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Old 17th November 2005, 04:19 PM   #1
meacho is offline meacho  United Kingdom
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Default Arcam Alpha 7 fuse keeps blowing

Hi
My amp stopped working so I pope her open to find the 1.25 A fuse blown. I changed the fuse and pop it went again!

So I figured Iíve got a problem! I disconnected the two orange and one blue wire from the transformer to the board and put in a new fuse (only had 3.15 A fuse left) and nothing happened reconnecting the three wires I turned it back on and some of the bits got a bit hot (it looked like it was the two ceramic 0.22ohm resistors next to the rhs amp chip).

Has anyone got any advice for things I can try to isolate the problem?

Any advice is much appreciated

Thanks, Matt
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Old 17th November 2005, 04:38 PM   #2
Netlist is offline Netlist  Belgium
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Matt,

The 0.22ohm resistors could be source resistors of the output transistors, indicating that these will probably be broken.
Don't keep pushing fuses in until the problem is solved, it will only make things worse and put you at risk.
I'm not sure how capable you are with electronics and the solder iron so be careful.

/Hugo
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Old 17th November 2005, 11:29 PM   #3
jaycee is offline jaycee  United Kingdom
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The outputs are MOSFETs if memory serves and it sounds like they have both failed short causing a +VE to -VE rail short.

If you want to check this with the power off, remove the fuses, make sure the supply caps are discharged and do a continuity test between source and drain on the mosfets. If you get a low resistance reading they are dead.

You should be able to email Arcam and get a service manual for this amplifier, if you know what you're doing it's invaluable as it includes the full schematic, biasing info, etc. If not send your amp to Arcam - they will still repair it even though the Alpha series is now obsolete. They still repair A60's to my knowlege, and they haven't been made since early 1990's

edit: just looked at the service manual and they are IRF540 devices. The two 0.22R ceramic resistors are in the -ve rail.
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Old 27th November 2005, 05:21 AM   #4
meacho is offline meacho  United Kingdom
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Hello
Thanks for the help!
I did a continuity check between the IRF540 units (Q101, Q102) and there is continuity between all the pins, on the other side of the amp (IRF540s Q1 and Q2) donít have this continuity so this is the problem?


If I replace them will they just blow again is there likely to be an underlying problem I need to fix first or do they sometimes just blow?


Thanks for all the help

Matt
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Old 27th November 2005, 06:56 AM   #5
jaycee is offline jaycee  United Kingdom
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Yep, that's the problem all right.

You may find you've got damage to the preceding stages. Typically the output MOSFETs dont fail unless the amplifier is being abused (short circuit, run into low impedance loads, played too loudly/into clipping). If none of that is the case, you may want to check your speaker cables aren't damaged or shorting at terminals.

If you really know what you're doing, ask Arcam for the service manual - they sent it to me for a friends amplifier no questions asked. Then you can see what parts comprise the RHS amplifier. The best part is the left channel still works so you have a good channel to refer to and take test measurements from.
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Old 29th November 2005, 05:00 PM   #6
meacho is offline meacho  United Kingdom
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Hi thanks for the help!

i got the two MOSFETs and put them in and kablamo it works!!!!

only thing is the right channel (the side with the replacement mosfets)now comes on before the left, it comes on distorted then the relay clicks the left channel comes on and by now the distortion on the right has gone.

Itís the same when you switch it off the right channel fades out as I guess a capacitor discharges. I was thinking that the relay has probably gone but donít really know.

Any ideas? Is it ok to keep using the amp like this or is it going to cause problems?

Thanks again!

Matt
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Old 29th November 2005, 07:24 PM   #7
anatech is offline anatech  Canada
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Hi Matt,
The relay contacts may be "welded" together on that channel. Inspect the relay and replace if defective.

-Chris
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Old 29th November 2005, 08:01 PM   #8
Bare is offline Bare  Canada
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Hmmm.. when My delta 290 "died"
Blown fuse, nothing energised, Turned out to be a Blown Diode. Suspect that yer problem was/is a faulty Diode (Replace all 4) leading to probs with the Output devices. Easy enough to try/fix.. but use at least the OEM quality.
Also it's V important that the Replacement fuse is of the 'Slo-Blow' type Don't ask:-)
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Old 29th November 2005, 09:22 PM   #9
meacho is offline meacho  United Kingdom
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Thanks
Well I put the meter across r3 and the relay a capacitor fills up after a second or two i have continuity on the other side between r103 and the relay the resistance just keeps going up and the meter never beeps?

Between the two pins on the relay on the working side there is no continuity on the dodgy side there is?

I guess your right and the pins have welded together?! But what does the lack of continuity suggest?

Any ideas about just keeping it like this and using the amp? Am I likely to cause any more damage?

Thanks

Matt
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Old 29th November 2005, 09:33 PM   #10
anatech is offline anatech  Canada
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Hi Matt,
Replace the relay. If the amp is okay, use it. I can't see it from here to test and make sure.

-Chris
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