

Twosheds.

It will be muc heasier to fault find once you have the schematic.

You can of course check obvious things with a multimeter.
Short cct caps. SHort output transistors etc etc
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Old 20th August 2008, 12:05 AM   #3
jaycee is offline jaycee  United Kingdom
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The 7.5A fuse is *definitely* wrong ! and you are correct it should be a T500mA fuse.

If Arcam don't get back to you, I have the service manual for the early Alpha's here.

Might be worth checking C212 in case it's gone short (it's an X-rated capacitor across the Live and Neutral terminals). Try powering up with the secondaries disconnected from the PCB. If the fuse still blows then that capacitor is probably bad (it'll have seen a lot of abuse!)
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Old 20th August 2008, 11:52 AM   #4
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Thanks for the replies, and the tip. I think that I'll replace that capacitor before removing the secondaries from the circuit, as this looks the easiest thing to do, and cost from Farnell is only 20p to boot.

This is a suppression capacitor isn't it? Excuse my lack of knowledge, but does this suppress the amplifier from creating interferance with other devices? Would it be safe to run the amplifier for test purposes without this capacitor do you think?

(I have not heard from Arcam yet.)

Thanks again,

Paul.
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Old 20th August 2008, 12:23 PM   #5
Mooly is online now Mooly  United Kingdom
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You mean this one Arcam Alpha 3 Left Channel broken...

We got there in the end.
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Old 20th August 2008, 12:56 PM   #6
AndrewT is offline AndrewT  Scotland
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disconnect the transformer from the downstream side as suggested.
Then plug in a mains bulb tester. power up the transformer through the mains bulb tester.
Tell us what you measure/find.

The X rated capacitor on the primary side is to suppress interference coming in from the mains. It is very unlikely that this has gone bad.
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Old 20th August 2008, 02:28 PM   #7
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Hello.

I managed to remove the secondary connections from the PCB, powered it up, and within seconds the fuse blew. I tested this twice, and twice it blew.

Therefore the problem has to be the transformer doesn't it, as it is blowing the fuse with no load?

I have seen on the forum that someone used an 18VA output torroidal as a substitute transformer.

I now have the service manual, but I can't see the spec of the transformer in the Arcam Alpha, although it does seem to indicate an output of 33V.

Would something like this or this do the trick?

Thanks everyone for your help so far.

As you'll have guessed, I'm a bit of a novice at this, but will be over the moon to get this working.
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Old 20th August 2008, 02:38 PM   #8
AndrewT is offline AndrewT  Scotland
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if you blew the fuse then you ignored my advice to power the transformer up through the mains bulb tester.

Do you want our help?
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Old 20th August 2008, 03:34 PM   #9
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Hi.

I really appreciate your help.

(I replaced the capacitor as suggested. This didn't help.)

I just thought I was doing the right thing. I disconnected the transformer from the downstream side and tested - or at least I thought I was.

I'm not sure what a mains bulb tester is, but I've been working on this problem with an electrician at work, and I guess he'll have one, so I'll see if I can get hold of one tomorrow morning to try out the transformer.

I'll let you know how I get on.
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Old 20th August 2008, 03:38 PM   #10
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By the way - I didn't ignore your advice. I had already tested in the way I said before I had chance to read your message.

Best wishes,

twosheds.
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