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Old 12th May 2010, 07:53 PM   #1
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Default Arcam Alpha 8 DEAD

I have an Arcam Alpha 8 integrated amplifier that blows the fuse on powerup. I have checked it over and it seems the output transistors on one side have shorted. The problem is that i am struggling to find replacements.
There are 4 output transistors in total (2 per side) and they are as follows; On one side there are 2 x IRF540 MOROCCO W841 and on the other side there are 2 x IRF540 MOROCCO 3731.
I have had a look on the internet to find replacements but i can't find the specific IRF540 that i am after.
In the past i have had amp's that have had outputs that have gone and it has been easy to find replacement's specific to each channel (npn/pnp).
Any help would be very much appreciated.
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Old 12th May 2010, 10:38 PM   #2
amc184 is offline amc184  New Zealand
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The label 'Morocco' and the labels 'W841' and '3731' are the place of manufacture and batch code respectively. There are no subtypes of IRF540. If you want to buy really exact replacements, get ST Microelectronics brand replacements, as that who fabs semiconductors in Morocco.

You'll also want to check the components upstream of the output MOSFETs, as just because the outputs are blown doesn't mean that they were the cause of the fault. There could still be other components that are bad that will cause the new IRF540s to blow as soon as you start the amp back up.
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Old 13th May 2010, 12:51 AM   #3
jaycee is offline jaycee  United Kingdom
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Any IRF540 can be used to repair this amp. I repaired a 7R (same circuit) using International Rectifier brand ones instead of the ST's that were originally there. It works fine.

Check Q(10)3, (10)4 and (10)5 in addition to the MOSFETs. Use a 60W bulb in place of the main fuse, or in series with the Live wire, when testing. If the bulb lights brightly, you still have a problem.

Email Arcam and they will quite happily send you the service manual.
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Old 13th May 2010, 06:56 AM   #4
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Thanks a lot for the help guy's.

There is actually only one pair of transistors blown, would you replace just the blown ones or all 4?

The amp's speaker cables were shorted and this is why the transisor's were blown.

Will check the other components as stated and let you know.
Thanks again.
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Old 13th May 2010, 07:19 AM   #5
Mooly is offline Mooly  United Kingdom
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Replace just the blown pair in the faulty channel... do as Jaycee says with the 60 or 100 watt bulb and make sure if there is a quiescent current adjustment that you adjust this correctly.
This adjustment sets the standing current in the output pair... the value will be in the manual, and where to measure (volt drop across drain or source resistor on outputs usually)... or compare with other channel and set to match.
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Old 13th May 2010, 11:27 PM   #6
jaycee is offline jaycee  United Kingdom
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The last one I repaired was damaged in a power surge, I replaced all 4 just to be sure, but if you know that they blew because of a dead short then it should be OK to replace just one channel.
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Old 14th May 2010, 03:03 PM   #7
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OK the amp is now working. Can someone explain how i would test and adjust. I haven't got the manual - surely there is a way to test the original channel and then set the new channel to the same value. I am not nearly as experienced as i want to be so i would prefer it to be spelled out to me so that i make no mistake as so far so good!
Thanks again for all the help so far.
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Old 16th May 2010, 01:27 AM   #8
jaycee is offline jaycee  United Kingdom
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Turn the trimpot, RV1 or RV101 depending on channel, down to minimum (fully anticlockwise). Connect a multimeter in millivolts over R1 /R101 depending on channel. Turn the amp on with no speakers, and with the signal input shorted to ground. Adjust the trim pot so that the reading across the resistor reads between 2.3-2.8mV.

You will need to adjust this over about 10 minutes as the amp warms up - the voltage will change as it warms up. Make sure it remains stable. I'd advise checking the other channel is set correctly also as it may have drifted with age. Adjust the trimpot slowly and carefully - careless adjustment can blow the MOSFETs. Use some insulated crocodile clip leads to connect over the resistor, and ensure nothing can short.
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Old 16th May 2010, 01:40 AM   #9
Bigun is offline Bigun  Canada
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Please feel free to send me away if this is off topic or threadjacking or something like that, but mention of Arcam caught my attention. I've heard very positive things about the sound quality of Arcam gear. I would be interested to learn why - circuit topology ??
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Old 16th May 2010, 02:13 AM   #10
jaycee is offline jaycee  United Kingdom
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The designs are nothing super special but they are well implemented, and the most important thing is that Arcam products are very well built with quality components. They certainly sound good too.
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