Arcam Alpha 9p blowing fuse

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#21
Thanks Mooly....you get about a bit...don't you!! 🙂
Wow....lot's of inconsistent behaviour across the 4 of 'em....comes down to 2 being kaputski, I think can be fairly deduced!!
When it comes to replacing these things...is quality important...or is it straight down to spec??
Additionally...given that, without MosFets, the unit powers on without blowing fuses...might that imply that the driver transistors are OK....or is that wishful thinking on my part?? 🙂
 
#22
Simple way to test MOSFETs:

1) Ensure you observe anti-static precautions always. Always store in conductive foam or similar.

2) short gate to source, measure source-drain in both directions and with diode test in both directions. It should look like a silicon diode. Power MOSFETs have body diodes as an integral part of their structure. (Compare to an ordinary diode if unsure how you meter behaves).

3) connect gate to drain, use a current-limited supply set to 100mA or so and no more than 10V to check it conducts and measure the gate-source voltage at that current. The polarity of the part determines the polarity of the supply, n-channel use +ve to drain/gate, -ve to source. This will warm up the device, don't dwell too long.

Static-damaged parts can sometimes still function, but typically have significant leakage current between gate and other terminals - so measure the resistance between gate and source using a low-voltage device like a modern multimeter. A gate-source zener or TVS diode is a great way to protect the gate oxide layer in-circuit.
 
#25
How much continuity? It should always read as a diode one way around and them provided there is no stray charge on the gate, open circuit the other way.

If you tag a high value resistor between G and S (to hold the gate OFF) then it should read open one way around.

If its reading just a few ohms D to S then its zapped.
 
#26
.....back on this now!!
Got myself 4 x IRF540bpf's from RS...and fitted this afternoon.
Amp now powers up...and doesn't take out a fuse 🙂
Have set the QI as per service manual....both channels now settle at 2.45 to 2.5mV, which is within required range of 2.3mV to 2.8mV.
Surprised at length of time taken for voltage to stabilise....creeps up from around 1.6mV and takes 3-4 minutes to settle finally....is this normal please??
 
#27
It's absolutely normal for bias current to wander about and this is due to thermal changes in the properties of the semiconductors.

Only when the whole amp is at a constant temperature can you accurately assess the bias.

And well done :up:
 
#28
Thanks Mooly!!!

Thanks for your patience Mooly....hoping that might be the end of Arcam trouble for now! (plan is to move out my 7R-CD, 8R/8P, and matching tuner, and go towards a better 2nd hand CD player 🙂 )

Now I can get on with re-capping and fettling 2 x JLH Monoblocks...hope to be hanging on to those tho'....😀


Love this forum!!! 🙂
 
#30
Phono stage now not working!!!

Thought I'd wake up this thread rather than start a new one!! 🙂

Believed that I'd successfully repaired the 8R integrated amp...but had occasion to use the Phono stage today....and realised that it isn't working correctly.
It's low in volume...and some distortion.

Perhaps...when the amp initially gave up the ghost, and needed O/P transistors replacing....it possibly 'took out' the phono stage at the same time??
Just wondering!!?

Hoping to get some pointers on getting said phono stage working again
.....any thoughts please???

(I have a decent DMM)

Thanks
 
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#31
As always... we need to see a circuit diagram.

Phono stages often use opamps (they almost certainly will not be at fault) but you need to check basics like supplies.

Also look how the input switching is done. If electronic then there may be some problem there. Scope checks tracing the signal should show where the problem is.
 
#33
That looks like the one. So if phono is distorted and yet the line inputs are not then you are looking around the opamp to begin with. The phono output just feeds into the same selector as all the line inputs.

Make sure that switch marked SW302 under the opamps (if fitted) is in the correct position. Looks to be for an external phono stage.

Check the supplies to the opamp.

Beyond that and you need to look at the opamp output with a scope and see what the signal looks like. It should be similar to a line level stage at that point.
 
#35
Aaagghh....describing Phono stage, service manual states:
"On the Alpha 8R this stage can be bypassed by use of switch SW302 to enable use as an extra line input."
...That's the switch....had no idea that was even there (had no use for an extra line-in!)
Suspect I've flipped that while PCB was out for the earlier repair...😱

Here's hoping for a simple/cheap fix!! 🙂
 
#36
Aaagghh....describing Phono stage, service manual states:
"On the Alpha 8R this stage can be bypassed by use of switch SW302 to enable use as an extra line input."
...That's the switch....had no idea that was even there (had no use for an extra line-in!)
Suspect I've flipped that while PCB was out for the earlier repair...😱

Here's hoping for a simple/cheap fix!! 🙂
 
#39
Alpha 9 Fixed for only a couple of minutes!

I got hold of an Alpha 9 integrated blowing fuses and sure enough, one channel's output transistors were short. I replaced them with apparent success but upon testing it only took a couple of minutes before the fuse blew and the other pair of o/p transistors have died.
The heatsinks seemed to have heated up far more than I'd expect with the low volume and short time I was testing.
Any ideas?
Off to order another pair of IRPF240's 🙄

Geoff
 
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