Arcam Alpha 8R Amp blowin fuses.......

Hi All:D

As it says....the 8R amp (Conf. with an 8P in Bi Amp mode!) is blowing fuses
....2 so far!!

Had a similar problem with a Mission Cyrus 1 recently, and cured it by replacing 4 X rectifier diodes in the PS....am hoping this might be a similar fix....?:mischiev:

I have a decent meter at hand....but have no idea how to test the O/P transistors....
What would members check first??:confused:

Ta
Kev/Leeds/UK
 
....testing MOSFETS......

With meter set for continuity...
....on LH both semi's show no shorting between pins

....on RH....one semi shorts between all 3 pins, and the other shorts between outer pins.....

Farnell lists several replacements for the IRF540....some with different specs:(

......

Browse for Products | Farnell United Kingdom

which one would suffice??? (I may as well replace all 4...??!)
Kev
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
It's old, sounds poor in your opinion Bare, so just get rid of it?

If someone wants to keep their gear it's usually for one of a few good reasons:
They happen to like the product, its features or brand associations.
They don't have or want to spend the cash on replacing it.
They have environmental concerns about consuming electronic goods like food.
They're DIY faithfuls, looking for solutions they can implement themselves.

Whatever the reason, you have to really hack or burn out an amplifier to prevent repairs being done, as long as the repairer does find the root of the problem and suitable spare parts are available.

So kev, how about posting the circumstances when the amp failed this time. Same shorted speaker leads or is this different? It would seem to me that that the VI limiter circuit has not being doing its job if this is down to shorted outputs but let's not assume so yet.
 
Thanks for that Ian......tbh, I have no intention of disposing of the thing, since, up to now at any rate, it has well served its purpose!!!

I'm using it in my AV system....(with A Yamaha RX-V771)...to deliver main front speakers.

On this occasion....I'm certain that 'shorting' was not the issue. No symptoms observed, just became aware that the amp was dead, and checked to find a blown internal fuse.

.....was tempted to look to rectifier diodes once again.....assume I need to lift one 'leg' of each, as they can't be tested in situ.

.........have a nice meter.......but would need coaching on how to use it for anything other than continuity testing:(
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I don't think the rectifier diodes were at fault last time - Is there something that suggests this? As you're using it for front speakers, I'd guess it has a harder workload than a lot of stereo systems but the protection should have kicked in last time too.

It's time to use the meter to test more than continuity. If you have the correct power supply voltages, the diodes must be operational and shouldn't need removal yet. Here's a basic tutorial on DMMs but the web is brimming with similar and more comprehensive instruction: 5 Ways to Use a Digital Multimeter - wikiHow

Set the meter (assuming a DMM) to 200V DC (look at the symbols for AC/DC selection), ground the -ve probe and test the voltage at the metal tabs (drain) of the output Mosfets with the +ve probe and be careful with slipping whilst using the meter. Clip, solder or use hook probes to secure at least the common ground probe connection where your attention will be divided and particularly when using blunt probes. Semis can vaporize in milliseconds so quick reflexes are of little help.

Look first at voltages on the Mosfet metal tabs (drain). [IMGDEAD]https://lh3.googleusercontent.com/gI70b6jRPrIsWRAkNdvjW8-AHJGIQQI-zmx-tXU-a_gLiqK46oV3yajewqdEtGgPyV_E1A=s91[/IMGDEAD] They should be close to +/- 37V depending whether they are Q1,101 or Q2,102
If there is power, the rectifier diodes are fine.

The service manual for this multi-model amplifier can be downloaded after registration here: Arcam Alpha 8 Manual - Stereo Integrated Amplifier - HiFi Engine
On pages 2-3 are all the test voltages which should be within 5 percent of correct if your local mains is reliable. With power off, check the Mosfets for the same open S-D connection using the meter set to a low resistance scale.

I suspect you have a repeat of the previous problem and that it likely stems from a different problem - perhaps with the protection circuit around IC Z203. Typically, the small caps around it and near the heatsinks will dry out due to local heat and it's operation is affected - sometimes remaining on, sometimes not protecting at all.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
On reading the tutorial again - the suggestion to measure mains AC at power points or anywhere, could be fatal unless the meter and its probes are rated (certified) at least to cat.2 safety level. https://en.wikipedia.org/wiki/Measurement_category
I have seen cheap meters burn out or even blow apart when used to test 240V mains - some idiots can't resist the urge to test everything in sight with a new toy. Also, the probes shown have no finger guard and should only be used with safe voltages, IMHO. "Don't do it!" is safest.

* As usual, I forgot about quasi-complementary Mosfets, where the lower Mosfet source (pin 3) for Q2 and Q102, is connected to the -37V supply instead of the drain. Please revise the suggestion for measuring the supply voltage from the topside and apologies.
 
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My Arcam did the same. Luckily it was new and was fixed under warranty...4 times.

The fourth time took a bit longer and I was told they were fixing a design fault in the PSU.
Didn't make it sound any better though. My Alpha 8 was the second worst amp I've ever owned, only an ancient Dynaco Stereo120 transistor amp sound worse.

On a lighter note I have a valve-powered Leslie sitting in my shed.
Interested?