Arcam Alpha 8 DEAD

[rhiollyrubgt]

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.

 

[amc184]

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.

 

[jaycee]

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.

 

[rhiollyrubgt]

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.

 

[Mooly]

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.

 

[jaycee]

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.

 

[rhiollyrubgt]

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.

 

[jaycee]

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.

 

[Bigun]

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 ??

 

[jaycee]

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.

 

[Bigun]

ah, found it !

http://www.diyaudio.com/forums/solid-state/153378-arcam-alpha-8r-schematic.html

Interestingly, one of their newer models"...features new output device technology which practically eliminates thermal modulation of audio signals to ensure a truly dynamic and transparent reproduction. "

which turns out to mean:

"includes new Sanken output devices and improved thermal coupling designed to offer extremely stable device temperatures for excellent linearity."

anyhow, sorry for busting in but this Arcam 8 schematic looks interesting... glad to hear the unit in question is almost repaired.

 

[jaycee]

If you want to see a fairly interesting topology in one of their amps, look at the DiVA A85 service manual. They use an opamp in an interesting way. Also pretty good on the DiVA A65 and A75 is the regulated frontend voltage.

 

[Charles Darwin]

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.

Used to own an Arcam Alpha 8 and that's not my experience at all.
Had to have it fixed three times under warranty and I found the bass quite mushy and bloated, almost as if it had a loudness function built in.
That said it worked ok with small speakers at low volumes which is why I made the mistake of buying it in the first place.

 

[rhiollyrubgt]

All Done! Jaycee you da man!
Many thanks for all your time.

 

[oncable]

I also have an Arcam Alpha 8 amplifier with the output transistors on one channel shorted. I presume it was a power surge. I plan to replace them myself but on reading the advice given here, I should also check the transistors Q103, 104 and 105 as being possible causes of the problem. How should I check them? I have tested continuity (in case it means anything) on these 3 transistors in circuit with the following results:
Q 103 - BC556B - continuous between B and E
Q 104 - BC546B - continuous between B and C
Q 105 - BC556B - continuous between B and E

I have found a circuit diagram for this amp but I have no experience reading it, although I have located the parts mentioned. Should I just automatically replace these 3 transistors as well?

thanks,
Jerry

 

[Mooly]

You need to learn to measure and how to test semiconductors. You can't normally test in circuit. Also continuous reading depends on what range your DVM is on. It should initially be on "diode check" If they read 0.00 or short on that then they are duff.

 

[oncable]

Thanks Mooly for your post. So I can't test the transistors in circuit. It would seem then that if I have to desolder them anyway I might as well replace them? Can anyone suggest a supply source online? I have checked Digi-Key, Jameco, Mouser and Electronic Goldmine. None of them have all three of the transistors that I need and some have minimum order requirements. The local electronics shops have none in stock. The amp is only 11 years old but already they tell me these parts are obsolete and special order. I guess things move faster than they used to Or does saying that date me? Oh well. Anyway, any help is much appreciated. I'm going to have a shot at fixing this thing myself, it really is too good to junk it.

 

[jaycee]

Parts obsolete ? Utter rubbish - there is nothing in the Alpha 8 that is obsolete. The IRF540 mosfets are made by at least 3 manufacturers (IR, ST, Fairchild) and the BC546/547/556/557 transistors are again very common.

If in doubt, fit new ones - they are so cheap you might as well. I used IR made IRF540 mosfets, but the ST or Fairchild ones will work just as well. Make sure you follow the service manual's guide to biasing after replacing these parts.

 

[Mooly]

You can't test them fully in circuit. If they genuinely read short on a low ohms/diode range then they are faulty. If you read them on a high meg/ohm range they would probably read "short" due to other circuit elements swamping the reading. Comes down to experience... so if in doubt you must at least unsolder two of the legs to isolate.

As mentioned these things are so cheap....
It is however useful to know if they have failed as that can determine whether other areas of the circuit may be damaged.

As always when powering up after repair use a bulb tester.

 

[oncable]

Well after a month of waiting for the order to be filled I have finally received the replacement parts. I replaced the two blown output mosfets as well as the 3 other transistors in the circuit just to be safe (2x BC556B and 1x BC546B). The two mosfets that were removed were shorted. I then turned down both IQ presets fully anticlockwise and turned power on. Fuse blew again. Not instantly but within a second or two. On checking the mosfets again, one of the new ones is now shorted. I thought that turning down the presets (R101 and R1) would have prevented a new short but no such luck. So something else is causing the outputs to short. Anyone have any suggestions?